Drill Bit Size Gauge in Metric and Inches
A free tool for naming a twist drill whose etched size has rubbed off. Bits are sold in four overlapping systems — fractional inches (1/16", 1/4"), metric millimeters, the numbered wire gauge (#1 down to #80) and the letter series (A to Z) — and they interleave so closely that a #29 sits between two fractions. Slide the cutting tip into a calibrated slot or against a printed scale and the tool reports the diameter in every system at once.
Identifying a drill bit step by step
Wipe swarf and old tape off the flutes so the lands sit flat against the scale or fully into the slot.
Lower the cutting tip through the holes; the smallest one it passes without forcing is its diameter.
No slots? Rest the tip on the calibrated scale and read the widest point across the two cutting lands.
The tool shows the fraction, the millimeter value, and the number or letter call-out, plus any tap it commonly drills.
Decode a worn bit across all four drill-size systems
Drill size for a tap
For a coarse metric thread the hole is drilled smaller than the nominal size by roughly the pitch. Take the row and the tap will cut without stripping the crests.
Fine pitch and imperial threads use different numbers. For a wood screw drill the shank diameter without the threads; for a wall plug, the plug diameter.
