Drill Bit Size Chart
Decimal-inch and millimetre equivalents for number (#1–#80), letter (A–Z) and fractional drill sizes. Search a decimal you measured and find the closest standard bit.
| Series | Size | Decimal (in) | Millimetre (mm) |
|---|
Standard: ANSI B94.11M · Data checked: 2026-10-14
Frequently asked questions
What do the letters mean in a drill index?
Letter drills run A to Z and fill the gaps between the numbered sizes and the fractional sizes, so a full index buys you finer steps. F is 0.257 in, for example. They are used mainly for tapping and for clearance holes that must be close.
Why do number drills run backwards — is #1 the biggest?
Yes. Number drills run from 80 (0.0135 in) up to 1 (0.228 in), so a smaller number is a bigger drill. Letter drills are the opposite way round from what people expect too — they get bigger as the letter advances.
How do I pick a drill for a tap?
Use the tap drill chart rather than guessing. For metric coarse threads the rule of thumb is nominal diameter minus pitch (M8 x 1.25 gives 6.75 mm), which produces about a 75% thread. Going smaller makes a stronger thread but risks breaking the tap.
Why is my drilled hole always slightly oversize?
Drill bits cut oversize by a few thousandths, and the amount grows with runout, poor sharpening and hand-held drilling. If a hole has to be exact, drill undersize and ream it, or bore it.