Getting your tap drill size right is one of the most critical steps in tapping. Too small and you risk breaking the tap; too large and you end up with an under-strength thread. This guide gives you the formulas, a ready-reference table, and practical tips for both metric and imperial tapping.
Why Tap Drill Size Matters
The tap drill creates the minor diameter of the thread. The goal is to leave enough material for the tap to cut full thread form while keeping torque manageable. Industry standard is to aim for approximately 75% thread engagement in steel — this achieves roughly 98% of full thread strength while keeping drill and tap loads reasonable.
Metric Tap Drill Formula
For metric threads (ISO standard):
Tap Drill Diameter = Nominal Diameter − Pitch
Example: M10 x 1.5 thread → 10 − 1.5 = 8.5 mm drill
Metric Tap Drill Reference Table
| Thread | Pitch (mm) | Tap Drill (mm) | Close Fit |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 2.5 |
| M4 | 0.7 | 3.3 | 3.3 |
| M5 | 0.8 | 4.2 | 4.2 |
| M6 | 1.0 | 5.0 | 5.0 |
| M8 | 1.25 | 6.75 | 6.8 |
| M10 | 1.5 | 8.5 | 8.5 |
| M12 | 1.75 | 10.25 | 10.2 |
| M16 | 2.0 | 14.0 | 14.0 |
| M20 | 2.5 | 17.5 | 17.5 |
| M24 | 3.0 | 21.0 | 21.0 |
Imperial (UNC / UNF) Tap Drill Reference
| Thread | TPI | Tap Drill | Decimal (in) |
|---|---|---|---|
| 1/4"-20 UNC | 20 | #7 | 0.201" |
| 1/4"-28 UNF | 28 | #3 | 0.213" |
| 5/16"-18 UNC | 18 | F | 0.257" |
| 3/8"-16 UNC | 16 | 5/16" | 0.313" |
| 1/2"-13 UNC | 13 | 27/64" | 0.422" |
| 1/2"-20 UNF | 20 | 29/64" | 0.453" |
| 5/8"-11 UNC | 11 | 17/32" | 0.531" |
| 3/4"-10 UNC | 10 | 21/32" | 0.656" |
Adjusting for Material
The 75% thread engagement rule is a starting point. Adjust based on material:
- Soft materials (aluminium, plastic, brass) — use a slightly smaller drill (higher engagement) because the material is weaker and you want more thread contact
- Hard/tough materials (stainless, tool steel, titanium) — consider 60–65% engagement (slightly larger drill) to reduce tap torque and breakage risk
- Through-holes vs blind holes — for blind holes, ensure you drill deep enough past the required thread depth to accommodate the tap’s chamfered lead (add 3–5 thread pitches of extra depth)
Tapping Tips
- Always use cutting fluid — even on aluminium. It reduces friction, improves finish, and extends tap life.
- Back off half a turn every 1–2 turns to break chips (especially in blind holes).
- Use a tapping stand or guide for hand tapping to keep the tap square to the workpiece.
- For CNC tapping, use a synchronised rigid tapping cycle and verify spindle/feed ratio before cutting.
Shop Taps and Drilling Tools at Tooltec
Find the right tap for your application in our tapping range, and pair it with a quality drill from our drilling collection. We stock metric and imperial taps including hand taps, spiral flute, and spiral point styles.