UNCcoarse60° flankASME B1.1

1/4-20 UNC Thread Dimensions

1/4-20 UNC is a coarse Unified National Coarse thread. There are 20 threads per inch on a 0.25 in major diameter, giving a metric-equivalent pitch of 1.27 mm. The recommended tap drill leaves roughly 75% thread engagement.

Major Ø0.25 in

Threads per inch20

Pitch (metric)1.27 mm

Pitch Ø0.2177 in

Tap drill0.2 in

Thread profile

millimetres unless stated

1/4-20 UNC — basic profile60° flank angle · Not to scale

Basic information

Designation: 1/4-20 UNC
Thread system: Unified National Coarse
Series: coarse
Standard: ASME B1.1
Basic profile: Unified basic profile
Flank angle: 60°

Dimensions

Major diameter: 0.25 in / 6.35 mm
Pitch (metric equivalent): 1.27 mm
Threads per inch: 20
Pitch diameter: 0.2177 in / 5.53 mm
Minor diameter (external): 0.1886 in / 4.79 mm
Minor diameter (internal): 0.1961 in / 4.98 mm
Thread depth: 0.0307 in / 0.779 mm
Recommended tap drill: Ø 0.2 in / 5.08 mm

How these values are derived

computed from the basic profile

These figures follow the Unified basic profile of ASME B1.1. A 60° basic profile uses a fundamental triangle height H = 0.866025 × P, with external minor diameter d₃ = d − 1.226869 × P and internal minor diameter D₁ = d − 1.082532 × P; the two differ because the external root is rounded while the internal crest is truncated. A 55° Whitworth profile uses H = 0.960491 × P and a thread height of 0.640327 × P. The recommended tap drill is given as nominal diameter minus pitch, which approximates 75% thread engagement — adjust it for tough materials or deep holes.

Compatible parts17 specification pages

All 1/4-20 fasteners

Every fastener below is threaded 1/4-20 UNC, so a nut, bolt or washer picked from this list will mate with it.

ASME B18.2.1 ASME B18.3 ASME B18.2.2

Specification	Length
ASME B18.2.1 1/4-20 × 0.5 Hex Head Cap Screw — Inch Series	0.0197 in
ASME B18.2.1 1/4-20 × 0.75 Hex Head Cap Screw — Inch Series	0.0295 in
ASME B18.2.1 1/4-20 × 1 Hex Head Cap Screw — Inch Series	0.0394 in
ASME B18.2.1 1/4-20 × 1.25 Hex Head Cap Screw — Inch Series	0.0492 in
ASME B18.2.1 1/4-20 × 1.5 Hex Head Cap Screw — Inch Series	0.0591 in
ASME B18.2.1 1/4-20 × 2 Hex Head Cap Screw — Inch Series	0.0787 in
ASME B18.2.1 1/4-20 × 2.5 Hex Head Cap Screw — Inch Series	0.0984 in
ASME B18.2.1 1/4-20 × 3 Hex Head Cap Screw — Inch Series	0.1181 in
ASME B18.3 1/4-20 × 0.5 Socket Head Cap Screw — Inch Series	0.0197 in
ASME B18.3 1/4-20 × 0.75 Socket Head Cap Screw — Inch Series	0.0295 in
ASME B18.3 1/4-20 × 1 Socket Head Cap Screw — Inch Series	0.0394 in
ASME B18.3 1/4-20 × 1.25 Socket Head Cap Screw — Inch Series	0.0492 in
ASME B18.3 1/4-20 × 1.5 Socket Head Cap Screw — Inch Series	0.0591 in
ASME B18.3 1/4-20 × 2 Socket Head Cap Screw — Inch Series	0.0787 in
ASME B18.3 1/4-20 × 2.5 Socket Head Cap Screw — Inch Series	0.0984 in
ASME B18.3 1/4-20 × 3 Socket Head Cap Screw — Inch Series	0.1181 in
ASME B18.2.2 1/4-20 Hex Nut — Inch Series	—

Neighbouring sizes in Unified National Coarse.

#6-32 UNC #8-32 UNC #10-24 UNC #12-24 UNC 5/16-18 UNC 3/8-16 UNC 7/16-14 UNC 1/2-13 UNC