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.
How to use the drill size calculator
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.
Questions
A historical wire-gauge convention. A #80 is 0.34 mm and a #1 is 5.79 mm.
Letter drills. A is just above #1, and Z is a little under 3/8 inch.
For a clearance hole, usually. For a tapped hole, no: a fraction of a millimetre changes the thread engagement significantly.
A thousandth of an inch, 0.0254 mm. Still the working unit in a lot of machining.
It gives a 0.4 mm step, fine enough for general work and coarse enough to keep the set to a manageable size.