Drilling Guide
Tap Drill Size Chart Explained
The hole you drill before tapping a thread — how the size is worked out, and the full metric coarse chart from M3 to M24.
Cutting an internal thread starts with a hole that's smaller than the thread itself — the tap then cuts the thread form into the walls of that hole as it's driven through. Get the tap drill size wrong and you either strip the thread (hole too big) or snap the tap (hole too small).
Full interactive tap drill table
See tap drill and clearance sizes for M3 to M24 alongside the rest of your drilling calculations.
The rule of thumb
This targets roughly 75% thread engagement — enough for a strong thread in most applications without excessive tapping torque. Full 100% engagement gains very little strength but greatly increases the force needed to cut the thread and the chance of snapping a tap, especially in blind holes.
Metric coarse tap drill chart
| Thread | Pitch (mm) | Tap drill | Clearance drill |
|---|---|---|---|
| M3 | 0.50 | 2.5 mm | 3.4 mm |
| M4 | 0.70 | 3.3 mm | 4.5 mm |
| M5 | 0.80 | 4.2 mm | 5.5 mm |
| M6 | 1.00 | 5.0 mm | 6.6 mm |
| M8 | 1.25 | 6.8 mm | 9.0 mm |
| M10 | 1.50 | 8.5 mm | 11.0 mm |
| M12 | 1.75 | 10.2 mm | 13.5 mm |
| M14 | 2.00 | 12.0 mm | 15.5 mm |
| M16 | 2.00 | 14.0 mm | 17.5 mm |
| M18 | 2.50 | 15.5 mm | 20.0 mm |
| M20 | 2.50 | 17.5 mm | 22.0 mm |
| M24 | 3.00 | 21.0 mm | 26.0 mm |
Always confirm the exact figure against your tap's own spec sheet — different tap styles (plug, taper, bottoming) and thread tolerance classes can shift the ideal drill size slightly.
Tap drill vs clearance drill
These two are often confused but serve opposite purposes. A tap drill is smaller than the thread, so a tap can cut threads into the hole wall. A clearance drill is larger than the thread, so a bolt of that size passes straight through without engaging any thread — used for the non-threaded side of a bolted joint.
Frequently asked questions
What is a tap drill size?
The hole diameter you drill before cutting an internal thread with a tap. For metric coarse threads it's approximately the thread diameter minus the pitch — an M6×1.0 thread uses a 5.0mm tap drill.
What is the tap drill size for M6?
For M6 × 1.0 metric coarse thread, the tap drill size is 5.0mm, giving roughly 75% thread engagement.
What's the difference between a tap drill and a clearance drill?
A tap drill is slightly smaller than the thread, so a tap can cut threads into it. A clearance drill is slightly larger, so a bolt passes straight through without threading.