Short answer: for a standard metric coarse thread, the tap drill size is the thread diameter minus the pitch.
M8 × 1.25 → 8 − 1.25 = 6.8 mm
That gives roughly 75–80% thread engagement, which is what you want for almost every job.
The full charts are below — coarse threads, fine threads, and clearance holes — along with the practical bits that actually stop taps snapping. We've supplied drill bits to the trade since 1985, and we stock HSS in 0.1 mm increments, so every tap drill size on this page is a size we actually sell.
The sizes people look up most
| Tap | Pitch | Tap drill size |
|---|---|---|
| M3 | 0.50 | 2.5 mm |
| M4 | 0.70 | 3.3 mm |
| M5 | 0.80 | 4.2 mm |
| M6 | 1.00 | 5.0 mm |
| M8 | 1.25 | 6.8 mm |
| M10 | 1.50 | 8.5 mm |
| M12 | 1.75 | 10.2 mm |
Full metric coarse tap drill chart (M1.6 – M30)
| Tap size | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M1.6 | 0.35 | 1.25 |
| M2 | 0.40 | 1.6 |
| M2.5 | 0.45 | 2.05 |
| M3 | 0.50 | 2.5 |
| M3.5 | 0.60 | 2.9 |
| M4 | 0.70 | 3.3 |
| M5 | 0.80 | 4.2 |
| M6 | 1.00 | 5.0 |
| M7 | 1.00 | 6.0 |
| M8 | 1.25 | 6.8 |
| M10 | 1.50 | 8.5 |
| M12 | 1.75 | 10.2 |
| M14 | 2.00 | 12.0 |
| M16 | 2.00 | 14.0 |
| M18 | 2.50 | 15.5 |
| M20 | 2.50 | 17.5 |
| M22 | 2.50 | 19.5 |
| M24 | 3.00 | 21.0 |
| M27 | 3.00 | 24.0 |
| M30 | 3.50 | 26.5 |
Metric fine tap drill chart
Fine threads have a shallower pitch, so the tap drill is larger than the coarse equivalent. Fine threads hold better in thin material and resist vibration — common in automotive and hydraulics.
| Tap size | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M8 × 1.0 | 1.00 | 7.0 |
| M10 × 1.25 | 1.25 | 8.8 |
| M12 × 1.5 | 1.50 | 10.5 |
| M14 × 1.5 | 1.50 | 12.5 |
| M16 × 1.5 | 1.50 | 14.5 |
| M18 × 1.5 | 1.50 | 16.5 |
| M20 × 1.5 | 1.50 | 18.5 |
| M22 × 1.5 | 1.50 | 20.5 |
| M24 × 2.0 | 2.00 | 22.0 |
Clearance hole chart (ISO 273)
A tap drill is for a hole you're going to thread. A clearance hole is for a bolt to pass through. Mixing the two up is the single most common mistake we hear about on the trade counter.
| Bolt size | Close fit | Normal fit | Loose fit |
|---|---|---|---|
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M14 | 15.0 | 15.5 | 16.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M20 | 21.0 | 22.0 | 24.0 |
All sizes in mm. Normal fit (highlighted) is the right choice for most general work. Use close fit where alignment matters, loose fit on structural work where you need room to line things up.
Which drill bit should you actually use?
The size is only half the job — the bit matters just as much, because tapping puts far more stress on a hole than ordinary drilling. Match the bit to the material:
- Mild steel, aluminium, plastic, timber — standard HSS drill bits are all you need.
- Stainless steel, steel and cast iron — step up to M35 cobalt (5%). Ordinary HSS work-hardens the surface of stainless and then stops cutting — which is what's really happening when a bit seems to "go blunt instantly."
- Hardened and high-alloy steels, or heavy repeat work — use M42 cobalt (8%). The higher cobalt content gives far greater heat and wear resistance, so it keeps its edge where M35 would start to struggle. It's the bit to reach for on the jobs that eat drill bits.
- Lots of repeat holes in ordinary material — titanium-coated HSS lasts noticeably longer than plain HSS.
- Above 20 mm — use blacksmith or reduced-shank bits, so they still fit a standard chuck.
M35 or M42? Both are cobalt-alloyed HSS and both handle stainless. M35 (5% cobalt) is the sensible everyday choice for stainless and tougher steels. M42 (8% cobalt) runs hotter for longer before losing its edge — worth the extra on hardened steel, high-alloy work, or when you're drilling all day.
Every size on this chart, in stock
We hold HSS drill bits in 0.1 mm increments and over a million bits in stock — so every tap drill and clearance size listed here is one we can dispatch the same day.
Five things that stop taps snapping
Frequently asked questions
What drill bit do I need for an M8 tap?
A 6.8 mm drill bit, for a standard M8 × 1.25 coarse thread. For an M8 × 1.0 fine thread, use a 7.0 mm drill.
What size drill bit for an M6 tap?
5.0 mm, for the standard M6 × 1.0 coarse thread.
How do you calculate tap drill size?
Subtract the pitch from the thread diameter. M10 × 1.5 → 10 − 1.5 = 8.5 mm. This gives about 75–80% thread engagement — the practical sweet spot: near-full strength, without the excessive torque (and snapped taps) that come from chasing 100%.
What's the difference between a tap drill and a clearance hole?
A tap drill is smaller than the bolt and leaves material for the tap to cut a thread into. A clearance hole is larger than the bolt so it passes straight through. For M8: tap drill 6.8 mm, clearance hole 9.0 mm.
Can I use an HSS drill bit for tapping stainless steel?
You can, but cobalt is far better. Stainless work-hardens as you drill, and standard HSS loses its edge quickly against it. M35 cobalt (5%) is the sensible choice for stainless and tougher steels; M42 cobalt (8%) steps up again for hardened and high-alloy steel, or sustained production drilling, because it resists heat and wear for longer.
What's the difference between M35 and M42 cobalt drill bits?
Both are HSS alloyed with cobalt for heat resistance. M35 contains 5% cobalt and handles stainless steel, steel and cast iron well — it's the everyday step up from plain HSS. M42 contains 8% cobalt, giving greater heat and wear resistance again, which makes it the better bit for hardened steel, high-alloy work and heavy repeat drilling.
Do you stock every size in these charts?
Yes. We hold HSS drill bits in 0.1 mm increments and over a million bits in stock, so every tap drill and clearance size listed here is available, with same-day dispatch and free next-working-day UK delivery on orders over £50.
UK Drills has supplied drill bits to UK trade and industry since 1985, from our trade counter in Hindley, Wigan. Need a hand choosing? Email info@ukdrills.com — we're happy to advise.
