Screw & Bolt Thread Size Identifier
Look up thread size, TPI, pitch, and tap drill size for Unified (UNC/UNF) and Metric threads. Identify unknown fasteners by measuring diameter.
| Size | Major dia (in) | UNC TPI | UNF TPI | Tap drill UNC | Tap drill UNF | Clearance (close) |
|---|---|---|---|---|---|---|
| #0 | 0.0600" | — | 80 | — | 3/64" | #51 |
| #1 | 0.0730" | 64 | 72 | #53 | 53 | #47 |
| #2 | 0.0860" | 56 | 64 | #50 | #50 | #42 |
| #3 | 0.0990" | 48 | 56 | #47 | #45 | #37 |
| #4 | 0.1120" | 40 | 48 | #43 | #42 | #31 |
| #5 | 0.1250" | 40 | 44 | #38 | #37 | 1/8" |
| #6 | 0.1380" | 32 | 40 | #36 | #33 | #27 |
| #8 | 0.1640" | 32 | 36 | #29 | #29 | #17 |
| #10 | 0.1900" | 24 | 32 | #25 | #21 | #7 |
| #12 | 0.2160" | 24 | 28 | #16 | #14 | 15/64" |
| 1/4" | 0.2500" | 20 | 28 | #7 | #3 | 17/64" |
| 5/16" | 0.3125" | 18 | 24 | F | I | 21/64" |
| 3/8" | 0.3750" | 16 | 24 | 5/16" | Q | 25/64" |
| 7/16" | 0.4375" | 14 | 20 | U | 25/64" | 29/64" |
| 1/2" | 0.5000" | 13 | 20 | 27/64" | 29/64" | 33/64" |
| 9/16" | 0.5625" | 12 | 18 | 31/64" | 33/64" | 37/64" |
| 5/8" | 0.6250" | 11 | 18 | 17/32" | 37/64" | 41/64" |
| 3/4" | 0.7500" | 10 | 16 | 21/32" | 11/16" | 49/64" |
| 7/8" | 0.8750" | 9 | 14 | 49/64" | 13/16" | 57/64" |
| 1" | 1.0000" | 8 | 12 | 7/8" | 59/64" | 1-1/64" |
| 1-1/8" | 1.1250" | 7 | 12 | 63/64" | 1-3/64" | 1-5/32" |
| 1-1/4" | 1.2500" | 7 | 12 | 1-7/64" | 1-11/64" | 1-9/32" |
How to identify an unknown thread
Use digital calipers to measure the outer diameter of the threaded shaft. For inch fasteners, count the number of thread crests in 1 inch (TPI). For metric, measure the distance between crests (pitch in mm). The tool matches within a 3% tolerance to account for wear and manufacturing variation.
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