UNC and UNF Thread Chart: Sizes and TPI

This chart lists the 47 Unified inch threads you are most likely to meet, from #0-80 to 1-1/2 inch, in the coarse (UNC) and fine (UNF) series. For each one it gives the threads per inch, the basic major, pitch and minor diameters, the tensile stress area and the tap drill for 75% thread. Every value is computed from the thread formulas listed under the chart, so you can check any row yourself. Click a column heading to sort, or type a size to filter.

Find a size

Size	Series	TPI	Major dia (in)	Major (mm)	Pitch dia (in)	Minor dia, nut (in)	Stress area (in²)	Tap drill, 75%
#0-80	UNF	80	0.0600	1.52	0.0519	0.0465	0.0018	3/64" (0.0469)
#1-64	UNC	64	0.0730	1.85	0.0629	0.0561	0.0026	#53 (0.0595)
#1-72	UNF	72	0.0730	1.85	0.0640	0.0580	0.0028	#53 (0.0595)
#2-56	UNC	56	0.0860	2.18	0.0744	0.0667	0.0037	#50 (0.0700)
#2-64	UNF	64	0.0860	2.18	0.0759	0.0691	0.0039	#50 (0.0700)
#3-48	UNC	48	0.0990	2.51	0.0855	0.0764	0.0049	#47 (0.0785)
#3-56	UNF	56	0.0990	2.51	0.0874	0.0797	0.0052	#45 (0.0820)
#4-40	UNC	40	0.1120	2.84	0.0958	0.0849	0.0060	#43 (0.0890)
#4-48	UNF	48	0.1120	2.84	0.0985	0.0894	0.0066	#42 (0.0935)
#5-40	UNC	40	0.1250	3.17	0.1088	0.0979	0.0080	#38 (0.1015)
#5-44	UNF	44	0.1250	3.17	0.1102	0.1004	0.0083	#37 (0.1040)
#6-32	UNC	32	0.1380	3.51	0.1177	0.1042	0.0091	#36 (0.1065)
#6-40	UNF	40	0.1380	3.51	0.1218	0.1109	0.0101	#33 (0.1130)
#8-32	UNC	32	0.1640	4.17	0.1437	0.1302	0.0140	#29 (0.1360)
#8-36	UNF	36	0.1640	4.17	0.1460	0.1339	0.0147	#29 (0.1360)
#10-24	UNC	24	0.1900	4.83	0.1629	0.1449	0.0175	#25 (0.1495)
#10-32	UNF	32	0.1900	4.83	0.1697	0.1562	0.0200	#21 (0.1590)
#12-24	UNC	24	0.2160	5.49	0.1889	0.1709	0.0242	#16 (0.1770)
#12-28	UNF	28	0.2160	5.49	0.1928	0.1773	0.0258	#14 (0.1820)
1/4-20	UNC	20	0.2500	6.35	0.2175	0.1959	0.0318	#7 (0.2010)
1/4-28	UNF	28	0.2500	6.35	0.2268	0.2113	0.0364	#3 (0.2130)
5/16-18	UNC	18	0.3125	7.94	0.2764	0.2524	0.0524	F (0.2570)
5/16-24	UNF	24	0.3125	7.94	0.2854	0.2674	0.0581	I (0.2720)
3/8-16	UNC	16	0.3750	9.52	0.3344	0.3073	0.0775	5/16" (0.3125)
3/8-24	UNF	24	0.3750	9.52	0.3479	0.3299	0.0878	Q (0.3320)
7/16-14	UNC	14	0.4375	11.11	0.3911	0.3602	0.1063	U (0.3680)
7/16-20	UNF	20	0.4375	11.11	0.4050	0.3834	0.1187	25/64" (0.3906)
1/2-13	UNC	13	0.5000	12.70	0.4500	0.4167	0.1419	27/64" (0.4219)
1/2-20	UNF	20	0.5000	12.70	0.4675	0.4459	0.1600	29/64" (0.4531)
9/16-12	UNC	12	0.5625	14.29	0.5084	0.4723	0.1819	31/64" (0.4844)
9/16-18	UNF	18	0.5625	14.29	0.5264	0.5024	0.2030	33/64" (0.5156)
5/8-11	UNC	11	0.6250	15.88	0.5660	0.5266	0.2260	17/32" (0.5312)
5/8-18	UNF	18	0.6250	15.88	0.5889	0.5649	0.2560	37/64" (0.5781)
3/4-10	UNC	10	0.7500	19.05	0.6850	0.6417	0.3345	21/32" (0.6562)
3/4-16	UNF	16	0.7500	19.05	0.7094	0.6823	0.3730	11/16" (0.6875)
7/8-9	UNC	9	0.8750	22.22	0.8028	0.7547	0.4617	49/64" (0.7656)
7/8-14	UNF	14	0.8750	22.22	0.8286	0.7977	0.5095	13/16" (0.8125)
1-8	UNC	8	1.0000	25.40	0.9188	0.8647	0.6057	7/8" (0.8750)
1-12	UNF	12	1.0000	25.40	0.9459	0.9098	0.6630	59/64" (0.9219)
1-1/8-7	UNC	7	1.1250	28.57	1.0322	0.9704	0.7633	63/64" (0.9844)
1-1/8-12	UNF	12	1.1250	28.57	1.0709	1.0348	0.8557	1-3/64" (1.0469)
1-1/4-7	UNC	7	1.2500	31.75	1.1572	1.0954	0.9691	1-7/64" (1.1094)
1-1/4-12	UNF	12	1.2500	31.75	1.1959	1.1598	1.0729	1-11/64" (1.1719)
1-3/8-6	UNC	6	1.3750	34.92	1.2667	1.1946	1.1549	1-7/32" (1.2188)
1-3/8-12	UNF	12	1.3750	34.92	1.3209	1.2848	1.3147	1-19/64" (1.2969)
1-1/2-6	UNC	6	1.5000	38.10	1.3917	1.3196	1.4053	1-11/32" (1.3438)
1-1/2-12	UNF	12	1.5000	38.10	1.4459	1.4098	1.5810	1-27/64" (1.4219)

Basic dimensions computed from the ASME B1.1 / FED-STD-H28 relationships. Pitch dia = D − 0.649519/n; minor dia (internal) = D − 1.082532/n; stress area = 0.7854 (D − 0.9743/n)². Tap drill = closest standard drill to D − 0.974/n.

How to read a Unified thread size

A Unified thread is named by its diameter, its threads per inch (TPI) and its series. In 1/4-20 UNC-2A, 1/4 is the nominal major diameter in inches, 20 is the number of threads per inch, UNC means Unified National Coarse, and 2A is the tolerance class (A for an external thread such as a bolt, B for an internal thread such as a nut; class 2 is the normal commercial fit). Sizes under 1/4 inch use machine screw numbers: a #10 screw has a major diameter of 0.060 + 0.013 x 10 = 0.190 inch, which is the formula behind every numbered size in the chart.

The pitch is the distance from one thread to the next, 1 / TPI. All the other dimensions follow from it, because the Unified thread has a fixed 60 degree profile. The fundamental triangle height is H = 0.866025 x pitch, and the standard subtracts fixed fractions of H from the major diameter to get the pitch diameter (where the thread and the gap are equally wide) and the minor diameter (the bottom of the internal thread).

Worked example: 1/4-20 compared with 1/4-28

For 1/4-20 UNC, the pitch is 1/20 = 0.0500 in. Pitch diameter = 0.2500 − 0.649519 x 0.05 = 0.2175 in. Minor diameter of the nut thread = 0.2500 − 1.082532 x 0.05 = 0.1959 in. Stress area = 0.7854 x (0.2500 − 0.9743 x 0.05)² = 0.0318 in². The 75% tap drill works out at 0.2500 − 0.974 x 0.05 = 0.2013 in, and the closest standard drill is #7 (0.2010 in), which gives 75% thread.

For 1/4-28 UNF the pitch is 0.0357 in, so the same arithmetic gives a stress area of 0.0364 in², 14% more than the coarse thread, because the shallower fine thread removes less metal from the core. The same pattern holds at every size: a 1/2-20 UNF bolt has 0.1600 in² against 0.1419 in² for 1/2-13 UNC, 13% more.

Coarse or fine: which to use

Coarse (UNC) is the default for general work and for anything that goes into wood, aluminium, cast iron or plastic. The deeper thread strips less easily in soft material, starts faster, and tolerates dirt, paint and slightly damaged threads.
Fine (UNF) is used where vibration could loosen a fastener, for fine adjustment (more turns per inch of travel), in thin walls where only a few threads engage, and where the extra stress area matters. Fine threads gall and cross-thread more easily, especially in stainless steel.
Do not mix them. A 1/4-28 nut will start on a 1/4-20 bolt for half a turn and then jam. If a nut is tight after one turn, stop and check the pitch.

Common mistakes

Measuring a bolt with calipers and reading the major diameter as smaller than nominal: real bolts are made slightly under size (class 2A allowance), so a 1/4-20 bolt typically measures 0.240 to 0.248 in, not 0.250.
Confusing #10-24 and #10-32: both have the same 0.190 in diameter. Count the threads over half an inch with a ruler or use a thread pitch gauge; 12 threads in half an inch is 24 TPI, 16 is 32 TPI.
Using the minor diameter of the bolt as the tap drill. The tap drill is close to the minor diameter of the nut thread, and choosing it from the chart (75% thread) avoids broken taps.

Frequently asked questions

What does UNC and UNF mean?

Unified National Coarse and Unified National Fine, the two main series of the Unified inch thread standard (ASME B1.1). Each nominal size has one coarse and usually one fine pitch, for example 1/4-20 UNC and 1/4-28 UNF.

What is the major diameter of a #10 screw?

0.190 inch (4.83 mm). Numbered machine screw sizes follow the formula 0.060 + 0.013 x number, so #6 is 0.138 in, #8 is 0.164 in and #12 is 0.216 in.

Is a fine thread stronger than a coarse thread?

In tension, yes, slightly: the fine thread has a larger stress area (0.0364 in² for 1/4-28 against 0.0318 in² for 1/4-20). In soft materials the coarse thread is usually better because its deeper teeth resist stripping.

How do I find the threads per inch of a bolt?

Use a thread pitch gauge, or lay a ruler along the bolt and count the crests in half an inch, then double it. Compare the diameter and TPI with the chart to identify the thread.

Sources and method

Every dimension on this page is computed from the formulas below, not copied from another chart, so you can check any row by hand. Values are basic (nominal) dimensions; real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric) and are slightly smaller (bolts) or larger (nuts and tapped holes).

ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form): basic profile, pitch-diameter and minor-diameter relationships, and the tensile stress area formula. The same relationships are published in the US federal standard FED-STD-H28/2B, Screw-Thread Standards for Federal Services, Section 2 (a US Government work).
ISO 68-1, ISO general purpose screw threads, basic profile; ISO 261, general plan of diameters and pitches; ISO 898-1, stress area of metric bolts.
ASME B94.11M, Twist Drills: number, letter and fractional drill diameters (the drill list used by our calculators).
ISO 273, Fasteners, clearance holes for bolts and screws (fine, medium, coarse series); inch close / normal / loose fits as published in the ASME B18.2.8 clearance classes and reproduced in fastener suppliers’ charts.
Percentage-of-thread formula for tap drills: Machinery’s Handbook (Industrial Press), as used by our tap drill size calculator.

More thread and fastener tools

Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.