UNCcoarse60° flank
1/4-20 UNC Thread Dimensions
1/4-20 UNC is a coarse pitch UNC thread. There are 20 threads per inch, giving a pitch of 1.27 mm. The recommended tap drill leaves roughly 75% thread engagement.
Nominal Ø6.35 mm
Pitch1.27 mm
Threads per inch20
Pitch Ø-14.6 mm
Tap drill-25.91 mm
Thread profile
millimetres unless stated- Designation
- 1/4-20 UNC
- Thread system
- UNC
- Series
- coarse
- Flank angle
- 60°
- Major diameter
- 6.35 mm / 0.25"
- Pitch
- 1.27 mm
- Threads per inch
- 20
- Pitch diameter
- -14.6 mm
- Minor diameter (external)
- -33.23 mm
- Minor diameter (internal)
- -28.57 mm
- Thread depth
- 19.79 mm
- Recommended tap drill
- Ø -25.91 mm / -1.02"
How these values are derived
Basic ISO thread geometry uses the fundamental triangle height H = 0.866025 × P. External minor diameter d₃ = d − 1.226869 × P, internal minor diameter D₁ = d − 1.082532 × P. Tap drill is given as nominal diameter minus pitch, which approximates a 75% thread engagement — adjust for tough materials or deep holes.