Drill sizes

The other way round from the charts. Instead of starting with a thread and finding a drill, start with the drill in your hand and find out what it is good for. Each page gives that drill's size in inches and millimetres, every thread it taps between 50% and 85% of thread, and whether it is the published tap drill, hard-material drill or clearance drill for anything.

Every drill below taps at least one thread. 360 of them have a page of their own; the other 129 are listed with their sizes but have too little to say for a page.

Or use the calculator's reverse mode →

Number drills 63

Letter drills 25

Fractional drills 120, 76 with a page

Metric drills 281, 196 with a page

How the drill series fit together

Four overlapping series cover the range between them, which is why a full index has so many sizes in it.

Number drills, #80 up to #1, run from 0.0135 in to 0.228 in. The numbers run backwards: a bigger number is a smaller drill. Most inch tap drills live here.
Letter drills, A to Z, pick up at 0.234 in and run to 0.413 in. These run the sensible way round, with Z the largest.
Fractional drills step in 64ths from 1/64 in upward, and are what most people own.
Metric drills step in tenths and twentieths of a millimetre. Every metric tap drill is one of these.

The series overlap heavily, which is the point: between them you can usually get within a couple of thousandths of any theoretical hole size. The calculator will tell you which set gets closest for a given thread, and let you rule out the ones you do not own.

Drill sizes: common questions

What is a number drill?

A drill sized by a gauge number from #1 down to #80, where a higher number means a smaller drill. #1 is 0.228 in and #80 is 0.0135 in. Most inch tap drills are number drills.

What is a letter drill?

A drill sized by a letter from A to Z, filling the gap between the largest number drill and the fractional sizes. A is 0.234 in and Z is 0.413 in. Here a later letter means a larger drill, the opposite of the number series.

Why do some drills below have no link?

There are 539 drills in the data and 489 of them tap something, but a page only earns its place if it has enough to say. Number and letter drills get one whenever they tap a thread at all, because people look those up by name. Fractional and metric sizes need to tap at least two. The other 129 are listed here with their sizes and nothing more.

How do I find out what my drill is for?

Find it below, or use the reverse mode of the calculator. Either way you get every thread that drill taps between 50% and 85% of thread.