Drill Bit Size Converter

Convert a drill bit size between fractional inches, letter sizes and metric — and find the nearest bit in each system.

How to use the Drill Bit Size Converter

Choose the system of the bit you have.
Enter its size (e.g. 1/4, F or 6.5).
Read the equivalent and nearest sizes.

How it works

US drill bits come in fractional sizes (1/64″ steps), letter sizes A–Z (0.234″ to 0.413″) and number sizes (#80 to #1, for small holes); metric bits are in 0.1–0.5 mm steps. Tap drill charts often specify letter or number bits because they fall between fractional sizes. Letter sizes here follow the standard ANSI table.

Fractional bits in mm

Fraction	Inches	mm
1/16″	0.0625	1.59
1/8″	0.125	3.18
3/16″	0.1875	4.76
1/4″	0.25	6.35
5/16″	0.3125	7.94
3/8″	0.375	9.53
7/16″	0.4375	11.11
1/2″	0.5	12.7

Frequently asked questions

What is a 1/4″ drill bit in mm?

6.35 mm.

What size is an F drill bit?

0.257″ (6.53 mm) — the tap drill for 5/16-18 threads.

What metric bit replaces 3/8″?

9.5 mm (3/8″ is 9.525 mm).

What is a letter drill?

A US drill size between 0.234″ (A) and 0.413″ (Z).