Drill Bit Size Converter
Convert drill bit sizes between fractional inches, decimal inches, mm, letter (A-Z), and number gauges (1-80).
Full cross-reference chart for metalworking.
Type a diameter or select a named size — all fields update instantly.
Drill bits come in four different sizing systems, which is exactly why a chart like this helps when a project or a tap-drill table calls for a size you don’t have in the system you’re used to.
Metric: diameter in millimeters (3.0 mm, 6.5 mm).
Fractional inch: common fractions (1/16", 1/4", 1/2"), with 1 inch = 25.4 mm exactly.
Number sizes (#1 to #80):
- #1 = 5.791 mm (0.228")
- #80 = 0.343 mm (0.0135")
Letter sizes (A to Z):
- A = 5.944 mm (0.234")
- Z = 10.490 mm (0.413")
The number and letter systems exist to fill the gaps between fractional-inch sizes, giving machinists fine steps for precision drilling and thread tapping. They’re mostly a North American thing; the rest of the world tends to work straight in millimeters.
Here’s the part that catches newcomers: the number sizes run backward. A higher number means a smaller bit, so #80 is a tiny 0.34 mm while #1 is nearly 6 mm. Letter sizes run the normal way, A smallest to Z largest, and they pick up right where the number sizes leave off. Keep a chart nearby, because guessing between systems is how you end up with a hole a hair too big or too small for the tap.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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