Tap and clearance drill chart
The drill for every machine screw thread a shop meets, the clearance hole for the piece the screw passes through, and what percentage of a full thread each one leaves.
| Screw | Major dia | Tap drill | Thread left | Clearance | Thread depth |
|---|---|---|---|---|---|
| #4-40 | 0.112" | #44 (0.0860") | 80% | 1/8" | 1/4" |
| #6-32 | 0.138" | #36 (0.1065") | 78% | 9/64" | 1/4" |
| #8-32 | 0.164" | #29 (0.1360") | 69% | 11/64" | 5/16" |
| #10-24 | 0.19" | #25 (0.1495") | 75% | 13/64" | 3/8" |
| #10-32 | 0.19" | #21 (0.1590") | 76% | 13/64" | 3/8" |
| 1/4-20 | 0.25" | #7 (0.2010") | 75% | 17/64" | 1/2" |
| 1/4-28 | 0.25" | #3 (0.2130") | 80% | 17/64" | 1/2" |
| 5/16-18 | 0.3125" | F (0.2570") | 77% | 21/64" | 5/8" |
| 3/8-16 | 0.375" | 5/16" (0.3125") | 77% | 25/64" | 3/4" |
| 1/2-13 | 0.5" | 27/64" (0.4219") | 78% | 33/64" | 1" |
Reading it
- The tap drill is the major diameter less one pitch, snapped to the nearest drill that exists. That reproduces the published chart at every size here but #4-40, where this lands on a #44 at about 80% thread and the chart prefers the gentler #43 — which is why the thread percentage is a column rather than left implied.
- Tap drills are number and letter sizes and clearance holes are fractions, and that is not an inconsistency: a tap drill has a thousandths-level target that the number series exists to hit, and a clearance hole only has to pass the screw.
- The tap drill goes in the piece that holds the thread and the clearance drill in the piece the screw passes through. Tapping both means the near threads fight the far ones and the joint never pulls tight.
- Thread depth here is two diameters, rather than the one and a half a machinist uses in steel — aluminium, hardwood and plywood all need more thread to reach the same holding. Drill deeper still to leave a plug tap its chip room.
For the case this table does not cover
- Tap and clearance drills
The drill for a machine screw or a threaded insert, and the clearance hole in the piece it passes through.
- Pilot hole calculator
The right drill bit for a wood screw, by gauge number and by softwood or hardwood.