Metric threads
Metric coarse (ISO 261) - tap drill and ISO 273 clearance holes, mm
| Thread | Pitch | Tap drill | Form tap* | Close | Medium | Free |
|---|---|---|---|---|---|---|
| M1.6 | 0.35 | 1.25 | 1.43 | 1.7 | 1.8 | 2 |
| M2 | 0.4 | 1.6 | 1.8 | 2.2 | 2.4 | 2.6 |
| M2.5 | 0.45 | 2.05 | 2.27 | 2.7 | 2.9 | 3.1 |
| M3 | 0.5 | 2.5 | 2.75 | 3.2 | 3.4 | 3.6 |
| M4 | 0.7 | 3.3 | 3.65 | 4.3 | 4.5 | 4.8 |
| M5 | 0.8 | 4.2 | 4.6 | 5.3 | 5.5 | 5.8 |
| M6 | 1 | 5 | 5.5 | 6.4 | 6.6 | 7 |
| M8 | 1.25 | 6.75 | 7.38 | 8.4 | 9 | 10 |
| M10 | 1.5 | 8.5 | 9.25 | 10.5 | 11 | 12 |
| M12 | 1.75 | 10.25 | 11.12 | 13 | 13.5 | 14.5 |
| M14 | 2 | 12 | 13 | 15 | 15.5 | 16.5 |
| M16 | 2 | 14 | 15 | 17 | 17.5 | 18.5 |
| M18 | 2.5 | 15.5 | 16.75 | 19 | 20 | 21 |
| M20 | 2.5 | 17.5 | 18.75 | 21 | 22 | 24 |
| M22 | 2.5 | 19.5 | 20.75 | 23 | 24 | 26 |
| M24 | 3 | 21 | 22.5 | 25 | 26 | 28 |
| M27 | 3 | 24 | 25.5 | 28 | 30 | 32 |
| M30 | 3.5 | 26.5 | 28.25 | 31 | 33 | 35 |
| M33 | 3.5 | 29.5 | 31.25 | 34 | 36 | 38 |
| M36 | 4 | 32 | 34 | 37 | 39 | 42 |
Metric fine - tap drill, mm
| Thread | Tap drill | Form tap* |
|---|---|---|
| M8 × 1 | 7 | 7.5 |
| M10 × 1.25 | 8.75 | 9.38 |
| M10 × 1 | 9 | 9.5 |
| M12 × 1.5 | 10.5 | 11.25 |
| M12 × 1.25 | 10.75 | 11.38 |
| M14 × 1.5 | 12.5 | 13.25 |
| M16 × 1.5 | 14.5 | 15.25 |
| M18 × 1.5 | 16.5 | 17.25 |
| M20 × 1.5 | 18.5 | 19.25 |
| M22 × 1.5 | 20.5 | 21.25 |
| M24 × 2 | 22 | 23 |
| M27 × 2 | 25 | 26 |
| M30 × 2 | 28 | 29 |
Rule of thumb: metric tap drill = nominal diameter − pitch (M10 × 1.5 → 8.5 mm).
* Form (roll) taps displace metal instead of cutting it and need a larger hole, roughly nominal − ½ pitch. Use the tap maker's figure where given.
Clearance holes (ISO 273): close for accurate location, medium for general work, free where holes may not line up exactly.
* Form (roll) taps displace metal instead of cutting it and need a larger hole, roughly nominal − ½ pitch. Use the tap maker's figure where given.
Clearance holes (ISO 273): close for accurate location, medium for general work, free where holes may not line up exactly.
Unified (inch) threads
Unified coarse (UNC)
| UNC | Tap drill (~75% thread) | Clearance - close | Clearance - free |
|---|---|---|---|
| #4-40 | #43 (0.0890" / 2.26 mm) | #32 (0.1160" / 2.95 mm) | #30 (0.1285" / 3.26 mm) |
| #6-32 | #36 (0.1065" / 2.71 mm) | #27 (0.1440" / 3.66 mm) | #25 (0.1495" / 3.80 mm) |
| #8-32 | #29 (0.1360" / 3.45 mm) | #18 (0.1695" / 4.31 mm) | #16 (0.1770" / 4.50 mm) |
| #10-24 | #25 (0.1495" / 3.80 mm) | #9 (0.1960" / 4.98 mm) | #7 (0.2010" / 5.11 mm) |
| #12-24 | #16 (0.1770" / 4.50 mm) | #2 (0.2210" / 5.61 mm) | #1 (0.2280" / 5.79 mm) |
| 1/4"-20 | #7 (0.2010" / 5.11 mm) | F (0.2570" / 6.53 mm) | H (0.2660" / 6.76 mm) |
| 5/16"-18 | F (0.2570" / 6.53 mm) | P (0.3230" / 8.20 mm) | Q (0.3320" / 8.43 mm) |
| 3/8"-16 | 5/16" (0.3125" / 7.94 mm) | W (0.3860" / 9.80 mm) | X (0.3970" / 10.08 mm) |
| 7/16"-14 | U (0.3680" / 9.35 mm) | 29/64" (0.4531" / 11.51 mm) | 15/32" (0.4688" / 11.91 mm) |
| 1/2"-13 | 27/64" (0.4219" / 10.72 mm) | 33/64" (0.5156" / 13.10 mm) | 17/32" (0.5312" / 13.49 mm) |
| 9/16"-12 | 31/64" (0.4844" / 12.30 mm) | 37/64" (0.5781" / 14.68 mm) | 19/32" (0.5938" / 15.08 mm) |
| 5/8"-11 | 17/32" (0.5312" / 13.49 mm) | 41/64" (0.6406" / 16.27 mm) | 21/32" (0.6562" / 16.67 mm) |
| 3/4"-10 | 21/32" (0.6562" / 16.67 mm) | 49/64" (0.7656" / 19.45 mm) | 25/32" (0.7812" / 19.84 mm) |
| 7/8"-9 | 49/64" (0.7656" / 19.45 mm) | 57/64" (0.8906" / 22.62 mm) | 29/32" (0.9062" / 23.02 mm) |
| 1"-8 | 7/8" (0.8750" / 22.22 mm) | 1-1/64" (1.0156" / 25.80 mm) | 1-1/32" (1.0312" / 26.19 mm) |
Unified fine (UNF) - clearance as UNC
| UNF | Tap drill (~75% thread) |
|---|---|
| #4-48 | #42 (0.0935" / 2.37 mm) |
| #6-40 | #33 (0.1130" / 2.87 mm) |
| #8-36 | #29 (0.1360" / 3.45 mm) |
| #10-32 | #21 (0.1590" / 4.04 mm) |
| #12-28 | #14 (0.1820" / 4.62 mm) |
| 1/4"-28 | #3 (0.2130" / 5.41 mm) |
| 5/16"-24 | I (0.2720" / 6.91 mm) |
| 3/8"-24 | Q (0.3320" / 8.43 mm) |
| 7/16"-20 | 25/64" (0.3906" / 9.92 mm) |
| 1/2"-20 | 29/64" (0.4531" / 11.51 mm) |
| 9/16"-18 | 33/64" (0.5156" / 13.10 mm) |
| 5/8"-18 | 37/64" (0.5781" / 14.68 mm) |
| 3/4"-16 | 11/16" (0.6875" / 17.46 mm) |
| 7/8"-14 | 13/16" (0.8125" / 20.64 mm) |
| 1"-12 | 59/64" (0.9219" / 23.42 mm) |
British and pipe threads
British Standard Whitworth
| BSW (TPI) | Tap drill mm |
|---|---|
| 1/8" - 40 | 2.55 |
| 3/16" - 24 | 3.7 |
| 1/4" - 20 | 5.1 |
| 5/16" - 18 | 6.5 |
| 3/8" - 16 | 7.9 |
| 7/16" - 14 | 9.3 |
| 1/2" - 12 | 10.5 |
| 5/8" - 11 | 13.5 |
| 3/4" - 10 | 16.25 |
| 7/8" - 9 | 19.25 |
| 1" - 8 | 22 |
British Standard Fine
| BSF (TPI) | Tap drill mm |
|---|---|
| 1/4" - 26 | 5.3 |
| 5/16" - 22 | 6.8 |
| 3/8" - 20 | 8.3 |
| 7/16" - 18 | 9.7 |
| 1/2" - 16 | 11.1 |
| 9/16" - 16 | 12.7 |
| 5/8" - 14 | 14 |
| 3/4" - 12 | 16.75 |
| 7/8" - 11 | 19.75 |
| 1" - 10 | 22.75 |
BSP parallel (G, ISO 228)
| BSP parallel | TPI | Thread OD mm | Tap drill mm |
|---|---|---|---|
| G 1/8" | 28 | 9.728 | 8.8 |
| G 1/4" | 19 | 13.157 | 11.8 |
| G 3/8" | 19 | 16.662 | 15.25 |
| G 1/2" | 14 | 20.955 | 19 |
| G 3/4" | 14 | 26.441 | 24.5 |
| G 1" | 11 | 33.249 | 30.75 |
| G 1-1/4" | 11 | 41.91 | 39.5 |
| G 1-1/2" | 11 | 47.803 | 45.25 |
| G 2" | 11 | 59.614 | 57 |
NPT taper pipe
| NPT | TPI | Tap drill (no reamer) |
|---|---|---|
| 1/8" NPT | 27 | R (0.3390" / 8.61 mm) |
| 1/4" NPT | 18 | 7/16" (0.4375" / 11.11 mm) |
| 3/8" NPT | 18 | 37/64" (0.5781" / 14.68 mm) |
| 1/2" NPT | 14 | 23/32" (0.7188" / 18.26 mm) |
| 3/4" NPT | 14 | 59/64" (0.9219" / 23.42 mm) |
| 1" NPT | 11.5 | 1-5/32" (1.1562" / 29.37 mm) |
| 1-1/4" NPT | 11.5 | 1-1/2" (1.5000" / 38.10 mm) |
| 1-1/2" NPT | 11.5 | 1-23/32" (1.7188" / 43.66 mm) |
| 2" NPT | 11.5 | 2-3/16" (2.1875" / 55.56 mm) |
BSPT (taper, R/Rc) uses the same sizes and TPI as BSP and is normally tapped with the BSP drill size, or one size smaller if a taper reamer isn't used. Pipe thread sizes refer to the nominal pipe bore, not the thread diameter.