All charts / Machining
Thread Size Chart
UNC, UNF and metric thread dimensions: major diameter, thread pitch and the minor (tap drill) diameter you need when cutting the thread.
| Thread | Type | Major dia. (in) | Major dia. (mm) | Pitch | Minor dia. (in) |
|---|
Standard: ISO 724 / ANSI B1.1 · Data checked: 2026-10-14
Frequently asked questions
What is the difference between UNC and UNF?
Both are Unified threads with a 60 degree profile; UNC is coarse and UNF is fine. Coarse threads are the default, are easier to start, and resist damage better. Fine threads have a larger minor diameter so they are stronger in tension and resist vibration loosening better, which is why they show up in automotive and aerospace work.
Is NPT the same as BSP?
No, and they are not interchangeable even when the nominal sizes match. NPT is the US tapered pipe thread at 60 degrees; BSP is the British standard at 55 degrees with a different pitch and diameter. Forcing one into the other damages both and it will leak. Use an adapter.
What does the second number in 1/4-20 mean?
Threads per inch. 1/4-20 is a quarter-inch nominal diameter with 20 threads per inch. Metric designations work the other way — M8 x 1.25 is 8 mm nominal with a 1.25 mm pitch, so a larger number means a coarser thread, not a finer one.
Why does a thread chart give both pitch diameter and minor diameter?
Because they are different things and different calculations use them. Pitch diameter is where the flanks of the thread meet, and it is what thread gauges measure. Minor diameter is the bottom of the thread, and it is what you use for the tensile stress area and for tapping.