UNCcoarse60° flankASME B1.1

1/4-20 UNC Thread Dimensions

1/4-20 UNC is a 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 is nominal diameter minus pitch.

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 has a fundamental triangle height H = 0.866025 × P, with pitch diameter d₂ = d − 0.649519 × P, external minor diameter d₃ = d − 1.226869 × P and internal minor diameter D₁ = d − 1.082532 × P; the two minor diameters differ because the external root is rounded while the internal crest is truncated. 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	0.5 in
ASME B18.2.1 1/4-20 × 0.75 Hex Head Cap Screw — Inch Series	0.75 in
ASME B18.2.1 1/4-20 × 1 Hex Head Cap Screw — Inch Series	1 in
ASME B18.2.1 1/4-20 × 1.25 Hex Head Cap Screw — Inch Series	1.25 in
ASME B18.2.1 1/4-20 × 1.5 Hex Head Cap Screw — Inch Series	1.5 in
ASME B18.2.1 1/4-20 × 2 Hex Head Cap Screw — Inch Series	2 in
ASME B18.2.1 1/4-20 × 2.5 Hex Head Cap Screw — Inch Series	2.5 in
ASME B18.2.1 1/4-20 × 3 Hex Head Cap Screw — Inch Series	3 in
ASME B18.3 1/4-20 × 0.5 Socket Head Cap Screw — Inch Series	0.5 in
ASME B18.3 1/4-20 × 0.75 Socket Head Cap Screw — Inch Series	0.75 in
ASME B18.3 1/4-20 × 1 Socket Head Cap Screw — Inch Series	1 in
ASME B18.3 1/4-20 × 1.25 Socket Head Cap Screw — Inch Series	1.25 in
ASME B18.3 1/4-20 × 1.5 Socket Head Cap Screw — Inch Series	1.5 in
ASME B18.3 1/4-20 × 2 Socket Head Cap Screw — Inch Series	2 in
ASME B18.3 1/4-20 × 2.5 Socket Head Cap Screw — Inch Series	2.5 in
ASME B18.3 1/4-20 × 3 Socket Head Cap Screw — Inch Series	3 in
ASME B18.2.2 1/4-20 Hex Nut — Inch Series	—

The dash in the Length column marks a nut or a washer: those parts have no length to quote, so the dash means not applicable rather than missing data.

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