Tap & Drill Size Chart
Look up the correct tap drill and clearance drill for any thread — metric (M) and imperial (UNC/UNF), with a to-scale diameter visual and the 75% thread rule.
Tap drill
5.0 mm
Screw major Ø
6 mm
Clearance drill
6.6 mm
M6 × 1.0
pitch 1 mm · tap drill 5.0 mm · clearance 6.6 mm
| Thread | Pitch | Tap drill | Clearance drill |
|---|---|---|---|
| M1.6 × 0.35 | 0.35 mm | 1.25 mm | 1.8 mm |
| M2 × 0.4 | 0.4 mm | 1.6 mm | 2.4 mm |
| M2.5 × 0.45 | 0.45 mm | 2.05 mm | 2.9 mm |
| M3 × 0.5 | 0.5 mm | 2.5 mm | 3.4 mm |
| M4 × 0.7 | 0.7 mm | 3.3 mm | 4.5 mm |
| M5 × 0.8 | 0.8 mm | 4.2 mm | 5.5 mm |
| M6 × 1.0 | 1 mm | 5.0 mm | 6.6 mm |
| M8 × 1.25 | 1.25 mm | 6.8 mm | 9 mm |
| M10 × 1.5 | 1.5 mm | 8.5 mm | 11 mm |
| M12 × 1.75 | 1.75 mm | 10.2 mm | 13.5 mm |
| M14 × 2.0 | 2 mm | 12.0 mm | 15.5 mm |
| M16 × 2.0 | 2 mm | 14.0 mm | 17.5 mm |
| M20 × 2.5 | 2.5 mm | 17.5 mm | 22 mm |
Tap drill vs. clearance drill
A tap drill makes the hole you cut threads into — it must be slightly smaller than the screw's major diameter so the tap has material to form the thread. A clearance drill makes a hole the screw passes straight through, so it is slightly larger than the major diameter.
Drill the tap hole too small and the tap binds or snaps; too large and the threads are shallow and strip easily. The values in this chart target the standard ~75% thread engagement used for general fastening.
The metric rule of thumb
For ISO metric threads the close approximation is tap drill = major diameter − pitch. An M6×1.0 needs a 5.0 mm bit; an M8×1.25 needs 6.8 mm. For imperial (UNC/UNF) the equivalent is major − (1 / TPI), then rounded to the nearest available number, letter, or fractional drill — which is why the chart lists named drills.
Thread engagement & tapping torque
~75% engagement
The chart default. Only marginally weaker than 100% but far easier to tap — the extra strength of a full thread is rarely worth the much higher torque and broken-tap risk.50–60% engagement
A slightly larger tap drill. Common in tough or hard materials and deep blind holes where reducing tapping torque matters more than the small loss of holding strength.Practical tapping notes
- Soft metals & plastics: stay close to the chart, or go a hair smaller — they form rather than cut and hold threads well.
- Hard steel & stainless: consider the next larger drill for lower torque, and use cutting fluid.
- Blind holes: the tap drill must go deeper than the thread depth to leave chip and lead-in clearance for a plug or bottoming tap.
- Always test first on scrap of the same material before committing to a real part.