What size drill do I need for an M6 tap?

For an M6 x 1.0 coarse thread the tap drill is 5.0 mm. That comes from the standard metric rule, tap drill = major diameter - pitch, which is 6.0 - 1.0 = 5.0 mm, and it gives about 77% thread engagement. For M6 x 0.75 fine thread use a 5.2 mm drill. If you are using a forming (roll) tap instead of a cutting tap, drill 5.5 mm.

What is the tap drill size for 1/4-20?

The tap drill for 1/4-20 UNC is a #7 drill, 0.201 inches. It comes from drill = D - (percentage/100) x 1.29904 / TPI, which at 75% thread is 0.250 - 0.75 x 1.29904 / 20 = 0.2013 inches. The nearest stock drill is #7. In metric a 5.1 mm drill is the closest equivalent. For 1/4-28 UNF use a #3 drill (0.213 inches).

How do you calculate tap drill size?

For 60 degree threads (ISO metric, UNC, UNF) the tap drill diameter is: drill = major diameter - (percent thread / 100) x 1.29904 x pitch, with pitch in mm, or x 1.29904 / TPI in inches. Rearranged, the engagement a given drill produces is: percent = 76.98 x (major diameter - drill) / pitch. The constant 1.29904 is 0.75 x the square root of 3. Some online calculators wrongly use 1.0825, which is the basic profile depth and gives the wrong drill.

Is the tap drill = diameter minus pitch rule 75% thread?

No. The metric rule of thumb, tap drill = major diameter - pitch, actually gives 76.98% thread engagement, not 75%. It is almost always described as 75% but the arithmetic says otherwise: 100 / 1.29904 = 76.98. Published metric charts are built on the D - P rule, while published inch charts are built at exactly 75%, which is why the two systems give slightly different engagements for equivalent sizes.

Why is 100% thread engagement a bad idea?

Because the strength gain is tiny and the cost is large. Going from 60% to 100% thread adds only a few percent of stripping strength, but roughly triples the tapping torque and dramatically increases the chance of breaking the tap. Most shops use 65% to 75% thread. In tough materials such as stainless steel or titanium, 50% to 60% is common and still develops full bolt strength, because the bolt itself fails before the thread strips when the engaged length is at least one diameter.

What drill size do I use for a roll form tap?

A forming or roll tap displaces material rather than cutting it, so it needs a larger hole than a cutting tap - roughly half the reduction. The hole is: drill = major diameter - (percent / 100) x 1.29904 x pitch / 2. For M6 x 1.0 that is 5.5 mm instead of 5.0 mm, for M8 x 1.25 it is 7.4 mm instead of 6.8 mm, and for M10 x 1.5 it is 9.3 mm instead of 8.5 mm. Roll tap holes are tuned by each tap maker, so confirm against your supplier data for production work.

Tap Drill Size Chart & Calculator

Metric • UNC • UNF • UNEF • BSW — tap drill, clearance drill & % thread engagement

Mode

Tap

Thread Nomenclature

Speeds & Feeds 📖 User Guide

Thread or drill size

Common:

Type a thread designation (M6, 1/4-20, #10-32) or a drill size (5.2 mm, #7, F) and this page answers in one step — both directions.

Tap drill values are the published chart convention: metric rows use the D − P rule (76.98% thread), inch rows use exactly 75%. The % column is the engagement the listed drill really produces. Sort by clicking a heading.

157 thread sizes
Thread	Series	Pitch	TPI	Tap drill	mm	inch	% thread	Clearance	Roll tap
ISO Metric Coarse — ISO 261
M1 × 0.25	Metric Coarse	0.25 mm	101.60	0.75 mm	0.75	0.0295	77.0%	—	0.9 mm
M1.1 × 0.25	Metric Coarse	0.25 mm	101.60	0.85 mm	0.85	0.0335	77.0%	—	1 mm
M1.2 × 0.25	Metric Coarse	0.25 mm	101.60	0.95 mm	0.95	0.0374	77.0%	—	1.1 mm
M1.4 × 0.3	Metric Coarse	0.3 mm	84.67	1.1 mm	1.10	0.0433	77.0%	—	1.25 mm
M1.6 × 0.35	Metric Coarse	0.35 mm	72.57	1.25 mm	1.25	0.0492	77.0%	1.8 mm	1.45 mm
M1.8 × 0.35	Metric Coarse	0.35 mm	72.57	1.45 mm	1.45	0.0571	77.0%	—	1.65 mm
M2 × 0.4	Metric Coarse	0.4 mm	63.50	1.6 mm	1.60	0.063	77.0%	2.4 mm	1.8 mm
M2.2 × 0.45	Metric Coarse	0.45 mm	56.44	1.75 mm	1.75	0.0689	77.0%	—	2 mm
M2.5 × 0.45	Metric Coarse	0.45 mm	56.44	2.05 mm	2.05	0.0807	77.0%	2.9 mm	2.3 mm
M3 × 0.5	Metric Coarse	0.5 mm	50.80	2.5 mm	2.50	0.0984	77.0%	3.4 mm	2.75 mm
M3.5 × 0.6	Metric Coarse	0.6 mm	42.33	2.9 mm	2.90	0.1142	77.0%	—	3.2 mm
M4 × 0.7	Metric Coarse	0.7 mm	36.29	3.3 mm	3.30	0.1299	77.0%	4.5 mm	3.7 mm
M4.5 × 0.75	Metric Coarse	0.75 mm	33.87	3.7 mm / 3.8 mm	3.70	0.1457	82.1%	—	4.1 mm
M5 × 0.8	Metric Coarse	0.8 mm	31.75	4.2 mm	4.20	0.1654	77.0%	5.5 mm	4.6 mm
M6 × 1	Metric Coarse	1 mm	25.40	5 mm	5.00	0.1969	77.0%	6.6 mm	5.5 mm
M7 × 1	Metric Coarse	1 mm	25.40	6 mm	6.00	0.2362	77.0%	—	6.5 mm
M8 × 1.25	Metric Coarse	1.25 mm	20.32	6.7 mm / 6.8 mm	6.70	0.2638	80.1%	9 mm	7.4 mm
M10 × 1.5	Metric Coarse	1.5 mm	16.93	8.5 mm	8.50	0.3346	77.0%	11 mm	9.3 mm
M12 × 1.75	Metric Coarse	1.75 mm	14.51	10.2 mm / 10.3 mm	10.20	0.4016	79.2%	13.5 mm	11.1 mm
M14 × 2	Metric Coarse	2 mm	12.70	12 mm	12.00	0.4724	77.0%	15.5 mm	13 mm
M16 × 2	Metric Coarse	2 mm	12.70	14 mm	14.00	0.5512	77.0%	17.5 mm	15 mm
M18 × 2.5	Metric Coarse	2.5 mm	10.16	15.5 mm	15.50	0.6102	77.0%	20 mm	16.8 mm
M20 × 2.5	Metric Coarse	2.5 mm	10.16	17.5 mm	17.50	0.689	77.0%	22 mm	18.8 mm
M22 × 2.5	Metric Coarse	2.5 mm	10.16	19.5 mm	19.50	0.7677	77.0%	24 mm	21 mm
M24 × 3	Metric Coarse	3 mm	8.47	21 mm	21.00	0.8268	77.0%	26 mm	22.5 mm
M27 × 3	Metric Coarse	3 mm	8.47	24 mm	24.00	0.9449	77.0%	30 mm	25.5 mm
M30 × 3.5	Metric Coarse	3.5 mm	7.26	26.5 mm	26.50	1.0433	77.0%	33 mm	28.5 mm
M33 × 3.5	Metric Coarse	3.5 mm	7.26	29.5 mm	29.50	1.1614	77.0%	36 mm	31.5 mm
M36 × 4	Metric Coarse	4 mm	6.35	32 mm	32.00	1.2598	77.0%	39 mm	34 mm
M39 × 4	Metric Coarse	4 mm	6.35	35 mm	35.00	1.378	77.0%	42 mm	37 mm
M42 × 4.5	Metric Coarse	4.5 mm	5.64	37.5 mm	37.50	1.4764	77.0%	45 mm	40 mm
M45 × 4.5	Metric Coarse	4.5 mm	5.64	40.5 mm	40.50	1.5945	77.0%	48 mm	43 mm
M48 × 5	Metric Coarse	5 mm	5.08	43 mm	43.00	1.6929	77.0%	52 mm	45.5 mm
M52 × 5	Metric Coarse	5 mm	5.08	47 mm	47.00	1.8504	77.0%	56 mm	49.5 mm
M56 × 5.5	Metric Coarse	5.5 mm	4.62	50.5 mm	50.50	1.9882	77.0%	62 mm	53.5 mm
M60 × 5.5	Metric Coarse	5.5 mm	4.62	54.5 mm	54.50	2.1457	77.0%	66 mm	57.5 mm
M64 × 6	Metric Coarse	6 mm	4.23	58 mm	58.00	2.2835	77.0%	70 mm	61 mm
ISO Metric Fine — ISO 261
M3 × 0.35	Metric Fine	0.35 mm	72.57	2.65 mm	2.65	0.1043	77.0%	3.4 mm	2.85 mm
M4 × 0.5	Metric Fine	0.5 mm	50.80	3.5 mm	3.50	0.1378	77.0%	4.5 mm	3.8 mm
M5 × 0.5	Metric Fine	0.5 mm	50.80	4.5 mm	4.50	0.1772	77.0%	5.5 mm	4.8 mm
M6 × 0.75	Metric Fine	0.75 mm	33.87	5.2 mm / 5.3 mm	5.20	0.2047	82.1%	6.6 mm	5.6 mm
M8 × 1	Metric Fine	1 mm	25.40	7 mm	7.00	0.2756	77.0%	9 mm	7.5 mm
M8 × 0.75	Metric Fine	0.75 mm	33.87	7.2 mm / 7.3 mm	7.20	0.2835	82.1%	9 mm	7.6 mm
M10 × 1.25	Metric Fine	1.25 mm	20.32	8.7 mm / 8.8 mm	8.70	0.3425	80.1%	11 mm	9.4 mm
M10 × 1	Metric Fine	1 mm	25.40	9 mm	9.00	0.3543	77.0%	11 mm	9.5 mm
M10 × 0.75	Metric Fine	0.75 mm	33.87	9.2 mm / 9.3 mm	9.20	0.3622	82.1%	11 mm	9.6 mm
M12 × 1.5	Metric Fine	1.5 mm	16.93	10.5 mm	10.50	0.4134	77.0%	13.5 mm	11.3 mm
M12 × 1.25	Metric Fine	1.25 mm	20.32	10.7 mm / 10.8 mm	10.70	0.4213	80.1%	13.5 mm	11.4 mm
M12 × 1	Metric Fine	1 mm	25.40	11 mm	11.00	0.4331	77.0%	13.5 mm	11.5 mm
M14 × 1.5	Metric Fine	1.5 mm	16.93	12.5 mm	12.50	0.4921	77.0%	15.5 mm	13.3 mm
M16 × 1.5	Metric Fine	1.5 mm	16.93	14.5 mm	14.50	0.5709	77.0%	17.5 mm	15.3 mm
M18 × 2	Metric Fine	2 mm	12.70	16 mm	16.00	0.6299	77.0%	20 mm	17 mm
M18 × 1.5	Metric Fine	1.5 mm	16.93	16.5 mm	16.50	0.6496	77.0%	20 mm	17.3 mm
M20 × 2	Metric Fine	2 mm	12.70	18 mm	18.00	0.7087	77.0%	22 mm	19 mm
M20 × 1.5	Metric Fine	1.5 mm	16.93	18.5 mm	18.50	0.7283	77.0%	22 mm	19.3 mm
M22 × 2	Metric Fine	2 mm	12.70	20 mm	20.00	0.7874	77.0%	24 mm	21 mm
M22 × 1.5	Metric Fine	1.5 mm	16.93	20.5 mm	20.50	0.8071	77.0%	24 mm	21.5 mm
M24 × 2	Metric Fine	2 mm	12.70	22 mm	22.00	0.8661	77.0%	26 mm	23 mm
M24 × 1.5	Metric Fine	1.5 mm	16.93	22.5 mm	22.50	0.8858	77.0%	26 mm	23.5 mm
M27 × 2	Metric Fine	2 mm	12.70	25 mm	25.00	0.9843	77.0%	30 mm	26 mm
M30 × 2	Metric Fine	2 mm	12.70	28 mm	28.00	1.1024	77.0%	33 mm	29 mm
M33 × 2	Metric Fine	2 mm	12.70	31 mm	31.00	1.2205	77.0%	36 mm	32 mm
M36 × 3	Metric Fine	3 mm	8.47	33 mm	33.00	1.2992	77.0%	39 mm	34.5 mm
M39 × 3	Metric Fine	3 mm	8.47	36 mm	36.00	1.4173	77.0%	42 mm	37.5 mm
M42 × 3	Metric Fine	3 mm	8.47	39 mm	39.00	1.5354	77.0%	45 mm	40.5 mm
M45 × 3	Metric Fine	3 mm	8.47	42 mm	42.00	1.6535	77.0%	48 mm	43.5 mm
M48 × 3	Metric Fine	3 mm	8.47	45 mm	45.00	1.7717	77.0%	52 mm	46.5 mm
M52 × 3	Metric Fine	3 mm	8.47	49 mm	49.00	1.9291	77.0%	56 mm	50.5 mm
M56 × 4	Metric Fine	4 mm	6.35	52 mm	52.00	2.0472	77.0%	62 mm	54 mm
M60 × 4	Metric Fine	4 mm	6.35	56 mm	56.00	2.2047	77.0%	66 mm	58 mm
M64 × 4	Metric Fine	4 mm	6.35	60 mm	60.00	2.3622	77.0%	70 mm	62 mm
Unified Coarse (UNC) — ASME B1.1
#1-64 UNC	UNC	0.397 mm	64	#53	1.51	0.0595	66.5%	#36	#51
#2-56 UNC	UNC	0.454 mm	56	#50	1.78	0.07	69.0%	#31	5/64"
#3-48 UNC	UNC	0.529 mm	48	#47	1.99	0.0785	75.7%	#29	#43
#4-40 UNC	UNC	0.635 mm	40	#43	2.26	0.089	70.8%	#27	#39
#5-40 UNC	UNC	0.635 mm	40	#38	2.58	0.1015	72.4%	5/32"	#33
#6-32 UNC	UNC	0.794 mm	32	#36	2.71	0.1065	77.6%	#18	1/8"
#8-32 UNC	UNC	0.794 mm	32	#29	3.45	0.136	69.0%	#9	#25
#10-24 UNC	UNC	1.058 mm	24	#25	3.80	0.1495	74.8%	#1	#18
#12-24 UNC	UNC	1.058 mm	24	#16	4.50	0.177	72.1%	1/4"	#9
1/4-20 UNC	UNC	1.27 mm	20	#7	5.11	0.201	75.4%	9/32"	#1
5/16-18 UNC	UNC	1.411 mm	18	F	6.53	0.257	76.9%	11/32"	9/32"
3/8-16 UNC	UNC	1.588 mm	16	5/16"	7.94	0.3125	77.0%	13/32"	11/32"
7/16-14 UNC	UNC	1.814 mm	14	U	9.35	0.368	74.9%	15/32"	Y
1/2-13 UNC	UNC	1.954 mm	13	27/64"	10.72	0.4219	78.2%	17/32"	15/32"
9/16-12 UNC	UNC	2.117 mm	12	31/64"	12.30	0.4844	72.2%	19/32"	33/64"
5/8-11 UNC	UNC	2.309 mm	11	17/32"	13.49	0.5313	79.4%	21/32"	37/64"
3/4-10 UNC	UNC	2.54 mm	10	21/32"	16.67	0.6563	72.2%	25/32"	45/64"
7/8-9 UNC	UNC	2.822 mm	9	49/64"	19.45	0.7656	75.8%	29/32"	53/64"
1-8 UNC	UNC	3.175 mm	8	7/8"	22.22	0.875	77.0%	1-1/32"	15/16"
1-1/8-7 UNC	UNC	3.629 mm	7	63/64"	25.00	0.9844	75.8%	1-5/32"	1-1/16"
1-1/4-7 UNC	UNC	3.629 mm	7	1-7/64"	28.18	1.1094	75.8%	1-9/32"	1-3/16"
1-3/8-6 UNC	UNC	4.233 mm	6	1-7/32"	30.96	1.2188	72.2%	1-13/32"	1-19/64"
1-1/2-6 UNC	UNC	4.233 mm	6	1-11/32"	34.13	1.3438	72.2%	1-17/32"	1-27/64"
1-3/4-5 UNC	UNC	5.08 mm	5	1-9/16"	39.69	1.5625	72.2%	1-25/32"	1-21/32"
2-4.5 UNC	UNC	5.644 mm	4.50	1-25/32"	45.24	1.7813	75.8%	—	1-57/64"
Unified Fine (UNF) — ASME B1.1
#0-80 UNF	UNF	0.318 mm	80	3/64"	1.19	0.0469	80.8%	#42	#54
#1-72 UNF	UNF	0.353 mm	72	#53	1.51	0.0595	74.8%	#36	#51
#2-64 UNF	UNF	0.397 mm	64	#50	1.78	0.07	78.8%	#31	#47
#3-56 UNF	UNF	0.454 mm	56	#45	2.08	0.082	73.3%	#29	#43
#4-48 UNF	UNF	0.529 mm	48	#42	2.37	0.0935	68.4%	#27	#38
#5-44 UNF	UNF	0.577 mm	44	#37	2.64	0.104	71.1%	5/32"	#33
#6-40 UNF	UNF	0.635 mm	40	#33	2.87	0.113	77.0%	#18	1/8"
#8-36 UNF	UNF	0.706 mm	36	#29	3.45	0.136	77.6%	#9	#25
#10-32 UNF	UNF	0.794 mm	32	#21	4.04	0.159	76.4%	#1	#17
#12-28 UNF	UNF	0.907 mm	28	#14	4.62	0.182	73.3%	1/4"	#8
1/4-28 UNF	UNF	0.907 mm	28	#3	5.41	0.213	79.8%	9/32"	A
5/16-24 UNF	UNF	1.058 mm	24	I	6.91	0.272	74.8%	11/32"	L
3/8-24 UNF	UNF	1.058 mm	24	Q	8.43	0.332	79.4%	13/32"	T
7/16-20 UNF	UNF	1.27 mm	20	25/64"	9.92	0.3906	72.2%	15/32"	Z
1/2-20 UNF	UNF	1.27 mm	20	29/64"	11.51	0.4531	72.2%	17/32"	15/32"
9/16-18 UNF	UNF	1.411 mm	18	33/64"	13.10	0.5156	65.0%	19/32"	17/32"
5/8-18 UNF	UNF	1.411 mm	18	37/64"	14.68	0.5781	65.0%	21/32"	19/32"
3/4-16 UNF	UNF	1.588 mm	16	11/16"	17.46	0.6875	77.0%	25/32"	23/32"
7/8-14 UNF	UNF	1.814 mm	14	13/16"	20.64	0.8125	67.4%	29/32"	27/32"
1-12 UNF	UNF	2.117 mm	12	59/64"	23.42	0.9219	72.2%	1-1/32"	61/64"
1-1/8-12 UNF	UNF	2.117 mm	12	1-3/64"	26.59	1.0469	72.2%	1-5/32"	1-5/64"
1-1/4-12 UNF	UNF	2.117 mm	12	1-11/64"	29.77	1.1719	72.2%	1-9/32"	1-13/64"
1-3/8-12 UNF	UNF	2.117 mm	12	1-19/64"	32.94	1.2969	72.2%	1-13/32"	1-21/64"
1-1/2-12 UNF	UNF	2.117 mm	12	1-27/64"	36.12	1.4219	72.2%	1-17/32"	1-29/64"
Unified Extra Fine (UNEF) — ASME B1.1
#12-32 UNEF	UNEF	0.794 mm	32	#13	4.70	0.185	76.4%	1/4"	#7
1/4-32 UNEF	UNEF	0.794 mm	32	7/32"	5.56	0.2188	77.0%	9/32"	15/64"
5/16-32 UNEF	UNEF	0.794 mm	32	9/32"	7.14	0.2813	77.0%	11/32"	19/64"
3/8-32 UNEF	UNEF	0.794 mm	32	11/32"	8.73	0.3438	77.0%	13/32"	23/64"
7/16-28 UNEF	UNEF	0.907 mm	28	Y	10.26	0.404	72.2%	15/32"	27/64"
1/2-28 UNEF	UNEF	0.907 mm	28	15/32"	11.91	0.4688	67.4%	17/32"	31/64"
9/16-24 UNEF	UNEF	1.058 mm	24	33/64"	13.10	0.5156	86.6%	19/32"	35/64"
5/8-24 UNEF	UNEF	1.058 mm	24	37/64"	14.68	0.5781	86.6%	21/32"	39/64"
11/16-24 UNEF	UNEF	1.058 mm	24	41/64"	16.27	0.6406	86.6%	23/32"	43/64"
3/4-20 UNEF	UNEF	1.27 mm	20	45/64"	17.86	0.7031	72.2%	25/32"	23/32"
13/16-20 UNEF	UNEF	1.27 mm	20	49/64"	19.45	0.7656	72.2%	27/32"	25/32"
7/8-20 UNEF	UNEF	1.27 mm	20	53/64"	21.03	0.8281	72.2%	29/32"	27/32"
15/16-20 UNEF	UNEF	1.27 mm	20	57/64"	22.62	0.8906	72.2%	31/32"	29/32"
1-20 UNEF	UNEF	1.27 mm	20	61/64"	24.21	0.9531	72.2%	1-1/32"	31/32"
British Standard Whitworth (BSW) — BS 84
1/8" × 40 BSW	BSW	0.635 mm	40	#38	2.58	0.1015	73.4%	5/32"	#33
3/16" × 24 BSW	BSW	1.058 mm	24	#26	3.73	0.147	75.9%	7/32"	#19
1/4" × 20 BSW	BSW	1.27 mm	20	#7	5.11	0.201	76.5%	9/32"	#1
5/16" × 18 BSW	BSW	1.411 mm	18	G	6.63	0.261	72.4%	11/32"	L
3/8" × 16 BSW	BSW	1.588 mm	16	O	8.03	0.316	73.7%	13/32"	11/32"
7/16" × 14 BSW	BSW	1.814 mm	14	U	9.35	0.368	76.0%	15/32"	Y
1/2" × 12 BSW	BSW	2.117 mm	12	27/64"	10.72	0.4219	73.2%	17/32"	29/64"
9/16" × 12 BSW	BSW	2.117 mm	12	31/64"	12.30	0.4844	73.2%	19/32"	33/64"
5/8" × 11 BSW	BSW	2.309 mm	11	17/32"	13.49	0.5313	80.5%	21/32"	37/64"
3/4" × 10 BSW	BSW	2.54 mm	10	21/32"	16.67	0.6563	73.2%	25/32"	45/64"
7/8" × 9 BSW	BSW	2.822 mm	9	49/64"	19.45	0.7656	76.9%	29/32"	53/64"
1" × 8 BSW	BSW	3.175 mm	8	7/8"	22.22	0.875	78.1%	1-1/32"	15/16"
British Standard Fine (BSF) — BS 84
3/16" × 32 BSF	BSF	0.794 mm	32	#22	3.99	0.157	76.2%	7/32"	#17
1/4" × 26 BSF	BSF	0.977 mm	26	#3	5.41	0.213	75.1%	9/32"	A
5/16" × 22 BSF	BSF	1.155 mm	22	H	6.76	0.266	79.9%	11/32"	L
3/8" × 20 BSF	BSF	1.27 mm	20	21/64"	8.33	0.3281	73.2%	13/32"	S
7/16" × 18 BSF	BSF	1.411 mm	18	W	9.80	0.386	72.4%	15/32"	Z
1/2" × 16 BSF	BSF	1.588 mm	16	7/16"	11.11	0.4375	78.1%	17/32"	15/32"
9/16" × 16 BSF	BSF	1.588 mm	16	1/2"	12.70	0.5	78.1%	19/32"	17/32"
5/8" × 14 BSF	BSF	1.814 mm	14	9/16"	14.29	0.5625	68.3%	21/32"	19/32"
3/4" × 12 BSF	BSF	2.117 mm	12	43/64"	17.07	0.6719	73.2%	25/32"	45/64"
7/8" × 11 BSF	BSF	2.309 mm	11	25/32"	19.84	0.7813	80.5%	29/32"	53/64"
1" × 10 BSF	BSF	2.54 mm	10	29/32"	23.02	0.9063	73.2%	1-1/32"	61/64"

Every size on this page

Jump straight to any thread. Each link opens that size in Find mode with its tap drill, clearance drill, all four drill series and the engagement it gives.

ISO Metric Coarse — ISO 261

M1 × 0.25 M1.1 × 0.25 M1.2 × 0.25 M1.4 × 0.3 M1.6 × 0.35 M1.8 × 0.35 M2 × 0.4 M2.2 × 0.45 M2.5 × 0.45 M3 × 0.5 M3.5 × 0.6 M4 × 0.7 M4.5 × 0.75 M5 × 0.8 M6 × 1 M7 × 1 M8 × 1.25 M10 × 1.5 M12 × 1.75 M14 × 2 M16 × 2 M18 × 2.5 M20 × 2.5 M22 × 2.5 M24 × 3 M27 × 3 M30 × 3.5 M33 × 3.5 M36 × 4 M39 × 4 M42 × 4.5 M45 × 4.5 M48 × 5 M52 × 5 M56 × 5.5 M60 × 5.5 M64 × 6

Unified Extra Fine (UNEF) — ASME B1.1

#12-32 UNEF 1/4-32 UNEF 5/16-32 UNEF 3/8-32 UNEF 7/16-28 UNEF 1/2-28 UNEF 9/16-24 UNEF 5/8-24 UNEF 11/16-24 UNEF 3/4-20 UNEF 13/16-20 UNEF 7/8-20 UNEF 15/16-20 UNEF 1-20 UNEF

British Standard Whitworth (BSW) — BS 84

1/8" × 40 BSW 3/16" × 24 BSW 1/4" × 20 BSW 5/16" × 18 BSW 3/8" × 16 BSW 7/16" × 14 BSW 1/2" × 12 BSW 9/16" × 12 BSW 5/8" × 11 BSW 3/4" × 10 BSW 7/8" × 9 BSW 1" × 8 BSW

British Standard Fine (BSF) — BS 84

3/16" × 32 BSF 1/4" × 26 BSF 5/16" × 22 BSF 3/8" × 20 BSF 7/16" × 18 BSF 1/2" × 16 BSF 9/16" × 16 BSF 5/8" × 14 BSF 3/4" × 12 BSF 7/8" × 11 BSF 1" × 10 BSF

User Guide — Tap Drill Size Chart & Calculator

1 Overview

This page answers the tap drill question in both directions. Given a thread it gives you the drill; given a drill it gives you every thread that drill will tap. It covers 157 thread sizes — ISO metric coarse (M1–M64), ISO metric fine, UNC, UNF, UNEF and Whitworth BSW/BSF — cross-referenced against 579 drill sizes in all four naming systems: fractional inch, number (wire gauge #1–#80), letter (A–Z) and metric.

Five modes sit on the Mode pills: Find (one box, instant answer), Chart (the full printable table), Reverse (drill → thread), Visualise (see the thread form change as you move the engagement) and Learn (where the numbers come from, including where published sources disagree). The Tap pills switch between a cutting tap and a forming (roll) tap, which needs a noticeably larger hole.

2 Getting Started

The page opens in Find mode. Type what you have into the single box. It accepts sloppy input on purpose: M6, m6x1, M6 x 1.0, 1/4-20, .25-20, #10-32, 10-32, 5.2mm, #7 and F all work. Press Enter or click Find. If you type a bare diameter like M6 the coarse pitch is assumed and the fine pitches are offered as suggestions underneath.

The chips under the box cover the sizes people look up most. Every result gets a permalink — the page URL updates to something like #M6x1, so you can bookmark or share a single size.

3 Find Mode — reading the answer

The big card gives the two numbers you actually need: the tap drill (the hole you drill before tapping) and the clearance drill (the hole in the part the screw passes through). Both are given in mm and inches.

Below it, the % thread slider changes the target engagement and everything recalculates live. Four readout badges track the consequences: the engagement, the exact theoretical hole diameter, the resulting stripping strength as a fraction of a 100% thread, and the relative tapping torque. The preset chips give sensible starting points by material.

Nearest stock drill in every series is the part no printed chart gives you: the closest fractional, number, letter and metric drill, each labelled with the engagement that specific drill produces. If your index only has letter drills, this tells you exactly what you get.

4 Chart Mode

The full table, and the part of this page that works without JavaScript — every row is real text in the HTML, so you can search it with Ctrl+F, copy it, or print it. Filter with the search box or the series pills, and click any column heading to sort.

Print card produces a clean shop-floor sheet with the site colours stripped out, sized for A4 and US Letter. CSV downloads whatever is currently filtered.

The tap drill column follows the published chart convention, and the two systems genuinely differ: metric charts are built on the D − P rule (76.98% thread) while inch charts are built at exactly 75%. Section 8 explains why this matters.

5 Reverse Mode

The question printed charts cannot answer: I have a 5 mm drill in my hand — what can I tap with it? Enter any drill size and you get every thread that drill will tap, sorted by how close each lands to a sensible engagement, with the engagement and a verdict for each: ideal, usable, tight (hard tapping, risk of snapping) or loose (weak thread).

The Accept slider sets the minimum engagement you are willing to consider. Lower it to see more options; raise it to see only strong threads.

6 Visualise Mode

A true-scale cross-section of the tapped hole. The drilled hole, the thread form the tap cuts, and the engaged flank are drawn from the same engine that produces the numbers — move the slider and the thread visibly gets shallower as the hole grows. The shaded wedge is the metal the tap has to remove, which is why torque climbs the way it does.

Below it, the strength-versus-torque graph. Right-click the canvas for Save as Image, Copy Reading and Reset.

7 Cutting vs Forming (roll) taps

The Tap pills in the control bar switch the whole page between the two. A cutting tap removes metal to make the thread. A forming (roll, or fluteless) tap displaces it, so the crest is pushed up out of the wall — which means the starting hole must be larger, roughly half the reduction of a cutting tap. Drill an M6 roll-tap hole at 5.0 mm instead of 5.5 mm and you will snap the tap.

Forming taps give a stronger, burr-free thread with no chips, but only work in ductile materials (aluminium, low-carbon steel, copper, many stainless grades) — never in cast iron or hardened steel. Roll-tap holes are tuned by each tap maker, so for production work confirm against your supplier's data. This page tells you when it is showing a formed hole.

8 The formula, and why calculators disagree

For 60° thread forms (ISO metric, UNC, UNF, UNEF):

drill = D − (E/100) × 1.29904 × P (P in mm)
drill = D − (E/100) × 1.29904 / TPI (inches)
E = 76.98 × (D − drill) / P

The constant 1.29904 is 0.75 × √3. Several online calculators use 1.0825 instead — that is the basic-profile depth (2 × ⅝H) and it is the wrong reference for the % thread convention. The test is easy: 1/4-20 UNC at 75% must come out as a #7 drill (0.201″), which is what every published chart in the world lists. 1.29904 gives 0.2013″. 1.0825 gives 0.2297″, a letter A drill, which no chart lists.

Whitworth (BSW/BSF) is a 55° form with depth 0.640327 P, so its constant is 1.280654. Published Whitworth charts are not built on a single engagement convention — see the Sources tab in Learn mode.

The myth worth unlearning: the metric rule “tap drill = diameter − pitch” is described everywhere as 75% thread. It is not. 100 / 1.29904 = 76.98%.

9 Choosing an engagement

Higher is not better. Going from 60% to 100% thread adds only a few percent of stripping strength but roughly triples the tapping torque, and torque is what breaks taps. Practical starting points:

50–60% — stainless, titanium, tough alloys, deep holes, hand tapping.
65–70% — production CNC tapping in steel and aluminium.
75–77% — the published chart default; a good general-purpose choice.
80–90% — thin sections, soft materials, or where engaged length is short.

As long as the engaged thread length is at least one diameter, the bolt breaks before the thread strips, so extra engagement buys nothing. That is why a 60% thread in a 1 D deep hole is not a compromise.

10 Clearance holes

The clearance drill is the hole in the part the screw passes through, not the tapped part. Metric clearance values on this page come from ISO 273 and are given in three fit classes: close, medium (normal) and coarse (loose).

Inch clearance values are common shop practice, not a standard table — close = D + 1/64″, free = D + 1/32″, loose = D + 3/64″, each rounded up to the next real drill size. The page labels them as such rather than implying a standard it is not quoting. For inch work to a drawing, use ASME B18.2.8.

11 Practice & Quiz

Practice gives you a thread and asks for the drill. Type your answer in whichever series you think in — 5 mm, #7, F and 27/64 are all accepted — and it marks anything within 3% engagement of the chart value as correct, because in a real shop it would be. The feedback always shows the chart drill, the engagement it gives, and the engagement your answer would have given. Your running score sits at the top; press Enter to check, then Enter again for a new question.

Quiz is five multiple-choice questions with four plausible drills each — the distractors are real neighbouring drill sizes, not obvious wrong answers. After each answer the correct option is highlighted and explained. At the end you get a score out of five and a star rating. Both modes draw from the sizes you actually meet in a workshop (M3–M24 coarse, and UNC/UNF from #4 upward).

12 Tips & common mistakes

Drill deeper than the thread. A plug tap needs 3–5 lead threads beyond the last full thread; a bottoming tap 1–2. Rule of thumb for a blind hole: drill depth = required thread depth + 3 × pitch.
Chase the hole size in tough material. Stainless work-hardens; at 75% thread an M6 tap in 316 will fight you. Drop to 60% and it cuts cleanly.
The drill is bigger than nominal. A worn or badly ground drill cuts oversize, which lowers the real engagement. If threads come out loose, check the actual hole before blaming the chart.
Do not use a cutting-tap hole for a roll tap — the single most common way to snap a forming tap.
Number and letter drills overlap. #1 (0.228″) is smaller than letter A (0.234″), and letter E is exactly 1/4″. The series card shows all four so you cannot pick the wrong one by accident.

Tap Drill Size Chart — Metric, UNC, UNF and Whitworth

A tap drill chart answers one question: before you cut an internal thread, how big should the hole be? Drill it too small and the tap has to remove more metal than it can stand, and it snaps in the part — the most expensive five seconds in a workshop. Drill it too large and the thread is shallow and strips under load. This page gives you the number for 157 thread sizes across ISO metric coarse and fine, UNC, UNF, UNEF and Whitworth BSW/BSF, cross-referenced against all 579 standard drill sizes in the fractional, number, letter and metric series — and it works in reverse, which a printed chart cannot.

How tap drill size is calculated

For 60° thread forms the tap drill diameter follows directly from the thread's major diameter D, its pitch P, and the percentage of thread engagement E you want:

drill = D − (E / 100) × 1.29904 × P (metric, mm)
drill = D − (E / 100) × 1.29904 / TPI (inch)

and rearranged, the engagement that any given drill actually produces is E = 76.98 × (D − drill) / P. The constant 1.29904 is 0.75 × √3, the reference depth the tap-chart convention is built on. It is worth checking any calculator you find against a value everyone agrees on: 1/4-20 UNC at 75% thread must give a #7 drill (0.201″). A number of online tap drill calculators use 1.0825 instead — the basic-profile depth from ISO 68-1 — and return 0.2297″, a letter A drill, which appears in no published chart.

Why metric and inch charts use different conventions

Here is something almost no tap drill page mentions. Published inch charts are built at exactly 75% thread. Published metric charts are built on the familiar rule of thumb tap drill = major diameter − pitch — and that rule is 76.98%, not the 75% it is universally claimed to be, because 100 / 1.29904 = 76.98. So an M6 hole and a 1/4-20 hole are not sized to quite the same engagement, even though both charts say “75%”. The difference is small in practice but it explains why calculators built on a single assumption disagree with printed charts on one system or the other. This page uses each system's own convention in the Chart tab, shows you the real engagement for every value, and lets you override it with the slider.

What percentage of thread should you actually use?

The instinct is that more thread means a stronger joint, so 100% must be best. The arithmetic says otherwise. Thread stripping strength rises steeply up to about 60% engagement and then flattens out, while the tapping torque keeps climbing almost linearly. Going from 60% to 100% buys a few percent of strength and costs roughly three times the torque — and torque is what breaks taps. This is why experienced shops tap at 65–75%, and drop to 50–60% in stainless steel, titanium and other tough or work-hardening alloys. As long as the engaged thread length is at least one nominal diameter, the bolt fails before the thread strips, so the extra engagement is doing no work at all. The Visualise tab plots both curves against each other so you can see exactly where the trade sits for your thread.

Cutting taps, forming taps and clearance holes

A forming or roll tap does not cut a thread — it displaces metal, pushing the crest up out of the hole wall. That means it needs a larger starting hole than a cutting tap, about half the reduction: drill = D − (E/100) × 1.29904 × P / 2. An M6 roll tap wants 5.5 mm, not the 5.0 mm a cutting tap wants; M8×1.25 wants 7.4 mm, not 6.8 mm. Using a cutting-tap hole for a roll tap is the most common way to snap one. Forming taps produce a stronger, chipless, burr-free thread but only work in ductile materials — never cast iron or hardened steel.

The clearance drill is a different hole entirely: the one in the part the screw passes through. Metric clearance values here come from ISO 273 in three fit classes (close, medium, coarse). Inch clearance values are common shop practice rather than a standard table, and this page says so rather than implying a standard it is not quoting.

Drill size series — fractional, number, letter and metric

Four drill naming systems are still in daily use, and tap drill charts move between them constantly. Fractional sizes run in 1/64″ steps. Number (wire gauge) sizes run from #1 (0.228″) down to #80 (0.0135″) — note that the numbers get smaller as the drills do. Letter sizes run A (0.234″) to Z (0.413″), and they overlap the number series: letter A is bigger than #1, and letter E is exactly 1/4″. Metric jobber drills come in 0.1 mm steps through the common range. This tool shows the nearest drill in all four series for any thread, each labelled with the engagement that particular drill gives — so if your index only has letter drills, you know precisely what you are getting rather than guessing.

Who Uses This Simulator?

Apprentices and engineering students use it to learn where the tap drill number comes from instead of copying it off a wall chart. Machinists and toolmakers use the reverse lookup and the four-series card when the ideal drill is not in the index. Maintenance fitters use it to identify what a hole was meant to be. Design engineers use the clearance-hole and engagement data when specifying tapped holes on a drawing. Makers, restorers and home-shop machinists use it for the Whitworth sizes that modern charts drop, and for the roll-tap holes that most charts never carried.

Explore Related Simulators

If this tap drill chart was useful, continue with the Thread Nomenclature simulator for the full geometry of a screw thread, the Speeds & Feeds Calculator for the drilling and tapping RPM to run, the Drilling Machine simulator for the operation itself, the Tolerance & Fits Calculator for hole and shaft limits, and the Bolted Joint Calculator for what the finished thread can actually carry.