Number and Letter Drill Bit Sizes: The Full #1–#80 and A–Z Chart in mm

The drawing says drill #11. The rack on the wall runs 1.0 to 13.0 mm in half-millimetre steps, and nothing on it says #11. This happens every week in workshops from Chennai to Sharjah, because number and letter gauges are still the working language of American drawings and imperial tap charts, while the bits on the shelf are metric. The answer for that specific case is 4.85 mm — order a 4.9 mm bit. The rest of the chart is below.

This converter has its own page too — the full drill size converter — if you want to bookmark it or send it to someone.

The gauge sizes people look up most

Nine sizes account for most of the number and letter drills anyone actually needs — seven of them are tap drills for standard UNC and UNF threads, and the other two are rivet and clearance work. If you memorise nothing else, memorise these:

Full imperial thread coverage — including the fractional tap drills above 3/8" — is in the UNC and UNF tap drill chart. Metric threads follow a different rule entirely; that one is in the metric tap drill guide.

Number drill sizes #1 to #40

Number drill sizes #41 to #80

Below #41 the sizes get small fast — #80 is 0.34 mm, thinner than a staple wire. These are micro-drill territory: PCB work, model engineering, instrument repair, jewellery. They break if you look at them wrong, and they are bought in tens, not ones.

From our range

Industrial 10-Piece Micro HSS Twist Drill Bit Set (0.8-3.0mm) – OEM

The 0.8–3.0 mm micro set covers the useful end of the #41–#70 range in metric steps.

Letter drill sizes A to Z

The number series stops at #1 (5.79 mm) and the letters pick up immediately above it — A is 5.94 mm, Z is 10.49 mm. Twenty-six sizes covering 4.5 mm of range, which is why letter drills feel absurdly fine-grained until you need one for a thread that will not tap cleanly on anything else.

Why the numbers run backwards, and why the steps are uneven

Both oddities come from the same place: the series was never designed as a drill chart. It is the Stubs Steel Wire Gauge, a 19th-century wire-drawing scale, where a higher number meant more passes through the die and therefore thinner wire. Drill makers adopted the wire sizes because that is what the stock was — so #1 is the fattest and #80 the finest, exactly backwards from any measurement you would design on purpose.

The uneven steps are the same story. Between #30 and #31 the jump is 0.21 mm; between #58 and #59 it is 0.03 mm. Nobody chose those intervals for engineering reasons — they are the historical wire sizes, frozen. This matters practically: you cannot interpolate. There is no rule that gets you from a gauge number to a diameter, which is why a chart is the only way and why every workshop that touches imperial drawings keeps one taped inside a cupboard door.

Swapping a metric bit for a gauge drill

The last column of every table above is the practical one. But "nearest" is not always "correct" — the direction of the error decides whether the substitution is safe:

• Tap drills: never go under. An undersized tap drill leaves too much material and the tap snaps in the hole. Rounding up by 0.05–0.1 mm costs a few percent of thread engagement and nothing else. Going 0.1 mm under can double the torque.
• Clearance holes: never go over by much. The bolt has to pass, but a sloppy hole loses the bearing area under the head. Up to about +0.2 mm is fine; beyond that use a washer.
• Reamed or dowel holes: do not substitute at all. If the drawing tolerances the hole, the gauge size is the size. Order it.
• Anything hand-held: stop worrying. A hand drill wanders more than the 0.03 mm between #58 and #59. For bracket holes and sheet work, the nearest 0.1 mm metric bit is the conversion.

From our range

Industrial 99-Piece Titanium HSS Drill Bit Master Set (Metal Case) – OEM

A 99-piece titanium HSS master set gives fine enough steps to stand in for most gauge drills without holding three separate index systems.

Which gauge sizes are worth stocking

If you sell into markets that run imperial drawings — the Gulf oilfield trade, Indian workshops doing US export work, Philippine and Vietnamese machine shops with American machinery — a full 115-piece index is dead capital. Six sizes move; the rest sit. Stock #7, #21, #25, #29, #36 and letter F as loose singles, and let the metric rack cover everything else. Those six are the tap drills for 1/4-20, 10-32, 10-24, 8-32, 6-32 and 5/16-18, which between them are most of the threaded holes in imperial-spec fabrication.

For the counter, the sales line is simple: a customer holding an American drawing does not want a lecture about gauge systems, they want the hole to be right the first time. Sell the six singles alongside a metric HSS index and they never come back annoyed. That mixed stocking is exactly the kind of thing we consolidate into one container for distributors — gauge singles, metric sets and taps on one packing list.