1/4-28 UNF Thread Specs
Standard: ASME B1.1 (UNF)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 1/4-28 UNF unified thread.
Compliant with the ASME B1.1 standard, this size has a nominal major diameter of 0.25 inches (6.35 mm) with a pitch of 0.03571 inches (TPI of 28).
For general engineering or fastening, the standard recommended tap drill is 0.2143 inches (5.44 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
1/4-28 UNF
Major Diameter (Nominal)
0.25 in(6.35 mm)
TPI (Threads per Inch)
28
Pitch
0.03571 in
Pitch Diameter
0.2268 in(5.76 mm)
Minor Diameter (Ext)
0.2113 in(5.37 mm)
Minor Diameter (Int)
0.2152 in(5.47 mm)
Thread Height
0.0309 in
Drilling & Strength Specs
Tap Drill Size
0.2143 in(5.44 mm)
Stress Area
0.0364 in²(23.47 mm²)
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Frequently Asked Questions
What is the tap drill size for 1/4-28 UNF?
The recommended tap drill size for a 1/4-28 UNF thread is 0.2143 inches (approx. 5.44 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 1/4-28 UNF bolt have?
A standard 1/4-28 UNF thread has exactly 28 threads per inch (TPI), with a corresponding linear pitch of 0.03571 inches.
What is the major diameter of a 1/4-28 UNF thread in millimeters?
The nominal major diameter of a 1/4-28 UNF thread is 0.25 inches, which converts to 6.35 mm.