Reading Threads Per Inch and Pitch
Inch threads are named by diameter and threads per inch (TPI): 1/4-20 is a 1/4″ bolt with 20 threads in every inch of length. Metric threads are named by diameter and pitch, the distance from one crest to the next: M6 × 1.0 is a 6 mm bolt with crests 1 mm apart.
To convert, divide 25.4 by either one: 20 TPI is a 1.27 mm pitch, and a 1 mm pitch is 25.4 TPI. That’s why a pitch gauge settles most inch-or-metric arguments in seconds.
Numbered machine screws (#0 to #12) follow a formula: the major diameter is 0.060″ + 0.013″ × the number, so a #10 is .190″.
Questions
What's the difference between UNC and UNF?
Same diameters, different thread spacing. UNC (coarse) is the everyday thread for bolts and screws. UNF (fine) has more threads per inch, which gives finer adjustment and a little more strength in thin walls, and shows up a lot in automotive and aircraft work.
How do I measure a bolt's thread?
Measure across the thread crests with calipers to get the outside diameter, then use a thread pitch gauge to find the pitch. Enter both in the identifier. A used or plated bolt will read a few thousandths under nominal, which is normal.
Is 1/4-20 the same as M6?
No. They’re close in size (6.35 mm vs 6 mm) but the pitch is different (1.27 mm vs 1 mm), so they won’t thread together. Forcing one into the other strips both.
