What drill for a 3-48 roll (form) tap?

About 0.0898 in; the nearest standard drill is #43. Check your tap maker's table.

What drill for a 3-48 Helicoil?

Drill #37 (0.1040 in), then tap with the 3-48 STI tap.

3-48 tap drill size

Use a #47 drill (0.0785 in) for a 3-48 cutting tap at 75% thread.

Drills for 3-48 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#47	0.0785
Cutting tap, 50% thread (hard material)	#44	0.0860
Roll / form tap, 65% thread	#43	0.0890
Helicoil / STI insert (#3-48×1D)	#37	0.1040

Pitch: 0.5292 mm (48 TPI)
Major Ø: 0.0990
Pitch Ø: 0.0855
Minor Ø: 0.0734

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 3-48 tap?: Use a #47 drill (0.0785 in) for a 3-48 cutting tap at 75% thread.
What drill for a 3-48 roll (form) tap?: About 0.0898 in; the nearest standard drill is #43. Check your tap maker's table.
What drill for a 3-48 Helicoil?: Drill #37 (0.1040 in), then tap with the 3-48 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.