Thread Size Chart — Metric, UNC & UNF Tap Drills | koeedmro
REFERENCE
Metric Coarse (ISO M)
| Designation | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M3 | 0.5 | 2.5 |
| M4 | 0.7 | 3.3 |
| M5 | 0.8 | 4.2 |
| M6 | 1.0 | 5 |
| M8 | 1.25 | 6.8 |
| M10 | 1.5 | 8.5 |
| M12 | 1.75 | 10.2 |
| M16 | 2.0 | 14 |
| M20 | 2.5 | 17.5 |
| M24 | 3.0 | 21 |
| M30 | 3.5 | 26.5 |
| M36 | 4.0 | 32 |
| M42 | 4.5 | 37.5 |
| M48 | 5.0 | 43 |
UNC — Unified Coarse
| Designation | Major Ø | TPI | Tap drill |
|---|---|---|---|
| #4-40 | 0.112" | 40 | #43 (0.0890") |
| #6-32 | 0.138" | 32 | #36 (0.1065") |
| #8-32 | 0.164" | 32 | #29 (0.1360") |
| #10-24 | 0.190" | 24 | #25 (0.1495") |
| 1/4-20 | 0.250" | 20 | #7 (0.2010") |
| 5/16-18 | 0.3125" | 18 | F (0.2570") |
| 3/8-16 | 0.375" | 16 | 5/16" (0.3125") |
| 7/16-14 | 0.4375" | 14 | U (0.3680") |
| 1/2-13 | 0.500" | 13 | 27/64" (0.4219") |
| 5/8-11 | 0.625" | 11 | 17/32" (0.5313") |
| 3/4-10 | 0.750" | 10 | 21/32" (0.6563") |
| 1-8 | 1.000" | 8 | 7/8" (0.8750") |
UNF — Unified Fine
| Designation | Major Ø | TPI | Tap drill |
|---|---|---|---|
| #6-40 | 0.138" | 40 | #33 (0.1130") |
| #8-36 | 0.164" | 36 | #29 (0.1360") |
| #10-32 | 0.190" | 32 | #21 (0.1590") |
| 1/4-28 | 0.250" | 28 | #3 (0.2130") |
| 5/16-24 | 0.3125" | 24 | F (0.2570") |
| 3/8-24 | 0.375" | 24 | Q (0.3320") |
| 7/16-20 | 0.4375" | 20 | 25/64" (0.3906") |
| 1/2-20 | 0.500" | 20 | 29/64" (0.4531") |
| 5/8-18 | 0.625" | 18 | 9/16" (0.5625") |
| 3/4-16 | 0.750" | 16 | 11/16" (0.6875") |
Tap drill values are for ~75% thread engagement — the standard starting point. Reduce engagement for hard or abrasive materials; increase for soft ones.
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FAQ
What tap drill gives 75% thread?
75% thread is the standard default — close to full fastener strength with manageable tapping torque. The UNC/UNF tables list 75% tap drills by size.
Metric coarse vs fine — which do I have?
Measure major diameter and pitch (crest-to-crest). M10 with 1.5 mm between crests is coarse; 1.25 or 1.0 mm is fine pitch.
Are UNC and UNF interchangeable?
No — same major diameter but different thread counts (1/4-20 vs 1/4-28). Verify TPI before ordering replacements.