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 millimetres, 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

Clean the tip

Wipe swarf and old tape off the flutes so the lands sit flat against the scale or fully into the slot.

Find the slot it just clears

Lower the cutting tip through the holes; the smallest one it passes without forcing is its diameter.

Or lay it across the rule

No slots? Rest the tip on the calibrated scale and read the widest point across the two cutting lands.

Read every system

The tool shows the fraction, the millimetre value, and the number or letter call-out, plus any tap it commonly drills.

Decode a worn bit across all four drill-size systems

What this gauge does

All four sizing systems

Reads fractional-inch, metric, number (wire) and letter drills together, so an odd-feeling bit lands as, say, a #21 rather than "between 5/32 and 3/16".

Slot-fit, not eyeballing

The tip drops into the smallest hole it clears, which pins the diameter far more reliably than laying a round bit on a flat rule.

Tap-drill context

Number and letter drills exist mainly as tapping sizes, so the tool flags when a diameter is the standard pilot for a common thread.

Closest-fraction fallback

For a metric bit with no exact inch twin it gives the nearest fraction and the gap, useful when only an imperial index is to hand.

Measures the cutting diameter

Reads across the lands at the tip, the true cutting size — not the narrower web or the shank, which can be turned down on some bits.

Why a four-system gauge beats a single ruler

A plain ruler tells you a bit is "about 4 millimetres" and stops there — useless when the real answer is a #21 tapping drill or a 5/32" fraction. Drill sizes interleave across four indices on purpose, so the only honest identification names the bit in each one. This gauge measures the cutting diameter once and resolves it to a fraction, a millimetre value and a number or letter, with the tap it commonly drills, so you can match a set or buy a single replacement with confidence.

Drill bit sizing questions

What are number and letter drill sizes?

They are two old gauge series that fill the gaps between fractions. The number drills run from #1 (about 5.8 mm) down to #80 (about 0.34 mm), and the letters run A (smallest, ~5.9 mm) through Z. Most exist so machinists can hit an exact tapping diameter.

Why are there so many sizes that are nearly identical?

Because three traditions overlap. A 5/32" bit, a 4 mm bit and a #22 bit are all within a few hundredths of a millimetre, yet each belongs to a different index. The tool measures once and tells you which name your bit answers to.

Should I measure the tip or the shank?

Always the cutting tip, across the two lands at their widest. Reduced-shank bits have a shank that is deliberately smaller than the hole they cut, so a shank reading would undersize them.

How do I pick a drill for tapping a thread?

A tap needs a hole near the thread's minor diameter — for example an M5 × 0.8 tap usually wants a 4.2 mm bit. When a measured diameter is a standard pilot, the tool names the thread it drills for so you can confirm the pairing.

My bit reads between two fractions — what is it really?

It is probably a number, letter or metric bit rather than a fraction. Check the number and letter columns first; if it still falls between everything, take it as the nearest metric size, since those are spaced most finely.

Can I tell a metric set from an imperial set this way?

Yes. Measure two or three bits: a set marked in round millimetres (3, 4, 5 mm) reads cleanly on the metric scale, while a fractional set lands on 1/16" steps. Mixed odd readings usually mean a number/letter index.

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Who is this tool for?

🔌 Electronics Enthusiasts

Published 18 May 2026 Authors: The Supreme