Drill Size Chart — Fraction, Number, Letter & Metric

What it is

North American drills come in three overlapping inch systems — wire-gauge number sizes, letter sizes and fractional sizes — plus metric sizes. Number and letter sizes fill the gaps between fractions, so a shop can drill an exact tap drill or clearance hole. This chart merges all three inch systems into one list sorted by diameter, with the millimeter equivalent of each.

How to identify

Number sizes run backward: #60 is the smallest on this chart and #1 the largest. Letter sizes start just above #1 (A) and end at Z. Fractions are shown in bold. Millimeter values are the inch size converted at 25.4 mm/in and rounded to 0.01 mm — they are conversions, not a list of metric drills. Metric drills are sold in their own millimeter steps; pick the metric drill whose size is closest to the value you need.

Chart

Key points

• Number sizes #61–#80 (micro drills) and fractional sizes above 1/2 in are not shown on this sheet.
• E and 1/4 in are the same nominal diameter; some neighbors (for example #28 and 9/64, K and 9/32) are within a few ten-thousandths of an inch and are often used interchangeably — confirm when the hole has a tight tolerance.
• Fractional decimals are rounded to four places; the exact value of a 64th is a repeating or five-place decimal.
• A drilled hole is normally slightly larger than the drill. Do not use this chart as a hole-size prediction.

Common mistakes

• Reading number sizes as if larger numbers meant larger drills.
• Ordering a "6.35 mm drill" when the customer needs a true 1/4 in, or the reverse, in a tight-tolerance application.
• Using a fractional drill as a tap drill when the tap drill chart calls for a number or letter size.

Governing standards: ANSI B94.11M (twist drill sizes and tolerances). Inch-to-mm conversion at exactly 25.4 mm per inch.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.