Tool Steel Reference Data

Tap Drill Size Chart

The drill to use before a tap, for UNC and UNF threads from #0 to 2-1/2 in., for ISO metric coarse threads from M1 to M68, and for machine screw sizes with their clearance drills. Drill sizes are given as the fraction, number or letter drill, with its decimal and millimetre equivalents.

Tap drills for UNC and UNF threads, fractional sizes Threads of American National Form, covering the common UNC and UNF pitches, with the older coarse and fine series included. The drill sizes allow approximately 75 per cent of a full thread. For the small sizes, choosing the larger hole sizes from the hole size limits table reduces tap breakage. Source, Table 3.

Thread	Root dia, in.	Tap drill	Drill dia, in.	Drill dia, mm
1/16 – 64	0.0422	3/64	0.0469	1.19
1/16 – 72	0.0445	3/64	0.0469	1.19
5/64 – 60	0.0563	1/16	0.0625	1.59
5/64 – 72	0.0601	52	0.0635	1.61
3/32 – 48	0.0667	49	0.0730	1.85
3/32 – 50	0.0678	49	0.0730	1.85
7/64 – 48	0.0823	43	0.0890	2.26
1/8 – 32	0.0844	3/32	0.0937	2.38
1/8 – 40	0.0925	38	0.1015	2.58
9/64 – 40	0.1081	32	0.1160	2.95
5/32 – 32	0.1157	1/8	0.1250	3.17
5/32 – 36	0.1202	30	0.1285	3.26
11/64 – 32	0.1313	9/64	0.1406	3.57
3/16 – 24	0.1334	26	0.1470	3.73
3/16 – 32	0.1469	22	0.1570	3.99
13/64 – 24	0.1490	20	0.1610	4.09
7/32 – 24	0.1646	16	0.1770	4.50
7/32 – 32	0.1782	12	0.1890	4.80
15/64 – 24	0.1806	10	0.1935	4.91
1/4 – 20	0.1850	7	0.2010	5.11
1/4 – 24	0.1959	4	0.2090	5.31
1/4 – 27	0.2019	3	0.2130	5.41
1/4 – 28	0.2036	3	0.2130	5.41
1/4 – 32	0.2094	7/32	0.2187	5.55
5/16 – 18	0.2403	F	0.2570	6.53
5/16 – 20	0.2476	17/64	0.2656	6.75
5/16 – 24	0.2584	I	0.2720	6.91
5/16 – 27	0.2644	J	0.2770	7.04
5/16 – 32	0.2719	9/32	0.2812	7.14
3/8 – 16	0.2938	5/16	0.3125	7.94
3/8 – 20	0.3100	21/64	0.3281	8.33
3/8 – 24	0.3209	Q	0.3320	8.43
3/8 – 27	0.3269	R	0.3390	8.61
7/16 – 14	0.3447	U	0.3680	9.35
7/16 – 20	0.3726	25/64	0.3906	9.92
7/16 – 24	0.3834	X	0.3970	10.08
7/16 – 27	0.3894	Y	0.4040	10.26
1/2 – 12	0.3918	27/64	0.4219	10.72
1/2 – 13	0.4001	27/64	0.4219	10.72
1/2 – 20	0.4351	29/64	0.4531	11.51
1/2 – 24	0.4459	29/64	0.4531	11.51
1/2 – 27	0.4519	15/32	0.4687	11.90
9/16 – 12	0.4542	31/64	0.4844	12.30
9/16 – 18	0.4903	33/64	0.5156	13.10
9/16 – 27	0.5144	17/32	0.5312	13.49
5/8 – 11	0.5069	17/32	0.5312	13.49
5/8 – 12	0.5168	35/64	0.5469	13.89
5/8 – 18	0.5528	37/64	0.5781	14.68
5/8 – 27	0.5769	19/32	0.5937	15.08
11/16 – 11	0.5694	19/32	0.5937	15.08
11/16 – 16	0.6063	5/8	0.6250	15.88
3/4 – 10	0.6201	21/32	0.6562	16.67
3/4 – 12	0.6418	43/64	0.6719	17.07
3/4 – 16	0.6688	11/16	0.6875	17.46
3/4 – 27	0.7019	23/32	0.7187	18.25
13/16 – 10	0.6826	23/32	0.7187	18.25
7/8 – 9	0.7307	49/64	0.7656	19.45
7/8 – 12	0.7668	51/64	0.7969	20.24
7/8 – 14	0.7822	13/16	0.8125	20.64
7/8 – 18	0.8028	53/64	0.8281	21.03
7/8 – 27	0.8269	27/32	0.8437	21.43
15/16 – 9	0.7932	53/64	0.8281	21.03
1 – 8	0.8376	7/8	0.8750	22.22
1 – 12	0.8918	59/64	0.9219	23.42
1 – 14	0.9072	15/16	0.9375	23.81
1 – 27	0.9519	31/32	0.9687	24.60
1 1/8 – 7	0.9394	63/64	0.9844	25.00
1 1/8 – 12	1.0168	1 3/64	1.0469	26.59
1 1/4 – 7	1.0644	1 7/64	1.1094	28.18
1 1/4 – 12	1.1418	1 11/64	1.1719	29.77
1 3/8 – 6	1.1585	1 7/32	1.2187	30.95
1 3/8 – 12	1.2668	1 19/64	1.2969	32.94
1 1/2 – 6	1.2835	1 11/32	1.3437	34.13
1 1/2 – 12	1.3918	1 27/64	1.4219	36.12
1 5/8 – 5 1/2	1.3888	1 29/64	1.4531	36.91
1 3/4 – 5	1.4902	1 9/16	1.5625	39.69
1 7/8 – 5	1.6152	1 11/16	1.6875	42.86
2 – 4 1/2	1.7113	1 25/32	1.7812	45.24
2 1/8 – 4 1/2	1.8363	1 29/32	1.9062	48.42
2 1/4 – 4 1/2	1.9613	2 1/32	2.0312	51.59
2 3/8 – 4	2.0502	2 1/8	2.1250	53.97
2 1/2 – 4	2.1752	2 1/4	2.2500	57.15

Drill dia, mm is converted from the inch value at 1 in. = 25.4 mm. Threads from #0 to 1 in. are also listed below with their clearance drills.

Tap drills and clearance drills for machine screws Numbered and small fractional machine screws: the tap drill for the thread, and the close fit and free fit clearance drills used when the screw passes through the part. Where a size carries more than one pitch, the values are listed for each pitch in the order shown. Source, Table 4.

Size	Dia, in.	Threads per in.	Tap drill	Drill dia, in.	Close fit drill	Dia, in.	Free fit drill	Dia, in.
0	.060	80	3⁄64	.0469	52	.0635	50	.0700
1	.073	64 72	53 53	.0595 .0595	48	.0760	46	.0810
2	.086	56 64	50 50	.0700 .0700	43	.0890	41	.0960
3	.099	48 56	47 45	.0785 .0820	37	.1040	35	.1100
4	.112	36a 40 48	44 43 42	.0860 .0890 .0935	32	.1160	30	.1285
5	.125	40 44	38 37	.1015 1040	30	.1285	29	.1360
6	.138	32 40	36 33	.1065 .1130	27	.1440	25	.1495
8	.164	32 36	29 29	.1360 .1360	18	.1695	16	.1770
10	.190	24 32	25 21	.1495 1590	9	.1960	7	.2010
12	.216	24 28	16 14	.1770 .1820	2	.2210	1	.2280
14	.242	20a 24a	10 7	.1935 .2010	D	.2460	F	.2570
1⁄4	.250	20 28	7 3	.2010 .2130	F	.2570	H	.2660
5⁄16	.3125	18 24	F I	.2570 .2720	P	.3230	Q	.3320
3⁄8	.375	16 24	5⁄16 Q	.3125 .3320	W	.3860	X	.3970
7⁄16	.4375	14 20	U 25⁄64	.3680 .3906	29⁄64	.4531	15⁄32	.4687
1⁄2	.500	13 20	27⁄64 29⁄64	.4219 .4531	33⁄64	.5156	17⁄32	.5312

Tap drills for ISO metric coarse threads Recommended and alternative tapping drill sizes for metric coarse pitch threads, in millimetres. The recommended sizes give a theoretical radial engagement of about 81 per cent, the alternative sizes about 70 to 75 per cent. The table is for fluted taps only, a fluteless (cold forming) tap takes a different size hole. Source, Table 6.

Thread	Recommended drill, mm	Radial engagement, %	Alternative drill, mm	Radial engagement, %
M1	0.75	81.5	0.78	71.7
M1.1	0.85	81.5	0.88	71.7
M1.2	0.95	81.5	0.98	71.7
M1.4	1.10	81.5	1.15	67.9
M1.6	1.25	81.5	1.30	69.9
M1.8	1.45	81.5	1.50	69.9
M2	1.60	81.5	1.65	71.3
M2.2	1.75	81.5	1.80	72.5
M2.5	2.05	81.5	2.10	72.5
M3	2.50	81.5	2.55	73.4
M3.5	2.90	81.5	2.95	74.7
M4	3.30	81.5	3.40	69.9b
M4.5	3.70	86.8	3.80	76.1
M5	4.20	81.5	4.30	71.3b
M6	5.00	81.5	5.10	73.4
M7	6.00	81.5	6.10	73.4
M8	6.80	78.5	6.90	71.7b
M9	7.80	78.5	7.90	71.7b
M10	8.50	81.5	8.60	76.1
M11	9.50	81.5	9.60	76.1
M12	10.20	83.7	10.40	74.5b
M14	12.00	81.5	12.20	73.4b
M16	14.00	81.5	14.25	71.3c
M18	15.50	81.5	15.75	73.4c
M20	17.50	81.5	17.75	73.4c
M22	19.50	81.5	19.75	73.4c
M24	21.00	81.5	21.25	74.7b
M27	24.00	81.5	24.25	74.7b
M30	26.50	81.5	26.75	75.7b
M33	29.50	81.5	29.75	75.7b
M36	32.00	81.5	…	…
M39	35.00	81.5	…	…
M42	37.50	81.5	…	…
M45	40.50	81.5	…	…
M48	43.00	81.5	…	…
M52	47.00	81.5	…	…
M56	50.50	81.5	…	…
M60	54.50	81.5	…	…
M64	58.00	81.5	…	…
M68	62.00	81.5	…	…

Footnotes in the source: (b) for tolerance class 6H and 7H threads only; (c) for tolerance class 7H threads only; the ellipsis marks sizes where no alternative drill is listed.

As a rule of thumb the tap drill diameter is the nominal diameter minus the pitch, tap drill = major diameter − pitch. The tables above give the recommended values, which build in the thread engagement the standard asks for.
Tapping is done at the lowest speed of any threading operation because the tap is fully engaged in the cut. Cutting speeds for tapping tool steels are in the cutting speeds and feeds chart.
Taps are made from high speed steel, and the grades used for them are grouped under high speed steels. Where a thread is formed by rolling instead of cutting, the die material is covered on the thread rolling dies page.
Hardened steel is tapped only as a repair. The difficulty of tapping rises with the hardness of the material, which follows the machinability ratings of the grades.

Choosing the drill

The tap drill decides the thread engagement. A drill at the top of the range gives a thread that cuts freely and a tap that lasts, at the cost of a slightly weaker thread. A drill under the range overloads the tap and is the usual cause of tap breakage in the small sizes.

Drill sizes list as fractions, numbers and letters on the same chart because the tap drill does not always land on a fraction: a 1/4-20 thread taps with a #7 drill, which is neither. The decimal equivalent column converts any of them to the diameter that goes into the machine.

Related reference pages

Selection guide for high speed steel cutting tools · High speed steel for cutting tools, hobs and broaches

Source: Machinery’s Handbook 28th Edition (Industrial Press), Tapping section: Table 3 (tap drill sizes for threads of American National Form), Table 4 (tap drills and clearance drills for machine screws), Table 6 (British Standard tapping drill sizes for ISO metric coarse pitch series threads, BS 1157). Millimetre equivalents of the inch drills are converted at 1 in. = 25.4 mm.