Drill Bit Size Chart: mm to Inch, Fractional, Number and Letter Sizes

Drill sizing never agreed on one language. Metric countries size in millimetres; American drawings arrive in fractions of an inch; older US tap charts call for number and letter gauge drills; and a workshop in Mumbai or Karachi will meet all four in the same week — a metric machine, an imperial pipe thread, a drawing that says "drill #7, tap 1/4-20". The conversions are not difficult. They are just never on the wall when the job is in the vice. So: the wall chart.

Fractional inch → metric

The third column is the honest one. A hand-held drill wanders more than the 0.05 mm difference between 4.76 and 4.8 — for anything except a reamed or toleranced fit, the nearest stock metric bit within about 0.1 mm is the conversion. Where the substitution direction matters (a clearance hole can be over, never under; an interference fit the reverse), the table errs the way the job usually needs.

Metric → decimal inch

Number and letter gauges: what they are and when you meet them

The number gauge runs from #80 (0.34 mm) up to #1 (5.79 mm), then hands over to letters A (5.94 mm) through Z (10.49 mm) — a legacy of American wire-gauge sizing that survives because US tap charts are written in it. You will meet these sizes almost exclusively as tap drills on imperial threads:

For metric threads, tap drills follow the diameter-minus-pitch rule — that full chart lives in the metric tap drill guide.

Clearance holes for metric bolts

Measuring a bit that lost its markings

1. 01Measure with calipers across the margins — the two raised lands that spiral up the flutes — a little behind the tip. That is the bit's true cutting diameter.
2. 02Do not measure at the very tip (the point grind narrows it) or high on a well-used shank (wear and chuck-chew lie in both directions).
3. 03A bit that measures 0.1 mm under its nominal size is not mislabelled — it is worn, and it will drill undersize and rub. Worn-under bits are the hidden cause of a lot of "blunt" complaints; the failure signatures are in why drill bits break.

One more sizing trap: chucks stop at 13 mm, drill sizes do not. Bits above 13 mm for hand drills come with a reduced shank — a 16 or 20 mm cutting diameter turned down to a 13 mm shank. The stamped size is the cutting end; the shank tells you nothing.

From our range

Industrial HSS 1/2" Reduced Shank Twist Drill Bit (Bright Finish) – OEM

Reduced-shank HSS bits — full cutting diameter, 13 mm chuck fit.

What a rational set looks like

Metric sets earn their keep by covering the tap and clearance tables above without gaps: 1–10 mm in 0.5 steps (19 pieces) handles fastener work through M10 and nearly every pilot; 1–13 mm (25 pieces) adds the clearance sizes; and the half-millimetre steps people are tempted to skip are precisely the tap-drill sizes — 4.2, 6.8, 8.5, 10.2 — that a set without them keeps failing to provide. For counters serving mixed metric/imperial demand, the honest stock answer is a metric set plus the handful of true imperial sizes with no metric twin: 6.35, 7.94, 9.53, 12.7.

From our range

Industrial 19-Piece HSS M2 Twist Drill Bit Set (Bright Finish, 1-10mm)

19-piece 1–10 mm set in 0.5 steps — the tap-drill sizes included, which is the point of it.

From our range

Industrial 25-Piece HSS Twist Drill Bit Set (1.0-13.0mm, Index Case) – OEM

25-piece 1–13 mm index — the full clearance range in one case.

Going deeper on one system

This page is the map. Where a single system needs its own detail, it has its own page: the complete #1–#80 and A–Z gauge tables with the metric substitute for each, the full mm-to-inch conversion chart covering every 64th and every tenth of a millimetre, and the metric clearance hole chart from M1.6 to M24 with counterbore sizes. Tap drills split by thread standard: metric and UNC/UNF.

Both of these live on their own pages as well — the drill size converter and the tap drill calculator.