Complete drill bit size chart with fractional, metric, letter and number gauge sizes.
A drill bit size reference covers metric (mm), imperial (inch), and numbered/lettered drill bit sizes used in metalworking, woodworking, and tap drill selection. The right drill bit produces a hole at the exact specification — important because over-drilling weakens fits and under-drilling makes the next operation (tapping, reaming, screwing) impossible.
The most common practical question is "what drill bit do I use to tap an M6 thread?" Tap drill diameter = nominal thread diameter − thread pitch. For M6 × 1.0 standard thread: 6 − 1 = 5 mm tap drill. The chart in this reference lists tap drill diameters for every common metric and imperial thread.
Inputs: Drilling a tap drill for an M8 × 1.25 standard thread
Fractional sizes go in 1/64 inch increments from 1/16 to 1+ inch — standard for general use. Numbered sizes (#1 to #80) fill the gaps between small fractional sizes for precision work. Lettered sizes (A to Z) cover the range between #1 and 1/2 inch. Tap drill charts list specific numbered/lettered drills because no fractional size matches exactly.
For thin sheet metal (under 4 mm), step drills (also called unibits) are vastly superior — they produce burr-free, round holes in a single pass. Twist drills tend to grab and produce triangular holes in thin material. For thick material (above 6 mm), twist drills with a pilot hole work better. Always clamp sheet metal firmly before drilling — never hand-held.
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