Unified inch · UNF

1/4-28 UNF tap drill size

For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.

Cut tap

7/32

0.2188" · 67% thread

Form tap

0.2340" · 66% thread

Open 1/4-28 UNF in the calculator

Pick your material for its % of thread, tapping RPM and the rigid-tap code.

Thread dimensions

Major Ø: 0.2500" (6.350 mm)
Threads per inch: 28 (pitch 0.0357")
Pitch Ø (basic): 0.2268" (5.761 mm)
Minor Ø (internal, basic): 0.2113" (5.368 mm)
Thread depth (internal): 0.0193" (0.491 mm)

Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.

Cut tap drills

For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Cut tap drills for 1/4-28 UNF
Drill	Ø in	Ø mm	Thread
5.4 mm	0.2126	5.40	81%
#3	0.2130	5.41	80%
5.5 mm	0.2165	5.50	72%
7/32suggested	0.2188	5.56	67%
5.6 mm	0.2205	5.60	64%
#2	0.2210	5.61	63%
5.7 mm	0.2244	5.70	55%

Form tap drills

A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Form tap drills for 1/4-28 UNF
Drill	Ø in	Ø mm	Thread
5.9 mm	0.2323	5.90	73%
Asuggested	0.2340	5.94	66%
15/64	0.2344	5.95	64%
6 mm	0.2362	6.00	57%

3-wire measurement

Best wire size: 0.0206" (0.524 mm)
Over wires, at basic pitch Ø: 0.2577" (6.546 mm)

For the external thread (the screw). The best wire touches the flanks at the pitch diameter.

Sources

ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size