Tap drill size chart
Find the correct drill size to bore before you cut a thread — for metric (coarse and fine), UNC and UNF. Pick a series or search a size; every value is shown in both millimetres and inches.
How the tap drill size is worked out
Before you cut an internal thread you first drill a plain hole — the tap drill — and its size decides how much thread is actually formed. The values here are the standard ~75% thread figures every workshop uses: for metric threads that is the major diameter minus the pitch, rounded to the nearest real drill; for UNC and UNF it is the standard published drill for each size. A 75% thread is the sweet spot — nearly as strong as a full thread but far easier to tap without snapping the tap. Everything is computed in your browser; nothing is sent anywhere.
What is a "tap drill"?
It is the drill you use to make the hole before running a tap through it to cut the thread. Too small and the tap jams or breaks; too big and the thread is shallow and weak. This chart gives the size that balances the two.
Why 75% thread and not 100%?
A full 100% thread is only marginally stronger but dramatically harder to tap — it roughly triples the cutting force and is the classic way to snap a tap in a hole. At 75% you keep about the same practical strength with far less risk, which is why it is the shop standard.
My drill index doesn't have that exact size — now what?
Use the nearest drill you do have. A hair larger gives a slightly lower thread percentage (easier tapping, a touch weaker); a hair smaller gives more thread but a heavier cut. For most work either side of the listed size is fine.
Does this cover clearance (through) holes?
No — these are tap drills, for cutting a thread. A clearance hole, where a bolt passes straight through, is drilled slightly larger than the bolt's major diameter (typically 0.5–1 mm over for metric) and is not tapped.
Metric or imperial — which drill do I actually buy?
Whichever matches your taps and stock. The chart shows every tap drill in both millimetres and inches, so you can pick from a metric drill set or an imperial number/letter/fraction set — whatever is on your bench.
This calculator gives you a starting point — always check it against your machine's real limits and your tool manufacturer's data. Start with a light test pass, then gradually work your way up as you find the sweet spot: quiet running, no vibration, no chatter. Clamp your workpiece securely, and wear the right protective gear — safety glasses, hearing protection, gloves suited to the material, and clothing that won't catch in moving parts.
