5/16-18 Tap Drill Size and Drill Bit You Need
Use a F drill bit
5/16-18 · 0.257 in / 6.53 mm · ~75% thread
5/16-18 is a coarse UNC thread used across furniture, fixtures and general machine work. A letter F bit is the standard tap drill.
At a glance
| Tap drill | F |
|---|---|
| Inches | 0.257 in |
| Millimetres | 6.53 mm |
| Major diameter | 0.3125 in |
| Pitch | 0.0556 in |
| Threads per inch | 18 |
Why this size
drillØ = major − (pitch × %thread) = 0.3125 − (0.0556 × 0.75) = 0.2708 in → next standard drill F (0.257 in / 6.53 mm)
The tap drill is letter F. For a clearance hole a 21/64 bit (0.3281 in) is the close fit; the bolt passes through freely, unthreaded.
Clearance holes for 5/16-18
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | F |
|---|---|
| Close fit clearance | 21/64 bit |
| Free fit clearance | 23/64 bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: