What drill for a 5/16-18 roll (form) tap?

About 0.2879 in; the nearest standard drill is L. Check your tap maker's table.

What drill for a 5/16-18 Helicoil?

Drill Q (0.3320 in), then tap with the 5/16-18 STI tap.

5/16-18 tap drill size

Use a F drill (0.2570 in) for a 5/16-18 cutting tap at 75% thread.

Drills for 5/16-18 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	F	0.2570
Cutting tap, 50% thread (hard material)	G	0.2610
Roll / form tap, 65% thread	L	0.2900
Helicoil / STI insert (5/16-18×1D)	Q	0.3320

Pitch: 1.4111 mm (18 TPI)
Major Ø: 0.3125
Pitch Ø: 0.2764
Minor Ø: 0.2443

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 5/16-18 tap?: Use a F drill (0.2570 in) for a 5/16-18 cutting tap at 75% thread.
What drill for a 5/16-18 roll (form) tap?: About 0.2879 in; the nearest standard drill is L. Check your tap maker's table.
What drill for a 5/16-18 Helicoil?: Drill Q (0.3320 in), then tap with the 5/16-18 STI tap.

Other UNC sizes

Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.