1/4-28 UNF Thread Dimensions
1/4-28 UNF is a Unified National Fine thread. There are 28 threads per inch on a 0.25 in major diameter, giving a metric-equivalent pitch of 0.907 mm. The recommended tap drill is nominal diameter minus pitch.
Thread profile
millimetres unless statedBasic information
- Designation
- 1/4-28 UNF
- Thread system
- Unified National Fine
- Series
- fine
- Standard
- ASME B1.1
- Basic profile
- Unified basic profile
- Flank angle
- 60°
Dimensions
- Major diameter
- 0.25 in / 6.35 mm
- Pitch (metric equivalent)
- 0.907 mm
- Threads per inch
- 28
- Pitch diameter
- 0.2268 in / 5.76 mm
- Minor diameter (external)
- 0.2063 in / 5.24 mm
- Minor diameter (internal)
- 0.2114 in / 5.37 mm
- Thread depth
- 0.0219 in / 0.556 mm
- Recommended tap drill
- Ø 0.2143 in / 5.443 mm
How these values are derived
computed from the basic profileThese 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 parts0 specification pages
No specification in this database uses 1/4-28 UNF yet.
Related
Neighbouring sizes in Unified National Fine.