Drill Size Conversion Chart: Number, Letter, Inch and mm

Number and letter drill gauges do not follow a formula — the sequence is irregular, so the only way to convert one is to look it up. This chart lists all 80 number sizes and all 26 letter sizes in one run, ordered smallest to largest, with the decimal inch and the exact millimetre equivalent for each.

How the gauge system works

Number drills run from #80 at the smallest to #1 at the largest, so a higher number means a smaller hole — the opposite of what most people expect the first time. Letter drills pick up above #1 and run A to Z, getting larger as they go. Between them they fill the gaps that fractional drills leave, which is why tap drill charts almost always specify a number or letter rather than a fraction.

Millimetre values below are exact conversions at 25.4 mm to the inch, rounded to three decimal places. A metric drill is rarely an exact match for a gauge size, so where a job specifies a gauge drill, use the gauge drill.

Complete drill size chart

Method and precision

Gauge sizes are the ASME/ANSI standard number and letter series and are listed as published, not derived. Millimetre values are exact conversions at 25.4 mm to the inch, rounded to three decimal places for display. Sizes are discrete standards, so the lookup above reports the closest listed size rather than interpolating between two of them — an interpolated value would name a drill that does not exist.

Choosing between two close sizes

When two sizes sit a few thousandths apart, the smaller one is usually the safer choice. A hole that is fractionally undersize can be opened up; one that is oversize cannot be closed, and in a threaded hole it costs you thread engagement. For clearance holes the reverse applies — err large enough that the fastener drops through without being driven.

Drill bits and the drivers that turn them are in rotary cutting tools and bits and drivers.