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

A #7 drill, 0.201 in. The nut minor diameter of 1/4-20 UNC is 0.1959 in.

What does #10-32 UNF mean?

A number 10 thread, 0.190 in (4.826 mm) in diameter, with 32 threads per inch in the fine series. Its tap drill is a #21 drill, 0.159 in.

UNC thread chart and UNF thread chart: unified inch threads with tap drill sizes

Unified inch threads in the coarse (UNC) and fine (UNF) series, from #0 to 1 1/2 in, with the diameters in inches and millimetres and the tap drill for each size.

UNC thread chart: unified coarse

Thread	TPIthreads per inch	dmajor, in	dmajor, mm	d2pitch dia., in	D1nut minor, in	Tap drillsize and in
#1-64	64	0.073	1.854	0.0629	0.0561	#53 · 0.0595
#2-56	56	0.086	2.184	0.0744	0.0667	#50 · 0.07
#3-48	48	0.099	2.515	0.0855	0.0764	#47 · 0.0785
#4-40	40	0.112	2.845	0.0958	0.0849	#43 · 0.089
#5-40	40	0.125	3.175	0.1088	0.0979	#38 · 0.1015
#6-32	32	0.138	3.505	0.1177	0.1042	#36 · 0.1065
#8-32	32	0.164	4.166	0.1437	0.1302	#29 · 0.136
#10-24	24	0.19	4.826	0.1629	0.1449	#25 · 0.1495
#12-24	24	0.216	5.486	0.1889	0.1709	#16 · 0.177
1/4-20	20	0.25	6.35	0.2175	0.1959	#7 · 0.201
5/16-18	18	0.3125	7.938	0.2764	0.2524	F · 0.257
3/8-16	16	0.375	9.525	0.3344	0.3073	5/16 · 0.3125
7/16-14	14	0.4375	–	0.3911	0.3602	U · 0.368
1/2-13	13	0.5	12.7	0.45	0.4167	27/64 · 0.4219
9/16-12	12	0.5625	14.288	0.5084	0.4723	31/64 · 0.4844
5/8-11	11	0.625	15.875	0.566	0.5266	17/32 · 0.5312
3/4-10	10	0.75	19.05	0.685	0.6417	21/32 · 0.6562
7/8-9	9	0.875	22.225	0.8028	0.7547	49/64 · 0.7656
1-8	8	1	25.4	0.9188	0.8647	7/8 · 0.875
1 1/8-7	7	1.125	28.575	1.0322	0.9704	63/64 · 0.9844
1 1/4-7	7	1.25	31.75	1.1572	1.0954	1 7/64 · 1.1094
1 3/8-6	6	1.375	34.925	1.2667	1.1946	–
1 1/2-6	6	1.5	38.1	1.3917	1.3196	1 11/32 · 1.3438

UNF thread chart: unified fine

Thread	TPIthreads per inch	dmajor, in	dmajor, mm	d2pitch dia., in	D1nut minor, in	Tap drillsize and in
#0-80	80	0.06	1.524	0.0519	0.0465	3/64 · 0.0469
#1-72	72	0.073	1.854	0.064	0.058	#53 · 0.0595
#2-64	64	0.086	2.184	0.0759	0.0691	#50 · 0.07
#3-56	56	0.099	2.515	0.0874	0.0797	#45 · 0.082
#4-48	48	0.112	2.845	0.0985	0.0894	#42 · 0.0935
#5-44	44	0.125	3.175	0.1102	0.1004	#37 · 0.104
#6-40	40	0.138	3.505	0.1218	0.1109	#33 · 0.113
#8-36	36	0.164	4.166	0.146	0.1339	#29 · 0.136
#10-32	32	0.19	4.826	0.1697	0.1562	#21 · 0.159
#12-28	28	0.216	5.486	0.1928	0.1773	#14 · 0.182
1/4-28	28	0.25	6.35	0.2268	0.2113	#3 · 0.213
5/16-24	24	0.3125	7.938	0.2854	0.2674	I · 0.272
3/8-24	24	0.375	9.525	0.3479	0.3299	Q · 0.332
7/16-20	20	0.4375	–	0.405	0.3834	25/64 · 0.3906
1/2-20	20	0.5	12.7	0.4675	0.4459	29/64 · 0.4531
9/16-18	18	0.5625	14.288	0.5264	0.5024	33/64 · 0.5156
5/8-18	18	0.625	15.875	0.5889	0.5649	37/64 · 0.5781
3/4-16	16	0.75	19.05	0.7094	0.6823	11/16 · 0.6875
7/8-14	14	0.875	22.225	0.8286	0.7977	13/16 · 0.8125
1-12	12	1	25.4	0.9459	0.9098	59/64 · 0.9219
1 1/8-12	12	1.125	28.575	1.0709	1.0348	1 3/64 · 1.0469
1 1/4-12	12	1.25	31.75	1.1959	1.1598	–
1 3/8-12	12	1.375	34.925	1.3209	1.2848	–
1 1/2-12	12	1.5	38.1	1.4459	1.4098	–

Basic dimensions. The pitch diameter is checked against the formula of the unified thread form; a dash means fewer than two independent sources gave that value.

Sources the values were checked against (7)

Imperial thread chart: how a unified thread is designated

A unified thread is written as size, threads per inch and series: 1/4-20 UNC is a 1/4 inch thread with 20 threads per inch in the coarse series, 1/4-28 UNF the same size in the fine series. Sizes under 1/4 inch use gauge numbers instead of fractions, from #0 to #12. The class that follows, such as 2A for a bolt or 2B for a nut, sets the tolerance; 2A and 2B are the usual ones.

The thread form has a 60° flank angle, the same as the ISO metric thread, but the sizes and pitches are different, so a UNC and a metric thread never fit each other. For metric threads, see the metric thread chart.

Standard UNC thread chart dimensions

The basic pitch diameter is d2 = d − 0.649519 / n, where n is the number of threads per inch; every value in the tables matches it. The nut minor diameter D1 is the smallest the tapped hole can be, and the tap drill is chosen to stay just above it. Drill sizes are given as the number, letter or fraction drill used in inch shops, with its diameter in inches.

UNC and UNF threads: common questions

What is the tap drill for 1/4-20 UNC?: A #7 drill, 0.201 in. The nut minor diameter of 1/4-20 UNC is 0.1959 in.
What is the tap drill for 1/2-13 UNC?: A 27/64 in drill, 0.4219 in.
What does #10-32 UNF mean?: A number 10 thread, 0.190 in (4.826 mm) in diameter, with 32 threads per inch in the fine series. Its tap drill is a #21 drill, 0.159 in.