What drill for a 1/4-28 roll (form) tap?

About 0.2342 in; the nearest standard drill is 15/64. Check your tap maker's table.

What drill for a 1/4-28 Helicoil?

Drill H (0.2660 in), then tap with the 1/4-28 STI tap.

1/4-28 tap drill size

Use a #3 drill (0.2130 in) for a 1/4-28 cutting tap at 75% thread.

Drills for 1/4-28 (UNF)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#3	0.2130
Cutting tap, 50% thread (hard material)	7/32"	0.2188
Roll / form tap, 65% thread	15/64	0.2344
Helicoil / STI insert (1/4-28×1D)	H	0.2660

Pitch: 0.9071 mm (28 TPI)
Major Ø: 0.2500
Pitch Ø: 0.2268
Minor Ø: 0.2062

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 1/4-28 tap?: Use a #3 drill (0.2130 in) for a 1/4-28 cutting tap at 75% thread.
What drill for a 1/4-28 roll (form) tap?: About 0.2342 in; the nearest standard drill is 15/64. Check your tap maker's table.
What drill for a 1/4-28 Helicoil?: Drill H (0.2660 in), then tap with the 1/4-28 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.