Metric tap drill size chart
Metric coarse tap drills follow one rule: drill diameter equals nominal diameter minus pitch. That gives roughly 75 percent thread engagement, which is the right target for almost all work. Going deeper than 75 percent adds very little strength and makes the tap far more likely to snap.
The clearance column is the hole for the bolt to pass through, not the hole to be tapped.
| Thread | Pitch (mm) | Tap drill (mm) | Clearance (mm) |
|---|---|---|---|
| M3 | 0.50 | 2.50 | 3.2 |
| M4 | 0.70 | 3.30 | 4.3 |
| M5 | 0.80 | 4.20 | 5.3 |
| M6 | 1.00 | 5.00 | 6.4 |
| M8 | 1.25 | 6.80 | 8.4 |
| M10 | 1.50 | 8.50 | 10.5 |
| M12 | 1.75 | 10.20 | 13.0 |
| M14 | 2.00 | 12.00 | 15.0 |
| M16 | 2.00 | 14.00 | 17.0 |
| M20 | 2.50 | 17.50 | 21.0 |
| M24 | 3.00 | 21.00 | 25.0 |
Drill size for 1/4-20 and other unified taps
Unified tap drills land on number, letter and fractional drills rather than a round figure, so the chart is the only reliable route. A 1/4-20 tap takes a number 7 drill at 0.201 inch. The 13/64 fractional at 0.2031 is close and often substituted, which cuts engagement to about 70 percent.
| Thread | Tap drill | Decimal (in) | Clearance drill |
|---|---|---|---|
| 6-32 UNC | #36 | 0.1065 | 9/64 |
| 8-32 UNC | #29 | 0.1360 | 11/64 |
| 10-24 UNC | #25 | 0.1495 | 3/16 |
| 10-32 UNF | #21 | 0.1590 | 3/16 |
| 1/4-20 UNC | #7 | 0.2010 | 17/64 |
| 1/4-28 UNF | #3 | 0.2130 | 17/64 |
| 5/16-18 UNC | F | 0.2570 | 21/64 |
| 5/16-24 UNF | I | 0.2720 | 21/64 |
| 3/8-16 UNC | 5/16 | 0.3125 | 25/64 |
| 3/8-24 UNF | Q | 0.3320 | 25/64 |
| 1/2-13 UNC | 27/64 | 0.4219 | 33/64 |
| 1/2-20 UNF | 29/64 | 0.4531 | 33/64 |
NPT pipe tap drill sizes
Pipe taps are tapered, so the drill is sized for the small end of the taper and the tap cuts progressively as it goes in. Depth of entry sets the thread, not the drill.
| NPT size | Threads per inch | Tap drill | Decimal (in) |
|---|---|---|---|
| 1/8-27 | 27 | R | 0.3390 |
| 1/4-18 | 18 | 7/16 | 0.4375 |
| 3/8-18 | 18 | 37/64 | 0.5781 |
| 1/2-14 | 14 | 23/32 | 0.7188 |
| 3/4-14 | 14 | 59/64 | 0.9219 |
| 1-11.5 | 11.5 | 1 5/32 | 1.1563 |
Drill bit conversion chart
Number drills run from 1 (0.228 inch) down to 80 (0.0135 inch), getting smaller as the number rises. Letter drills run A (0.234 inch) to Z (0.413 inch), getting larger. Both overlap the fractional sizes, which is why a full index has all three.
| Drill | Decimal (in) | mm | Nearest fraction |
|---|---|---|---|
| #36 | 0.1065 | 2.71 | 7/64 |
| #29 | 0.1360 | 3.45 | 9/64 |
| #25 | 0.1495 | 3.80 | 5/32 |
| #21 | 0.1590 | 4.04 | 5/32 |
| #7 | 0.2010 | 5.11 | 13/64 |
| #3 | 0.2130 | 5.41 | 7/32 |
| F | 0.2570 | 6.53 | 1/4 |
| I | 0.2720 | 6.91 | 17/64 |
| Q | 0.3320 | 8.43 | 21/64 |
| R | 0.3390 | 8.61 | 11/32 |
