Drill Bit Size Converter
Convert between fractional inches, decimal inches, mm, letter sizes, and number sizes for drill bits.
Type a diameter or select a named size — all fields update instantly.
Drill bits are sized in four different systems.
Metric: Diameter in millimeters (e.g., 3.0 mm, 6.5 mm).
Fractional inches: Common fractions (e.g., 1/16", 1/4", 1/2").
- 1 inch = 25.4 mm (exact)
Number sizes (#1–#80): Smaller numbers = larger bits.
- #1 = 5.791 mm (0.228")
- #80 = 0.343 mm (0.0135")
Letter sizes (A–Z): A = smallest, Z = largest.
- A = 5.944 mm (0.234")
- Z = 10.490 mm (0.413")
Number and letter drill sizes fill in the gaps between fractional inch sizes. They are commonly used in North America for precision drilling and tapping.