A 20 TPI thread has 1.27 mm between thread crests.
UNC UNF Thread Size Converter
Convert common inch thread callouts to major diameter, threads per inch, and metric pitch. Handy for checking bolts, taps, dies, machine screws, repair notes, and parts listings.
Convert inch thread size
Choose a thread size and series, or enter a custom TPI, to see diameter and pitch in inch and metric units.
Callout1/4-20 UNC
Series meaningUnified National Coarse
Nearest metric diameterAbout M6
Use the converted values to identify a fastener, then confirm the exact standard before buying taps, dies, nuts, or replacement bolts.
How to use this converter
- Select the inch thread size stamped on the drawing, tap, package, or fastener listing.
- Choose UNC for coarse threads or UNF for fine threads.
- Read the major diameter in inches and millimeters, plus pitch in TPI and millimeters.
- If your thread gauge shows a different TPI, enter it in the custom field.
What UNC and UNF mean
UNC means Unified National Coarse. It is common for general bolts, hardware, and repairs.
UNF means Unified National Fine. Fine threads have more threads per inch and a smaller pitch distance.
Metric pitch is calculated with: pitch in mm = 25.4 ÷ TPI.
Common UNC and UNF thread equivalents
Reference values for common Unified inch machine screw and bolt sizes. Custom or specialty threads may differ.
| Size | Major diameter | UNC coarse | UNF fine | Metric diameter |
|---|
UNC and UNF thread FAQ
Is #10-24 the same as M5?
No. A #10 screw is about 0.190 inch or 4.83 mm major diameter, which is close to M5 diameter, but #10-24 has a 1.058 mm pitch. M5 coarse is usually 0.8 mm pitch.
What is the metric pitch of 1/4-20?
1/4-20 has 20 threads per inch. Divide 25.4 by 20 to get a 1.27 mm pitch.
What is the difference between UNC and UNF?
UNC is the coarse thread series and UNF is the fine thread series. For the same diameter, UNF has more threads per inch and a shorter distance between each thread crest.