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.
Choosing the hole for the job
The chart gives the standard drill for a roughly 75% thread, which is the right answer in the great majority of cases. It is worth knowing when it is not. In a soft material such as aluminium or brass you can go slightly smaller for a fuller, stronger thread, because the tap cuts easily. In tough stainless or a hard alloy you go the other way: a slightly larger drill takes load off the tap, and a tap snapped off inside a nearly finished part is a far more expensive mistake than a thread at 65% instead of 75%.
Blind holes need extra depth
A tap cannot cut a full thread right down to the bottom of a hole: the lead-in at its tip has to go somewhere, and the chips have nowhere to escape. Drill a blind hole deeper than the thread you need — a few millimetres past the last usable thread is a sensible minimum — and clear the chips as you go rather than at the end. Where the thread must run close to the bottom, start with a taper tap and finish with a bottoming tap.
Depth is not where the strength is
The instinct is to tap as deep as possible. In practice a thread engaged over about one and a half times the bolt diameter in steel is already stronger than the bolt itself: beyond that the bolt breaks before the thread strips, so extra depth buys nothing. In aluminium, allow roughly two diameters for the same reason. Getting the drill size and the alignment right is worth far more than depth — a thread started crooked is weak however deep it goes.
