Thread Size Chart: UNC, UNF & Metric with Tap Drills

A 1/4-20 UNC thread has a 0.2500 in major diameter, 20 threads per inch and takes a #7 tap drill; an M8 x 1.25 metric thread has an 8 mm major diameter, a 1.25 mm pitch and takes a 6.8 mm tap drill. This chart lists those figures for every standard Unified (UNC, UNF and UNEF) size from #0 to 1-1/2 in and ISO metric coarse and fine threads from M1.6 to M48.

For each thread you get the pitch (with threads per inch or the equivalent pitch), the basic major, pitch and minor diameters, a recommended tap drill and close and normal clearance hole sizes. Use the buttons to show one series, the search box to jump to a size such as 3/8-16 or M12, and the unit buttons to show everything in inches or millimetres.

Thread Dimensions, Tap Drills and Clearance Holes

All UNC UNF UNEF Metric coarse Metric fine

Native units All inch All mm

Unified and metric thread dimensions
Thread	Series	Pitch	Major dia	Pitch dia	Minor dia (basic)	Tap drill	Clearance close	Clearance normal
#0-80 UNF	UNF	80 TPI0.318 mm pitch	0.0600 in	0.0519 in	0.0465 in	3/64 0.0469 in 81% thread	#52 0.0635 in	#50 0.0700 in
#1-64 UNC	UNC	64 TPI0.397 mm pitch	0.0730 in	0.0629 in	0.0561 in	#53 0.0595 in 67% thread	#48 0.0760 in	#46 0.0810 in
#1-72 UNF	UNF	72 TPI0.353 mm pitch	0.0730 in	0.0640 in	0.0580 in	#53 0.0595 in 75% thread	#48 0.0760 in	#46 0.0810 in
#2-56 UNC	UNC	56 TPI0.454 mm pitch	0.0860 in	0.0744 in	0.0667 in	#50 0.0700 in 69% thread	#43 0.0890 in	#41 0.0960 in
#2-64 UNF	UNF	64 TPI0.397 mm pitch	0.0860 in	0.0759 in	0.0691 in	#50 0.0700 in 79% thread	#43 0.0890 in	#41 0.0960 in
#3-48 UNC	UNC	48 TPI0.529 mm pitch	0.0990 in	0.0855 in	0.0764 in	#47 0.0785 in 76% thread	#37 0.1040 in	#35 0.1100 in
#3-56 UNF	UNF	56 TPI0.454 mm pitch	0.0990 in	0.0874 in	0.0797 in	#45 0.0820 in 73% thread	#37 0.1040 in	#35 0.1100 in
#4-40 UNC	UNC	40 TPI0.635 mm pitch	0.1120 in	0.0958 in	0.0849 in	#43 0.0890 in 71% thread	#32 0.1160 in	#31 0.1200 in
#4-48 UNF	UNF	48 TPI0.529 mm pitch	0.1120 in	0.0985 in	0.0894 in	#42 0.0935 in 68% thread	#32 0.1160 in	#31 0.1200 in
#5-40 UNC	UNC	40 TPI0.635 mm pitch	0.1250 in	0.1088 in	0.0979 in	#38 0.1015 in 72% thread	#30 0.1285 in	#29 0.1360 in
#5-44 UNF	UNF	44 TPI0.577 mm pitch	0.1250 in	0.1102 in	0.1004 in	#37 0.1040 in 71% thread	#30 0.1285 in	#29 0.1360 in
#6-32 UNC	UNC	32 TPI0.794 mm pitch	0.1380 in	0.1177 in	0.1042 in	#36 0.1065 in 78% thread	#27 0.1440 in	#25 0.1495 in
#6-40 UNF	UNF	40 TPI0.635 mm pitch	0.1380 in	0.1218 in	0.1109 in	#33 0.1130 in 77% thread	#27 0.1440 in	#25 0.1495 in
#8-32 UNC	UNC	32 TPI0.794 mm pitch	0.1640 in	0.1437 in	0.1302 in	#29 0.1360 in 69% thread	#18 0.1695 in	#16 0.1770 in
#8-36 UNF	UNF	36 TPI0.706 mm pitch	0.1640 in	0.1460 in	0.1339 in	#29 0.1360 in 78% thread	#18 0.1695 in	#16 0.1770 in
#10-24 UNC	UNC	24 TPI1.058 mm pitch	0.1900 in	0.1629 in	0.1449 in	#25 0.1495 in 75% thread	#9 0.1960 in	#7 0.2010 in
#10-32 UNF	UNF	32 TPI0.794 mm pitch	0.1900 in	0.1697 in	0.1562 in	#21 0.1590 in 76% thread	#9 0.1960 in	#7 0.2010 in
#12-24 UNC	UNC	24 TPI1.058 mm pitch	0.2160 in	0.1889 in	0.1709 in	#16 0.1770 in 72% thread	#2 0.2210 in	#1 0.2280 in
#12-28 UNF	UNF	28 TPI0.907 mm pitch	0.2160 in	0.1928 in	0.1773 in	#14 0.1820 in 73% thread	#2 0.2210 in	#1 0.2280 in
#12-32 UNEF	UNEF	32 TPI0.794 mm pitch	0.2160 in	0.1957 in	0.1822 in	#13 0.1850 in 76% thread	#2 0.2210 in	#1 0.2280 in
1/4-20 UNC	UNC	20 TPI1.270 mm pitch	0.2500 in	0.2175 in	0.1959 in	#7 0.2010 in 75% thread	F 0.2570 in	H 0.2660 in
1/4-28 UNF	UNF	28 TPI0.907 mm pitch	0.2500 in	0.2268 in	0.2113 in	#3 0.2130 in 80% thread	F 0.2570 in	H 0.2660 in
1/4-32 UNEF	UNEF	32 TPI0.794 mm pitch	0.2500 in	0.2297 in	0.2162 in	7/32 0.2188 in 77% thread	F 0.2570 in	H 0.2660 in
5/16-18 UNC	UNC	18 TPI1.411 mm pitch	0.3125 in	0.2764 in	0.2524 in	F 0.2570 in 77% thread	P 0.3230 in	Q 0.3320 in
5/16-24 UNF	UNF	24 TPI1.058 mm pitch	0.3125 in	0.2854 in	0.2674 in	I 0.2720 in 75% thread	P 0.3230 in	Q 0.3320 in
5/16-32 UNEF	UNEF	32 TPI0.794 mm pitch	0.3125 in	0.2922 in	0.2787 in	9/32 0.2812 in 77% thread	P 0.3230 in	Q 0.3320 in
3/8-16 UNC	UNC	16 TPI1.587 mm pitch	0.3750 in	0.3344 in	0.3073 in	5/16 0.3125 in 77% thread	W 0.3860 in	X 0.3970 in
3/8-24 UNF	UNF	24 TPI1.058 mm pitch	0.3750 in	0.3479 in	0.3299 in	Q 0.3320 in 79% thread	W 0.3860 in	X 0.3970 in
3/8-32 UNEF	UNEF	32 TPI0.794 mm pitch	0.3750 in	0.3547 in	0.3412 in	11/32 0.3438 in 77% thread	W 0.3860 in	X 0.3970 in
7/16-14 UNC	UNC	14 TPI1.814 mm pitch	0.4375 in	0.3911 in	0.3602 in	U 0.3680 in 75% thread	29/64 0.4531 in	15/32 0.4688 in
7/16-20 UNF	UNF	20 TPI1.270 mm pitch	0.4375 in	0.4050 in	0.3834 in	25/64 0.3906 in 72% thread	29/64 0.4531 in	15/32 0.4688 in
7/16-28 UNEF	UNEF	28 TPI0.907 mm pitch	0.4375 in	0.4143 in	0.3988 in	Y 0.4040 in 72% thread	29/64 0.4531 in	15/32 0.4688 in
1/2-13 UNC	UNC	13 TPI1.954 mm pitch	0.5000 in	0.4500 in	0.4167 in	27/64 0.4219 in 78% thread	33/64 0.5156 in	17/32 0.5312 in
1/2-20 UNF	UNF	20 TPI1.270 mm pitch	0.5000 in	0.4675 in	0.4459 in	29/64 0.4531 in 72% thread	33/64 0.5156 in	17/32 0.5312 in
1/2-28 UNEF	UNEF	28 TPI0.907 mm pitch	0.5000 in	0.4768 in	0.4613 in	15/32 0.4688 in 67% thread	33/64 0.5156 in	17/32 0.5312 in
9/16-12 UNC	UNC	12 TPI2.117 mm pitch	0.5625 in	0.5084 in	0.4723 in	31/64 0.4844 in 72% thread	37/64 0.5781 in	39/64 0.6094 in
9/16-18 UNF	UNF	18 TPI1.411 mm pitch	0.5625 in	0.5264 in	0.5024 in	33/64 0.5156 in 65% thread	37/64 0.5781 in	39/64 0.6094 in
9/16-24 UNEF	UNEF	24 TPI1.058 mm pitch	0.5625 in	0.5354 in	0.5174 in	33/64 0.5156 in 87% thread	37/64 0.5781 in	39/64 0.6094 in
5/8-11 UNC	UNC	11 TPI2.309 mm pitch	0.6250 in	0.5660 in	0.5266 in	17/32 0.5312 in 79% thread	41/64 0.6406 in	21/32 0.6562 in
5/8-18 UNF	UNF	18 TPI1.411 mm pitch	0.6250 in	0.5889 in	0.5649 in	37/64 0.5781 in 65% thread	41/64 0.6406 in	21/32 0.6562 in
5/8-24 UNEF	UNEF	24 TPI1.058 mm pitch	0.6250 in	0.5979 in	0.5799 in	37/64 0.5781 in 87% thread	41/64 0.6406 in	21/32 0.6562 in
3/4-10 UNC	UNC	10 TPI2.540 mm pitch	0.7500 in	0.6850 in	0.6417 in	21/32 0.6562 in 72% thread	49/64 0.7656 in	25/32 0.7812 in
3/4-16 UNF	UNF	16 TPI1.587 mm pitch	0.7500 in	0.7094 in	0.6823 in	11/16 0.6875 in 77% thread	49/64 0.7656 in	25/32 0.7812 in
3/4-20 UNEF	UNEF	20 TPI1.270 mm pitch	0.7500 in	0.7175 in	0.6959 in	45/64 0.7031 in 72% thread	49/64 0.7656 in	25/32 0.7812 in
7/8-9 UNC	UNC	9 TPI2.822 mm pitch	0.8750 in	0.8028 in	0.7547 in	49/64 0.7656 in 76% thread	57/64 0.8906 in	59/64 0.9219 in
7/8-14 UNF	UNF	14 TPI1.814 mm pitch	0.8750 in	0.8286 in	0.7977 in	13/16 0.8125 in 67% thread	57/64 0.8906 in	59/64 0.9219 in
7/8-20 UNEF	UNEF	20 TPI1.270 mm pitch	0.8750 in	0.8425 in	0.8209 in	53/64 0.8281 in 72% thread	57/64 0.8906 in	59/64 0.9219 in
1-8 UNC	UNC	8 TPI3.175 mm pitch	1.0000 in	0.9188 in	0.8647 in	7/8 0.8750 in 77% thread	1-1/64 1.0156 in	1-5/64 1.0781 in
1-12 UNF	UNF	12 TPI2.117 mm pitch	1.0000 in	0.9459 in	0.9098 in	59/64 0.9219 in 72% thread	1-1/64 1.0156 in	1-5/64 1.0781 in
1-20 UNEF	UNEF	20 TPI1.270 mm pitch	1.0000 in	0.9675 in	0.9459 in	61/64 0.9531 in 72% thread	1-1/64 1.0156 in	1-5/64 1.0781 in
1-1/8-7 UNC	UNC	7 TPI3.629 mm pitch	1.1250 in	1.0322 in	0.9704 in	63/64 0.9844 in 76% thread	—	—
1-1/8-12 UNF	UNF	12 TPI2.117 mm pitch	1.1250 in	1.0709 in	1.0348 in	1-3/64 1.0469 in 72% thread	—	—
1-1/8-18 UNEF	UNEF	18 TPI1.411 mm pitch	1.1250 in	1.0889 in	1.0649 in	1-5/64 1.0781 in 65% thread	—	—
1-1/4-7 UNC	UNC	7 TPI3.629 mm pitch	1.2500 in	1.1572 in	1.0954 in	1-7/64 1.1094 in 76% thread	—	—
1-1/4-12 UNF	UNF	12 TPI2.117 mm pitch	1.2500 in	1.1959 in	1.1598 in	1-11/64 1.1719 in 72% thread	—	—
1-1/4-18 UNEF	UNEF	18 TPI1.411 mm pitch	1.2500 in	1.2139 in	1.1899 in	1-13/64 1.2031 in 65% thread	—	—
1-3/8-6 UNC	UNC	6 TPI4.233 mm pitch	1.3750 in	1.2667 in	1.1946 in	1-7/32 1.2188 in 72% thread	—	—
1-3/8-12 UNF	UNF	12 TPI2.117 mm pitch	1.3750 in	1.3209 in	1.2848 in	1-19/64 1.2969 in 72% thread	—	—
1-3/8-18 UNEF	UNEF	18 TPI1.411 mm pitch	1.3750 in	1.3389 in	1.3149 in	1-21/64 1.3281 in 65% thread	—	—
1-1/2-6 UNC	UNC	6 TPI4.233 mm pitch	1.5000 in	1.3917 in	1.3196 in	1-11/32 1.3438 in 72% thread	—	—
1-1/2-12 UNF	UNF	12 TPI2.117 mm pitch	1.5000 in	1.4459 in	1.4098 in	1-27/64 1.4219 in 72% thread	—	—
1-1/2-18 UNEF	UNEF	18 TPI1.411 mm pitch	1.5000 in	1.4639 in	1.4399 in	1-29/64 1.4531 in 65% thread	—	—
M1.6 x 0.35	Metric coarse	0.35 mm72.57 TPI	1.600 mm	1.373 mm	1.221 mm	1.25 mm D - P	1.70 mm ISO 273 fine	1.80 mm ISO 273 medium
M2 x 0.4	Metric coarse	0.4 mm63.50 TPI	2.000 mm	1.740 mm	1.567 mm	1.60 mm D - P	2.20 mm ISO 273 fine	2.40 mm ISO 273 medium
M2.5 x 0.45	Metric coarse	0.45 mm56.44 TPI	2.500 mm	2.208 mm	2.013 mm	2.05 mm D - P	2.70 mm ISO 273 fine	2.90 mm ISO 273 medium
M3 x 0.5	Metric coarse	0.5 mm50.80 TPI	3.000 mm	2.675 mm	2.459 mm	2.50 mm D - P	3.20 mm ISO 273 fine	3.40 mm ISO 273 medium
M4 x 0.7	Metric coarse	0.7 mm36.29 TPI	4.000 mm	3.545 mm	3.242 mm	3.30 mm D - P	4.30 mm ISO 273 fine	4.50 mm ISO 273 medium
M5 x 0.8	Metric coarse	0.8 mm31.75 TPI	5.000 mm	4.480 mm	4.134 mm	4.20 mm D - P	5.30 mm ISO 273 fine	5.50 mm ISO 273 medium
M5 x 0.5	Metric fine	0.5 mm50.80 TPI	5.000 mm	4.675 mm	4.459 mm	4.50 mm D - P	5.30 mm ISO 273 fine	5.50 mm ISO 273 medium
M6 x 1	Metric coarse	1 mm25.40 TPI	6.000 mm	5.350 mm	4.917 mm	5.00 mm D - P	6.40 mm ISO 273 fine	6.60 mm ISO 273 medium
M6 x 0.75	Metric fine	0.75 mm33.87 TPI	6.000 mm	5.513 mm	5.188 mm	5.25 mm D - P	6.40 mm ISO 273 fine	6.60 mm ISO 273 medium
M8 x 1.25	Metric coarse	1.25 mm20.32 TPI	8.000 mm	7.188 mm	6.647 mm	6.75 mm D - P	8.40 mm ISO 273 fine	9.00 mm ISO 273 medium
M8 x 1	Metric fine	1 mm25.40 TPI	8.000 mm	7.350 mm	6.917 mm	7.00 mm D - P	8.40 mm ISO 273 fine	9.00 mm ISO 273 medium
M10 x 1.5	Metric coarse	1.5 mm16.93 TPI	10.000 mm	9.026 mm	8.376 mm	8.50 mm D - P	10.50 mm ISO 273 fine	11.00 mm ISO 273 medium
M10 x 1.25	Metric fine	1.25 mm20.32 TPI	10.000 mm	9.188 mm	8.647 mm	8.75 mm D - P	10.50 mm ISO 273 fine	11.00 mm ISO 273 medium
M10 x 1	Metric fine	1 mm25.40 TPI	10.000 mm	9.350 mm	8.917 mm	9.00 mm D - P	10.50 mm ISO 273 fine	11.00 mm ISO 273 medium
M12 x 1.75	Metric coarse	1.75 mm14.51 TPI	12.000 mm	10.863 mm	10.106 mm	10.25 mm D - P	13.00 mm ISO 273 fine	13.50 mm ISO 273 medium
M12 x 1.5	Metric fine	1.5 mm16.93 TPI	12.000 mm	11.026 mm	10.376 mm	10.50 mm D - P	13.00 mm ISO 273 fine	13.50 mm ISO 273 medium
M12 x 1.25	Metric fine	1.25 mm20.32 TPI	12.000 mm	11.188 mm	10.647 mm	10.75 mm D - P	13.00 mm ISO 273 fine	13.50 mm ISO 273 medium
M14 x 2	Metric coarse	2 mm12.70 TPI	14.000 mm	12.701 mm	11.835 mm	12.00 mm D - P	15.00 mm ISO 273 fine	15.50 mm ISO 273 medium
M14 x 1.5	Metric fine	1.5 mm16.93 TPI	14.000 mm	13.026 mm	12.376 mm	12.50 mm D - P	15.00 mm ISO 273 fine	15.50 mm ISO 273 medium
M16 x 2	Metric coarse	2 mm12.70 TPI	16.000 mm	14.701 mm	13.835 mm	14.00 mm D - P	17.00 mm ISO 273 fine	17.50 mm ISO 273 medium
M16 x 1.5	Metric fine	1.5 mm16.93 TPI	16.000 mm	15.026 mm	14.376 mm	14.50 mm D - P	17.00 mm ISO 273 fine	17.50 mm ISO 273 medium
M18 x 2.5	Metric coarse	2.5 mm10.16 TPI	18.000 mm	16.376 mm	15.294 mm	15.50 mm D - P	19.00 mm ISO 273 fine	20.00 mm ISO 273 medium
M18 x 1.5	Metric fine	1.5 mm16.93 TPI	18.000 mm	17.026 mm	16.376 mm	16.50 mm D - P	19.00 mm ISO 273 fine	20.00 mm ISO 273 medium
M20 x 2.5	Metric coarse	2.5 mm10.16 TPI	20.000 mm	18.376 mm	17.294 mm	17.50 mm D - P	21.00 mm ISO 273 fine	22.00 mm ISO 273 medium
M20 x 1.5	Metric fine	1.5 mm16.93 TPI	20.000 mm	19.026 mm	18.376 mm	18.50 mm D - P	21.00 mm ISO 273 fine	22.00 mm ISO 273 medium
M22 x 2.5	Metric coarse	2.5 mm10.16 TPI	22.000 mm	20.376 mm	19.294 mm	19.50 mm D - P	23.00 mm ISO 273 fine	24.00 mm ISO 273 medium
M22 x 1.5	Metric fine	1.5 mm16.93 TPI	22.000 mm	21.026 mm	20.376 mm	20.50 mm D - P	23.00 mm ISO 273 fine	24.00 mm ISO 273 medium
M24 x 3	Metric coarse	3 mm8.47 TPI	24.000 mm	22.051 mm	20.752 mm	21.00 mm D - P	25.00 mm ISO 273 fine	26.00 mm ISO 273 medium
M24 x 2	Metric fine	2 mm12.70 TPI	24.000 mm	22.701 mm	21.835 mm	22.00 mm D - P	25.00 mm ISO 273 fine	26.00 mm ISO 273 medium
M27 x 3	Metric coarse	3 mm8.47 TPI	27.000 mm	25.051 mm	23.752 mm	24.00 mm D - P	28.00 mm ISO 273 fine	30.00 mm ISO 273 medium
M27 x 2	Metric fine	2 mm12.70 TPI	27.000 mm	25.701 mm	24.835 mm	25.00 mm D - P	28.00 mm ISO 273 fine	30.00 mm ISO 273 medium
M30 x 3.5	Metric coarse	3.5 mm7.26 TPI	30.000 mm	27.727 mm	26.211 mm	26.50 mm D - P	31.00 mm ISO 273 fine	33.00 mm ISO 273 medium
M30 x 2	Metric fine	2 mm12.70 TPI	30.000 mm	28.701 mm	27.835 mm	28.00 mm D - P	31.00 mm ISO 273 fine	33.00 mm ISO 273 medium
M33 x 3.5	Metric coarse	3.5 mm7.26 TPI	33.000 mm	30.727 mm	29.211 mm	29.50 mm D - P	34.00 mm ISO 273 fine	36.00 mm ISO 273 medium
M33 x 2	Metric fine	2 mm12.70 TPI	33.000 mm	31.701 mm	30.835 mm	31.00 mm D - P	34.00 mm ISO 273 fine	36.00 mm ISO 273 medium
M36 x 4	Metric coarse	4 mm6.35 TPI	36.000 mm	33.402 mm	31.670 mm	32.00 mm D - P	37.00 mm ISO 273 fine	39.00 mm ISO 273 medium
M36 x 3	Metric fine	3 mm8.47 TPI	36.000 mm	34.051 mm	32.752 mm	33.00 mm D - P	37.00 mm ISO 273 fine	39.00 mm ISO 273 medium
M39 x 4	Metric coarse	4 mm6.35 TPI	39.000 mm	36.402 mm	34.670 mm	35.00 mm D - P	40.00 mm ISO 273 fine	42.00 mm ISO 273 medium
M39 x 3	Metric fine	3 mm8.47 TPI	39.000 mm	37.051 mm	35.752 mm	36.00 mm D - P	40.00 mm ISO 273 fine	42.00 mm ISO 273 medium
M42 x 4.5	Metric coarse	4.5 mm5.64 TPI	42.000 mm	39.077 mm	37.129 mm	37.50 mm D - P	43.00 mm ISO 273 fine	45.00 mm ISO 273 medium
M42 x 3	Metric fine	3 mm8.47 TPI	42.000 mm	40.051 mm	38.752 mm	39.00 mm D - P	43.00 mm ISO 273 fine	45.00 mm ISO 273 medium
M45 x 4.5	Metric coarse	4.5 mm5.64 TPI	45.000 mm	42.077 mm	40.129 mm	40.50 mm D - P	46.00 mm ISO 273 fine	48.00 mm ISO 273 medium
M45 x 3	Metric fine	3 mm8.47 TPI	45.000 mm	43.051 mm	41.752 mm	42.00 mm D - P	46.00 mm ISO 273 fine	48.00 mm ISO 273 medium
M48 x 5	Metric coarse	5 mm5.08 TPI	48.000 mm	44.752 mm	42.587 mm	43.00 mm D - P	50.00 mm ISO 273 fine	52.00 mm ISO 273 medium
M48 x 3	Metric fine	3 mm8.47 TPI	48.000 mm	46.051 mm	44.752 mm	45.00 mm D - P	50.00 mm ISO 273 fine	52.00 mm ISO 273 medium

— = inch clearance sizes above 1 in are not tabulated; a hole 1/16 in over nominal is a common normal fit. Native units show inch threads in inches and metric threads in mm.

How to Read the Thread Chart

Pitch. Unified threads are specified in threads per inch (TPI); pitch = 1 / TPI. Metric threads are specified by pitch in mm. The small grey figure converts one to the other so you can spot near matches such as M5 x 0.8 (31.75 TPI) and 10-32 (0.794 mm pitch).
Diameters are basic. Pitch diameter = D - 0.649519 P and basic minor diameter = D - 1.082532 P, the same formulas for Unified (ASME B1.1) and ISO metric (ISO 68-1/724) 60° threads. Bolts are made under and nuts over these sizes by the tolerance class.
Tap drill. Inch rows show the standard number, letter or fractional drill nearest to 75% thread, with the actual percentage: % thread = TPI × (D - drill) / 1.299038 × 100. Metric rows use D - P, about 77% thread.
Clearance holes. Close and normal (free) fit, from the clearance and counterbore chart. Metric values are ISO 273 fine and medium series.

Worked example (inch): 3/8-16 UNC. Pitch = 1/16 = 0.0625 in. Pitch diameter = 0.375 - 0.649519 × 0.0625 = 0.3344 in. Basic minor = 0.375 - 1.082532 × 0.0625 = 0.3073 in. For 75% thread the drill is 0.375 - 0.974279 × 0.0625 = 0.3141 in, so a 5/16 in (0.3125 in) drill gives 77% thread.
Worked example (metric): M10 x 1.5. Pitch diameter = 10 - 0.649519 × 1.5 = 9.026 mm; basic minor = 10 - 1.082532 × 1.5 = 8.376 mm; tap drill = 10 - 1.5 = 8.5 mm.

Shop tips

Go to 60 to 70% thread in hard or tough materials (stainless, titanium, tool steel). It barely reduces strip strength but greatly cuts tapping torque and tap breakage.
Form (roll) taps need a larger drill, roughly the pitch diameter. See the Tap & Drill Size Reference or the Tap Drill Size Calculator for roll-tap and STI sizes.
Check the pitch before you tap. 1/4-20 vs M6 x 1 and 10-32 vs M5 x 0.8 look similar; a thread gauge or the three-wire calculator settles it.
For blind holes, drill deeper than the full thread length; the Blind Hole Tap Depth Checker allows for the tap's chamfer.

Frequently Asked Questions

Use a #7 drill (0.2010 in, 5.105 mm). It gives about 75% thread, the usual target for cut taps in steel and aluminium. A 13/64 in drill (0.2031 in) is a common substitute and gives about 72% thread.

Subtract the pitch from the nominal diameter: tap drill = D - P. An M8 x 1.25 thread needs 8 - 1.25 = 6.8 mm and an M10 x 1.5 needs 8.5 mm. This gives roughly 75 to 80% thread engagement.

UNC (Unified National Coarse) has fewer threads per inch than UNF (Unified National Fine) for the same diameter, for example 1/4-20 UNC and 1/4-28 UNF. Coarse threads assemble faster and suit soft materials; fine threads are stronger in tension, resist vibration better and allow finer adjustment.

M8 x 1.25 is an ISO metric thread with an 8 mm major diameter and a 1.25 mm pitch, the distance from one thread crest to the next. 1.25 mm is the standard coarse pitch for M8, so it is often written simply as M8. M8 x 1 is the fine-pitch version.

No. The major, pitch and minor diameters are basic (theoretical) sizes from the thread formulas. Real bolts are made slightly under and real nuts slightly over these sizes by the tolerance class, such as 2A/2B or 6g/6H, so use the tolerance tables in ASME B1.1 or ISO 965 for gauging limits.

Basic diameters calculated from the 60° basic profile formulas of ASME B1.1 (Unified inch screw threads) and ISO 68-1 / ISO 724 (ISO metric); sizes and pitches per ASME B1.1 and ISO 261/262. Inch tap drills are the standard drill nearest to 75% thread (ASME B94.11M drill sizes); metric tap drills are D − P. Clearance holes: common inch close/free-fit practice and ISO 273. Tolerance classes (2A/2B, 6g/6H, ISO 965) are not included; check critical values against the standard or the tap manufacturer.

Thread Size Chart: UNC, UNF, UNEF and Metric

Thread Dimensions, Tap Drills and Clearance Holes

How to Read the Thread Chart

Shop tips

Frequently Asked Questions

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

How do you work out a metric tap drill size?

What is the difference between UNC and UNF?

What does M8 x 1.25 mean?

Are the diameters in this chart the sizes I should machine to?

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