Drill size calculator
Three imperial drill series overlap rather than nest: fractional in 64ths of an inch, number gauges running 80 down to 1, and letters A to Z. This converts any of them to millimetres, inches and thou, and names the nearest metric drill so you know whether the bit you own will do.
How to use the drill size calculator
The three series exist because each solved a different problem and none was ever retired. Fractional drills step in 64ths of an inch, 0.397 mm at a time, which is fine for general holes and useless for anything small. Number drills descend from a wire gauge, so they run backwards: #80 is 0.343 mm, #1 is 5.79 mm, and at the fine end adjacent sizes are under 0.05 mm apart. Letters pick up where the numbers stop, A at 5.94 mm through Z at 10.49 mm, covering the range where fractional steps are still too coarse to be useful.
Laid end to end they give a finer progression than metric drills do, which is why machinists in metric shops keep an imperial index anyway. The coverage is uneven, though: around 5 mm you have #9, #8, #7 and A within about a millimetre of one another, while between 1/2 inch and Z there is nothing at all.
The nearest-metric figure is where this earns its place, and the answer depends entirely on what the hole is for. For a clearance hole a tenth of a millimetre is irrelevant and the substitution is free. For a tap drill it is not: a #7 at 5.11 mm and a 5.0 mm drill sit around ten points of thread engagement apart on an M6, which is the difference between a tap that cuts and a tap that shears off in the work.
What people use it for
- Finding which metric bit substitutes for a #7 in an American plan
- Reading an old workshop drawing that specifies letter drills
- Buying the single bit missing from a metric index
- Checking whether a fractional bit will clear a bolt shank
- Converting a thou dimension off a lathe setup sheet
Questions
They descend from a wire gauge, where the number counted drawing passes. A #80 is 0.343 mm and a #1 is 5.79 mm.
Letter drills. A is 5.94 mm, a shade over #1, and Z is 10.49 mm, a shade under 7/16 inch.
A #9 at 4.98 mm. A #8 is 5.06 and a #7 is 5.11, so four imperial sizes fall within about a millimetre of the metric one.
For a clearance hole, usually. For a tapped hole, no — a tenth of a millimetre moves thread engagement by several points, and that is what breaks taps.
A thousandth of an inch, 0.0254 mm. Still the working unit across a lot of machining.
It gives a 0.397 mm step: fine enough for general work and coarse enough to keep a set to a manageable number of bits.