Drill size calculator

Converts between the three imperial drill series — fractional, number gauge and letter — and millimetres, plus the nearest metric drill you probably actually own.

Three overlapping series exist for historical reasons and they interleave rather than nest. Fractional sizes run in 64ths of an inch and are the coarse series. Number drills run 80 down to 1, getting *larger* as the number falls, and cover the small sizes fractional cannot. Letter drills run A to Z, getting larger, and fill the gap between the largest number drill and the fractional sizes. Between them they give a much finer progression than metric drills do, which is why machinists keep all three even in metric shops. The nearest-metric figure is the practical one: it tells you whether the drill in your index is close enough for the job.