Metric threads
A metric thread is written M8 × 1.25: M for metric, 8 mm the outside (major) diameter, and 1.25 mm the pitch, the distance from one thread to the next. When no pitch is given, as in plain "M8", the standard coarse pitch is meant. Fine threads such as M8 × 1 have a smaller pitch.
Imperial threads (UNC and UNF)
An imperial thread is written 1/4-20 UNC: 1/4 inch diameter and 20 threads per inch (TPI). UNC is the coarse series and UNF the fine one, so 1/4-28 UNF is the fine thread of the same diameter. Sizes under 1/4 inch use numbers, such as #10-24.
Telling them apart
- Measure the outside diameter with calipers. M6 is 6.0 mm and 1/4 inch is 6.35 mm, close enough to fool you by eye.
- Check the pitch with a thread gauge, or measure the length of 10 threads: 12.5 mm means 1.25 mm pitch, while 1/2 inch (12.7 mm) means 20 TPI.
- Never force a metric bolt into an imperial nut or the other way round. Similar sizes may start to turn and then strip.
Converting pitch and TPI
TPI = 25.4 ÷ pitch in mm, and pitch = 25.4 ÷ TPI. A 1.25 mm pitch is 20.3 TPI and 20 TPI is a 1.27 mm pitch: close, but not interchangeable.
Tap drill sizes
For a metric thread, the tap drill is roughly diameter − pitch: M8 × 1.25 takes a 6.8 mm drill, M6 × 1 a 5 mm drill. For imperial threads use a chart: 1/4-20 UNC takes a #7 drill (0.201 in).
Chart
The Screw Thread Chart lists metric coarse and fine threads and UNC and UNF sizes with their diameter, pitch or threads per inch, and tap drill.