1/4-20 UNC Thread Specs
Standard: ASME B1.1 (UNC)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 1/4-20 UNC 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 0.05 inches (TPI of 20).
For general engineering or fastening, the standard recommended tap drill is 0.2 inches (5.08 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
1/4-20 UNC
Major Diameter (Nominal)
0.25 in(6.35 mm)
TPI (Threads per Inch)
20
Pitch
0.05 in
Pitch Diameter
0.2175 in(5.53 mm)
Minor Diameter (Ext)
0.1959 in(4.98 mm)
Minor Diameter (Int)
0.2013 in(5.11 mm)
Thread Height
0.0433 in
Drilling & Strength Specs
Tap Drill Size
0.2 in(5.08 mm)
Stress Area
0.0318 in²(20.53 mm²)
Quick Unit Converter
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Frequently Asked Questions
What is the tap drill size for 1/4-20 UNC?
The recommended tap drill size for a 1/4-20 UNC thread is 0.2 inches (approx. 5.08 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 1/4-20 UNC bolt have?
A standard 1/4-20 UNC thread has exactly 20 threads per inch (TPI), with a corresponding linear pitch of 0.05 inches.
What is the major diameter of a 1/4-20 UNC thread in millimeters?
The nominal major diameter of a 1/4-20 UNC thread is 0.25 inches, which converts to 6.35 mm.