F Drill Bit Size and Diameter
A F drill bit measures 0.2570 in
6.53 mm · letter drill · used as the 5/16-18 tap drill
The F is a letter-size drill bit in the American gauge series, measuring 0.2570 inches (6.53 mm). It sits between a 1/4 in bit (0.2500 in) and a G bit (0.2610 in). It is best known as the standard tap drill for a 5/16-18 UNC coarse thread.
At a glance
| Drill size | F |
|---|---|
| Diameter | 0.2570 in |
| Millimetres | 6.53 mm |
| Series | Letter drill (A–Z) |
| Closest fractional | 1/4 in (0.2500 in, slightly smaller) |
What the F is used for
5/16-18 tap drill: drillØ = major − (pitch × 0.75) = 0.3125 − (0.0556 × 0.75) = 0.2708 in → nearest standard bit = F (0.2570 in)
An F hole before a 5/16-18 tap gives the standard ~75% thread engagement. For a clearance hole a 21/64 in bit is the close fit — the bolt passes through unthreaded.
Related sizes
Nearby sizes
The threads and drill bits the F sits alongside: