UNF unified fine threads

The UNF series (Unified National Fine Thread) is part of the unified thread system defined by the ANSI/ASME B1.1 standard. It is characterized by a finer pitch compared to the UNC coarse series, providing greater resistance to vibration, better fit, and thinner thread walls. The flank angle is 60°, identical to ISO metric threads, but all dimensions are expressed in inches. UNF threads are used in precision applications, hydraulic connections, aerospace systems, and automotive applications where high strength and self-locking capability are required.

Nomenclature and designation

A UNF thread is designated by the nominal diameter (in inches or with a gauge number for sizes below ¼“), the number of threads per inch (TPI), and the suffix UNF. Optionally, the tolerance class (1, 2, 3) and whether the fit is external (A) or internal (B) are added. For example:

1/4-28 UNF 2A → external UNF thread ¼“, 28 threads per inch, class 2 (general use).
#10-32 UNF 3B → #10 gauge screw, 32 threads per inch, class 3 (precision fit, internal).

Class 2A/2B is the most common in general applications; classes 3A/3B are reserved for assemblies requiring high precision and minimal clearance.

UNF Dimensions Table

Nominal Size	Threads per Inch (TPI)	Major Diameter (in / mm)	Pitch Diameter (in / mm)	Minor Diameter (in / mm)	Pitch (in / mm)	Tensile Stress Area (in² / mm²)	Tap Drill Size (in / mm)
#0 - 80	80	0.0600 / 1.524	0.0519 / 1.318	0.0465 / 1.181	0.0125 / 0.317	0.00180 / 1.16	0.049 / 1.25
#1 - 72	72	0.0730 / 1.854	0.0640 / 1.625	0.0580 / 1.473	0.0139 / 0.353	0.00278 / 1.79	0.061 / 1.55
#2 - 64	64	0.0860 / 2.184	0.0759 / 1.927	0.0691 / 1.755	0.0156 / 0.397	0.00393 / 2.54	0.075 / 1.90
#3 - 56	56	0.0990 / 2.515	0.0874 / 2.220	0.0797 / 2.024	0.0179 / 0.453	0.00523 / 3.37	0.085 / 2.15
#4 - 48	48	0.1120 / 2.845	0.0985 / 2.502	0.0894 / 2.272	0.0208 / 0.529	0.00660 / 4.26	0.094 / 2.40
#5 - 44	44	0.1250 / 3.175	0.1102 / 2.800	0.1004 / 2.550	0.0227 / 0.577	0.00831 / 5.36	0.106 / 2.70
#6 - 40	40	0.1380 / 3.505	0.1218 / 3.094	0.1109 / 2.818	0.0250 / 0.635	0.0101 / 6.54	0.116 / 2.95
#8 - 36	36	0.1640 / 4.166	0.1460 / 3.708	0.1339 / 3.402	0.0278 / 0.706	0.0147 / 9.50	0.138 / 3.50
#10 - 32	32	0.1900 / 4.826	0.1697 / 4.310	0.1562 / 3.967	0.0313 / 0.794	0.0200 / 12.9	0.161 / 4.10
#12 - 28	28	0.2160 / 5.486	0.1928 / 4.897	0.1773 / 4.504	0.0357 / 0.907	0.0258 / 16.6	0.185 / 4.70
1/4″ - 28	28	0.2500 / 6.350	0.2268 / 5.761	0.2113 / 5.368	0.0357 / 0.907	0.0364 / 23.5	0.217 / 5.50 (7/32 in)
5/16″ - 24	24	0.3125 / 7.938	0.2854 / 7.250	0.2674 / 6.792	0.0417 / 1.058	0.0581 / 37.5	0.272 / 6.90 (17/64 in)
3/8″ - 24	24	0.3750 / 9.525	0.3479 / 8.838	0.3299 / 8.380	0.0417 / 1.058	0.0878 / 56.7	0.335 / 8.50
7/16″ - 20	20	0.4375 / 11.113	0.4050 / 10.287	0.3834 / 9.738	0.0500 / 1.270	0.1187 / 76.6	0.390 / 9.90 (25/64 in)
1/2″ - 20	20	0.5000 / 12.700	0.4675 / 11.875	0.4459 / 11.325	0.0500 / 1.270	0.1599 / 103.2	0.453 / 11.50 (29/64 in)
9/16″ - 18	18	0.5625 / 14.288	0.5264 / 13.370	0.5024 / 12.761	0.0556 / 1.411	0.2029 / 130.9	0.508 / 12.90
5/8″ - 18	18	0.6250 / 15.875	0.5889 / 14.958	0.5649 / 14.348	0.0556 / 1.411	0.2559 / 165.1	0.571 / 14.50
3/4″ - 16	16	0.7500 / 19.050	0.7094 / 18.019	0.6823 / 17.331	0.0625 / 1.588	0.3729 / 240.6	0.689 / 17.50 (11/16 in)
7/8″ - 14	14	0.8750 / 22.225	0.8286 / 21.047	0.7977 / 20.261	0.0714 / 1.814	0.5097 / 328.9	0.803 / 20.40 (13/16 in)
1″ - 12	12	1.0000 / 25.400	0.9459 / 24.026	0.9098 / 23.107	0.0833 / 2.117	0.6630 / 427.7	0.915 / 23.25 (29/32 in)
1 1/8″ - 12	12	1.1250 / 28.575	1.0709 / 27.201	1.0348 / 26.284	0.0833 / 2.117	0.8555 / 552.0	1.043 / 26.50
1 1/4″ - 12	12	1.2500 / 31.750	1.1959 / 30.375	1.1598 / 29.459	0.0833 / 2.117	1.0729 / 692.2	1.161 / 29.50
1 3/8″ - 12	12	1.3750 / 34.925	1.3209 / 33.551	1.2848 / 32.634	0.0833 / 2.117	1.3145 / 848.1	1.289 / 32.75
1 1/2″ - 12	12	1.5000 / 38.100	1.4459 / 36.726	1.4098 / 35.809	0.0833 / 2.117	1.5810 / 1020.1	1.417 / 36.00

Tap Drill Sizes

The recommended drill bit for tapping UNF threads at 75% thread depth is indicated in the dimensions table. For tougher materials, a slightly larger drill bit can be used (up to 3% larger), while for light alloys, the diameter can be reduced to obtain a higher thread percentage. The following summarizes the fractional or decimal equivalences of the most common holes:

Nominal Size	Recommended Drill (mm)	Equivalent Drill (in)
#0 - 80	1.25	0.049
#1 - 72	1.55	0.061
#2 - 64	1.90	0.075
#3 - 56	2.15	0.085
#4 - 48	2.40	0.094
#5 - 44	2.70	0.106
#6 - 40	2.95	0.116
#8 - 36	3.50	0.138
#10 - 32	4.10	0.161
#12 - 28	4.70	0.185
1/4 - 28	5.50	7/32
5/16 - 24	6.90	17/64
3/8 - 24	8.50	0.335
7/16 - 20	9.90	25/64
1/2 - 20	11.50	29/64
9/16 - 18	12.90	0.508
5/8 - 18	14.50	0.571
3/4 - 16	17.50	11/16
7/8 - 14	20.40	13/16
1 - 12	23.25	29/32
1 1/8 - 12	26.50	1.043
1 1/4 - 12	29.50	1.161
1 3/8 - 12	32.75	1.289
1 1/2 - 12	36.00	1.417

Allowable Loads

The allowable loads are based on the proof load for the most commonly used SAE J429 fastener grades in UNF threads. The values correspond to a static axial load; in dynamic or fatigue applications, an appropriate safety factor must be applied.

Nominal Size	Stress Area (mm² / in²)	Proof Load Grade 2 (kN / lbf)	Proof Load Grade 5 (kN / lbf)	Proof Load Grade 8 (kN / lbf)
#0 - 80	1.16 / 0.00180	0.38 / 86	0.68 / 153	0.96 / 216
#1 - 72	1.79 / 0.00278	0.59 / 133	1.05 / 236	1.49 / 334
#2 - 64	2.54 / 0.00393	0.84 / 189	1.48 / 334	2.11 / 474
#3 - 56	3.37 / 0.00523	1.11 / 251	1.97 / 444	2.80 / 630
#4 - 48	4.26 / 0.00660	1.41 / 317	2.49 / 561	3.54 / 795
#5 - 44	5.36 / 0.00831	1.77 / 399	3.13 / 706	4.45 / 1001
#6 - 40	6.54 / 0.0101	2.16 / 486	3.83 / 862	5.43 / 1223
#8 - 36	9.50 / 0.0147	3.14 / 706	5.56 / 1250	7.89 / 1773
#10 - 32	12.9 / 0.0200	4.26 / 960	7.55 / 1700	10.7 / 2400
#12 - 28	16.6 / 0.0258	5.49 / 1238	9.73 / 2193	13.8 / 3096
1/4 - 28	23.5 / 0.0364	7.75 / 1747	13.7 / 3094	19.5 / 4368
5/16 - 24	37.5 / 0.0581	12.4 / 2789	21.9 / 4939	31.1 / 6972
3/8 - 24	56.7 / 0.0878	18.7 / 4214	33.2 / 7463	47.1 / 10536
7/16 - 20	76.6 / 0.1187	25.3 / 5698	44.8 / 10090	63.6 / 14244
1/2 - 20	103.2 / 0.1599	34.1 / 7675	60.4 / 13590	85.7 / 19188
9/16 - 18	130.9 / 0.2029	43.2 / 9739	76.6 / 17247	108.6 / 24348
5/8 - 18	165.1 / 0.2559	54.5 / 12283	96.6 / 21750	137.0 / 30708
3/4 - 16	240.6 / 0.3729	79.4 / 17899	140.8 / 31600	199.7 / 44748
7/8 - 14	328.9 / 0.5097	108.5 / 24466	192.4 / 43325	273.0 / 61364
1 - 12	427.7 / 0.6630	141.1 / 31824	250.2 / 56355	355.0 / 79760
1 1/8 - 12	552.0 / 0.8555	182.2 / 41064	322.9 / 72718	458.1 / 102960
1 1/4 - 12	692.2 / 1.0729	228.4 / 51499	405.0 / 91197	574.6 / 129148
1 3/8 - 12	848.1 / 1.3145	279.9 / 63096	496.1 / 111733	703.8 / 158240
1 1/2 - 12	1020.1 / 1.5810	336.6 / 75888	596.8 / 134385	846.5 / 190320

Recommended Tightening Torques

The torque values were calculated for bolts and nuts with clean and dry threads (friction coefficient K≈0.20), taking a preload of 75% of the proof load. The maximum recommended torques are indicated for Grades 2, 5, and 8. In applications with lubrication or special coatings, the values should be reduced.

Nominal Size	Torque Grade 2 (Nm / lb‑in / lb‑ft)	Torque Grade 5 (Nm / lb‑in / lb‑ft)	Torque Grade 8 (Nm / lb‑in / lb‑ft)
#0 - 80	0.04 Nm / 0.34 lb‑in	0.07 Nm / 0.60 lb‑in	0.10 Nm / 0.86 lb‑in
#1 - 72	0.08 Nm / 0.68 lb‑in	0.14 Nm / 1.20 lb‑in	0.19 Nm / 1.70 lb‑in
#2 - 64	0.13 Nm / 1.15 lb‑in	0.23 Nm / 2.04 lb‑in	0.33 Nm / 2.89 lb‑in
#3 - 56	0.20 Nm / 1.81 lb‑in	0.37 Nm / 3.26 lb‑in	0.52 Nm / 4.62 lb‑in
#4 - 48	0.29 Nm / 2.60 lb‑in	0.53 Nm / 4.70 lb‑in	0.75 Nm / 6.66 lb‑in
#5 - 44	0.41 Nm / 3.65 lb‑in	0.74 Nm / 6.56 lb‑in	1.05 Nm / 9.30 lb‑in
#6 - 40	0.54 Nm / 4.82 lb‑in	0.98 Nm / 8.68 lb‑in	1.39 Nm / 12.3 lb‑in
#8 - 36	0.94 Nm / 8.32 lb‑in	1.69 Nm / 14.9 lb‑in	2.39 Nm / 21.2 lb‑in
#10 - 32	1.48 Nm / 13.1 lb‑in / 1.09 lb‑ft	2.67 Nm / 23.6 lb‑in / 1.97 lb‑ft	3.79 Nm / 33.5 lb‑in / 2.79 lb‑ft
#12 - 28	2.17 Nm / 19.2 lb‑in / 1.60 lb‑ft	3.92 Nm / 34.7 lb‑in / 2.89 lb‑ft	5.57 Nm / 49.3 lb‑in / 4.11 lb‑ft
1/4 - 28	3.56 Nm / 31.5 lb‑in / 2.62 lb‑ft	6.43 Nm / 56.9 lb‑in / 4.74 lb‑ft	9.12 Nm / 80.7 lb‑in / 6.72 lb‑ft
5/16 - 24	7.12 Nm / 63.0 lb‑in / 5.25 lb‑ft	12.9 Nm / 114 lb‑in / 9.51 lb‑ft	18.3 Nm / 162 lb‑in / 13.5 lb‑ft
3/8 - 24	12.9 Nm / 114 lb‑in / 9.53 lb‑ft	23.4 Nm / 207 lb‑in / 17.3 lb‑ft	33.2 Nm / 294 lb‑in / 24.5 lb‑ft
7/16 - 20	20.4 Nm / 15.0 lb‑ft	36.9 Nm / 27.2 lb‑ft	52.3 Nm / 38.6 lb‑ft
1/2 - 20	31.4 Nm / 23.2 lb‑ft	56.9 Nm / 42.0 lb‑ft	80.7 Nm / 59.5 lb‑ft
9/16 - 18	45.0 Nm / 33.2 lb‑ft	81.5 Nm / 60.1 lb‑ft	115.6 Nm / 85.3 lb‑ft
5/8 - 18	63.1 Nm / 46.6 lb‑ft	114.3 Nm / 84.3 lb‑ft	162.1 Nm / 119.6 lb‑ft
3/4 - 16	110.5 Nm / 81.5 lb‑ft	200.0 Nm / 147.5 lb‑ft	283.7 Nm / 209.3 lb‑ft
7/8 - 14	176.1 Nm / 129.9 lb‑ft	318.9 Nm / 235.2 lb‑ft	452.3 Nm / 333.6 lb‑ft
1 - 12	262.1 Nm / 193.3 lb‑ft	474.6 Nm / 350.0 lb‑ft	673.1 Nm / 496.4 lb‑ft
1 1/8 - 12	380.4 Nm / 280.6 lb‑ft	688.7 Nm / 508.0 lb‑ft	976.6 Nm / 720.3 lb‑ft
1 1/4 - 12	530.5 Nm / 391.3 lb‑ft	960.3 Nm / 708.4 lb‑ft	1362 Nm / 1005 lb‑ft
1 3/8 - 12	717.8 Nm / 529.4 lb‑ft	1299 Nm / 958 lb‑ft	1843 Nm / 1360 lb‑ft
1 1/2 - 12	946.6 Nm / 698.2 lb‑ft	1714 Nm / 1264 lb‑ft	2431 Nm / 1793 lb‑ft

Equivalences between standards (DIN, ISO, ANSI)

UNF threads are standardized exclusively under the ANSI/ASME B1.1 system and do not have a direct equivalent in the DIN or ISO metric standards. While UNF threads are based on inches and use pitch as threads per inch, DIN/ISO standards use pitch in millimeters and diameter in millimeters. Although dimensionally close combinations may exist (e.g., UNF 1/4-28 and a hypothetical M6×0.5), they are not interchangeable or substitutable. In projects that combine American and European specifications, adapters should be used or the design should be re-evaluated using a single system.

Frequently Asked Questions (FAQ)

What is the pitch of a UNF 1/4-28 thread?

The pitch is 0.907 mm, equivalent to 28 threads per inch (0.0357 in).

What drill diameter is needed to machine an internal UNF 3/8-24 thread?

It is recommended to use a drill of 8.50 mm (0.335 in) to obtain approximately 75% thread depth.

How much load can a UNF 1/2-20 Grade 5 bolt withstand before permanent deformation?

The proof load is 60.4 kN (13,590 lbf), which corresponds to the practical yield strength of the material in direct tension.

What tightening torque should be applied to a UNF 5/8-18 Grade 8 bolt in dry assembly?

The maximum recommended torque is 162.1 Nm (119.6 lb‑ft), assuming a friction coefficient K=0.20 and no lubrication.

What is the stress area of a UNF 3/4-16 thread?

The tensile stress area is 240.6 mm² (0.3729 in²), a value used to calculate the maximum load on the bolt.

Is there any metric thread identical to the UNF series?

There are no identical metric equivalents. UNF threads are defined by ANSI/ASME B1.1 in inches; any dimensional similarity with fine metric threads (DIN 13 / ISO 724) is coincidental and does not guarantee interchangeability.

References

engineeringtoolbox.com: https://www.engineeringtoolbox.com/unified-screw-threads-unc-unf-d_1809.html
bossard.com: https://www.bossard.com/global-en/knowledge-hub/resources/online-calculators-and-converters/thread-engagement-length-calculator/
mcmaster.com: https://www.mcmaster.com/products/thread-size-identifiers/thread-type~unf-1/