UNC and UNF Thread Chart with Tap Drills
Look up unified inch screw threads (UNC coarse and UNF fine): threads per inch, major diameter and the common tap drill for each size.
Unified thread chart (ASME B1.1)
| Size | Major dia (in) | Major dia (mm) | UNC TPI | UNC tap drill | UNF TPI | UNF tap drill |
|---|---|---|---|---|---|---|
| #4 | 0.1120 | 2.84 | 40 | #43 | 48 | #42 |
| #6 | 0.1380 | 3.51 | 32 | #36 | 40 | #33 |
| #8 | 0.1640 | 4.17 | 32 | #29 | 36 | #29 |
| #10 | 0.1900 | 4.83 | 24 | #25 | 32 | #21 |
| 1/4" | 0.2500 | 6.35 | 20 | #7 | 28 | #3 |
| 5/16" | 0.3125 | 7.94 | 18 | F | 24 | I |
| 3/8" | 0.3750 | 9.52 | 16 | 5/16" | 24 | Q |
| 7/16" | 0.4375 | 11.11 | 14 | U | 20 | 25/64" |
| 1/2" | 0.5000 | 12.70 | 13 | 27/64" | 20 | 29/64" |
| 9/16" | 0.5625 | 14.29 | 12 | 31/64" | 18 | 33/64" |
| 5/8" | 0.6250 | 15.88 | 11 | 17/32" | 18 | 37/64" |
| 3/4" | 0.7500 | 19.05 | 10 | 21/32" | 16 | 11/16" |
| 7/8" | 0.8750 | 22.22 | 9 | 49/64" | 14 | 13/16" |
| 1" | 1.0000 | 25.40 | 8 | 7/8" | 12 | 59/64" |
Reading the Designation
Pitch (mm) = 25.4 ÷ TPI
TPI is threads per inch. Tap drills shown give about 75 % thread engagement, the common shop choice. Numbered and lettered drills are standard US drill sizes.
Worked example
1/4"-20 UNC: major diameter 0.250 in (6.35 mm), pitch 1.27 mm, tap drill #7 (0.201 in).
Frequently Asked Questions
What does 1/4-20 UNC mean?
A 1/4 inch diameter coarse thread with 20 threads per inch.
When is UNF used?
Where finer adjustment or thin walls need more threads, and in some instrument and aerospace parts.
Are UNC and NPT the same?
No, NPT is a tapered pipe thread; UNC and UNF are straight fastener threads.
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