Drill size chart

Fractional drill sizes

Number drill sizes (#1 to #40)

Letter drill sizes (A to Z)

Number and letter sizes are the standard published decimal equivalents; fractional sizes are exact (the fraction times one inch). Millimetre values are the decimal inch times 25.4.

The tap drill column is computed, not looked up: for every drill we take each UNC and UNF thread in the common range and work out the engagement that drill would leave, keeping any between 60% and 85% and showing the two closest to the usual 75%. Of the 98 sizes here, 63 are the tap drill for at least one standard thread. Below about 60% the thread is weak; above about 85% the tap is fighting the hole and likely to break. A dash means no common Unified thread lands in that band for that drill.

The same drill under two names

The three series were not designed together, so they overlap, and in a few places two sizes are the same drill with two labels. 13 cross-series pairs land within 2.0 thousandths of each other, which is inside the runout of most drill chucks. When the index is empty where you are reaching, this is the column that says what else will do.

1/4 in and letter E are not close, they are the same size: both are exactly 0.2500 in, so a drawing calling out an E drill and one calling out a quarter inch specify the same hole. Treat the rest as substitutes for clearance and tapping work, not for a hole that will be gauged.

What to do when the set is metric

A metric index covers this chart almost completely. 96 of the 98 inch sizes have a metric drill within 0.05 mm, which is two thousandths of an inch. A shop with a metric set in tenths of a millimetre can hit nearly any number, letter or fractional call-out on this page without owning a single inch drill. The exceptions are the ones where the substitution costs you something:

An oversize metric drill takes thread engagement away and an undersize one raises tapping torque. Check the tap drill column above before substituting on a hole that gets threaded.

Why there are three series, and where they stop

The series exist to fill each other's gaps, and the arithmetic shows exactly where. Number drills run 0.0980 to 0.2280 in, letter drills carry on to 0.4130 in at letter Z, and across that span the combined set steps by a median of 3.5 thousandths. There is almost always a drill within half a thousandth of what you want.

Above Z the other two series are finished and only fractional sizes remain, stepping 15.6 thousandths at a time. That is the most useful fact on this page and no chart states it: below 0.413 in you pick a drill, above it you pick the nearest fraction and open the hole if you need more. It is also why tap drill call-outs for small threads are number and letter sizes while large ones are always fractions.