What drill for a 5/16-24 roll (form) tap?

About 0.2941 in; the nearest standard drill is M. Check your tap maker's table.

What drill for a 5/16-24 Helicoil?

Drill S (0.3480 in), then tap with the 5/16-24 STI tap.

5/16-24 tap drill size

Use a I drill (0.2720 in) for a 5/16-24 cutting tap at 75% thread.

Drills for 5/16-24 (UNF)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	I	0.2720
Cutting tap, 50% thread (hard material)	J	0.2770
Roll / form tap, 65% thread	M	0.2950
Helicoil / STI insert (5/16-24×1D)	S	0.3480

Pitch: 1.0583 mm (24 TPI)
Major Ø: 0.3125
Pitch Ø: 0.2854
Minor Ø: 0.2614

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 5/16-24 tap?: Use a I drill (0.2720 in) for a 5/16-24 cutting tap at 75% thread.
What drill for a 5/16-24 roll (form) tap?: About 0.2941 in; the nearest standard drill is M. Check your tap maker's table.
What drill for a 5/16-24 Helicoil?: Drill S (0.3480 in), then tap with the 5/16-24 STI tap.

Other UNF 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.