UNEF Thread Chart (Unified Extra Fine)
All 25 Unified extra fine threads from #12-32 to 1-11/16-18, with diameters, tap drill and tensile stress area. UNEF is used where the thread must be shallow: thin-walled tubes, adjusting screws, bearing retaining nuts and instrument fittings.
| Thread | TPI | Major Ø (in) | Pitch Ø (in) | Minor Ø D1 (in) | Tap drill | Drill (in) | Drill (mm) | % thread | Stress area (in²) |
|---|---|---|---|---|---|---|---|---|---|
| #12-32 UNEF | 32 | 0.2160 | 0.1957 | 0.1822 | #13 | 0.1850 | 4.70 | 76% | 0.0270 |
| 1/4-32 UNEF | 32 | 0.2500 | 0.2297 | 0.2162 | 7/32" | 0.2188 | 5.56 | 77% | 0.0379 |
| 5/16-32 UNEF | 32 | 0.3125 | 0.2922 | 0.2787 | 9/32" | 0.2812 | 7.14 | 77% | 0.0625 |
| 3/8-32 UNEF | 32 | 0.3750 | 0.3547 | 0.3412 | 11/32" | 0.3438 | 8.73 | 77% | 0.0932 |
| 7/16-28 UNEF | 28 | 0.4375 | 0.4143 | 0.3988 | Y | 0.4040 | 10.26 | 72% | 0.1274 |
| 1/2-28 UNEF | 28 | 0.5000 | 0.4768 | 0.4613 | 15/32" | 0.4688 | 11.91 | 67% | 0.170 |
| 9/16-24 UNEF | 24 | 0.5625 | 0.5354 | 0.5174 | 33/64" | 0.5156 | 13.10 | 87% | 0.214 |
| 5/8-24 UNEF | 24 | 0.6250 | 0.5979 | 0.5799 | 37/64" | 0.5781 | 14.68 | 87% | 0.268 |
| 11/16-24 UNEF | 24 | 0.6875 | 0.6604 | 0.6424 | 41/64" | 0.6406 | 16.27 | 87% | 0.329 |
| 3/4-20 UNEF | 20 | 0.7500 | 0.7175 | 0.6959 | 45/64" | 0.7031 | 17.86 | 72% | 0.386 |
| 13/16-20 UNEF | 20 | 0.8125 | 0.7800 | 0.7584 | 49/64" | 0.7656 | 19.45 | 72% | 0.458 |
| 7/8-20 UNEF | 20 | 0.8750 | 0.8425 | 0.8209 | 53/64" | 0.8281 | 21.03 | 72% | 0.536 |
| 15/16-20 UNEF | 20 | 0.9375 | 0.9050 | 0.8834 | 57/64" | 0.8906 | 22.62 | 72% | 0.620 |
| 1-20 UNEF | 20 | 1.0000 | 0.9675 | 0.9459 | 61/64" | 0.9531 | 24.21 | 72% | 0.711 |
| 1-1/16-18 UNEF | 18 | 1.0625 | 1.0264 | 1.0024 | 1-1/64" | 1.0156 | 25.80 | 65% | 0.799 |
| 1-1/8-18 UNEF | 18 | 1.1250 | 1.0889 | 1.0649 | 1-5/64" | 1.0781 | 27.38 | 65% | 0.901 |
| 1-3/16-18 UNEF | 18 | 1.1875 | 1.1514 | 1.1274 | 1-9/64" | 1.1406 | 28.97 | 65% | 1.009 |
| 1-1/4-18 UNEF | 18 | 1.2500 | 1.2139 | 1.1899 | 1-13/64" | 1.2031 | 30.56 | 65% | 1.123 |
| 1-5/16-18 UNEF | 18 | 1.3125 | 1.2764 | 1.2524 | 1-17/64" | 1.2656 | 32.15 | 65% | 1.244 |
| 1-3/8-18 UNEF | 18 | 1.3750 | 1.3389 | 1.3149 | 1-21/64" | 1.3281 | 33.73 | 65% | 1.37 |
| 1-7/16-18 UNEF | 18 | 1.4375 | 1.4014 | 1.3774 | 1-25/64" | 1.3906 | 35.32 | 65% | 1.503 |
| 1-1/2-18 UNEF | 18 | 1.5000 | 1.4639 | 1.4399 | 1-29/64" | 1.4531 | 36.91 | 65% | 1.64 |
| 1-9/16-18 UNEF | 18 | 1.5625 | 1.5264 | 1.5024 | 1-33/64" | 1.5156 | 38.50 | 65% | 1.79 |
| 1-5/8-18 UNEF | 18 | 1.6250 | 1.5889 | 1.5649 | 1-37/64" | 1.5781 | 40.08 | 65% | 1.94 |
| 1-11/16-18 UNEF | 18 | 1.6875 | 1.6514 | 1.6274 | 1-41/64" | 1.6406 | 41.67 | 65% | 2.1 |
About the UNEF series
The extra fine series has the most threads per inch of the three standard series. At 1 inch it has 20 threads per inch against 12 for UNF and 8 for UNC, so the thread is less than half as deep as the coarse one and can be cut in a wall that a coarse thread would break through.
Half of the UNEF sizes are secondary sizes in ASME B1.1 (the odd sixteenths such as 11/16, 13/16 and 1-1/16). Above 1-11/16 in the standard continues the series with the 16-thread series (16 UN).
Threads per inch by series
Common questions
What is the tap drill for 1/2-28?
The tap drill for 1/2-28 UNEF is 15/32 in, 0.4688 in (11.91 mm), for about 67% thread.
What is the tap drill for 1/4-32?
The tap drill for 1/4-32 UNEF is 7/32 in, 0.2188 in (5.56 mm), for about 77% thread.
Where are UNEF threads used?
On thin-walled parts and for fine adjustment: instrument and optical mounts, bearing lock nuts, hydraulic tube fittings and electrical connectors.
Other charts and tools
Sources: ASME B1.1 (series, basic dimensions and stress areas) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.