Identifying a Thread
Metric, UNC, UNF, Whitworth, BSP and NPT threads can look almost the same - and the wrong nut or fitting will start, then bind and strip. Four measurements tell them apart: the diameter, the pitch, the thread angle, and whether it is parallel or tapered.
Thread identification chartTap drill chart
What you need
- Calipers (vernier, dial or digital) for the diameter.
- A thread pitch gauge with both metric (mm) and inch (TPI) leaves.
- A screw-cutting gauge to tell a 60° thread from a 55° one.
- A wire brush - gauges only read properly on a clean thread.
Digital calipersThread pitch gaugesScrew-cutting gauges
1. Measure the diameter
Bolt or stud: measure across the tops of the thread. A used bolt usually measures a little under its nominal size - 0.1 to 0.2 mm under is normal, so an M10 bolt may read 9.8 mm and a 3/8" bolt about 9.4 mm.
Nut or tapped hole: measure across the tops of the thread inside the hole; the nominal size is roughly that figure plus one pitch. Easier still, if you have a bolt you know, see what fits.
Pipe threads are the odd one out. BSP and NPT sizes are named after an old pipe bore, not the thread, so the thread is much bigger than the name suggests:
| Pipe size | BSP thread measures | NPT thread measures |
|---|---|---|
| 1/8" | 9.7 mm | 10.3 mm |
| 1/4" | 13.2 mm | 13.7 mm |
| 3/8" | 16.7 mm | 17.1 mm |
| 1/2" | 21.0 mm | 21.3 mm |
| 3/4" | 26.4 mm | 26.7 mm |
| 1" | 33.2 mm | 33.4 mm |
If a thread measures one of these sizes, it is almost certainly a pipe thread.
2. Gauge the pitch
Try leaves of the pitch gauge until one sits in the thread with no light showing between the teeth along its whole length. Metric leaves are marked in mm between threads (1.5, 1.25...); inch leaves in threads per inch (TPI).
Converting between mm and TPI
Metric threads are given as the pitch - the distance from one thread to the next, in mm. Inch threads are given as threads per inch (TPI). There are 25.4 mm in an inch, so:
pitch in mm = 25.4 ÷ TPI
TPI = 25.4 ÷ pitch in mm
For example, 16 TPI is 25.4 ÷ 16 = 1.588 mm, and a 1.5 mm pitch is 25.4 ÷ 1.5 = 16.9 TPI - close, but not the same.
Measuring the pitch without a gauge
Lay the calipers along the thread and measure from the top of one thread to the top of another, as many threads apart as you can. Then divide by the number of gaps, not the number of tops: eleven tops have ten gaps between them.
pitch = length measured ÷ number of gaps
Example: eleven thread tops measure 17.5 mm from first to last. 17.5 ÷ 10 = 1.75 mm, which is the coarse pitch of an M12. A 14 TPI thread (1.814 mm) would have measured 18.1 mm over the same ten gaps.
Measuring over many threads matters for two reasons. A caliper reading that is 0.05 mm out becomes only 0.005 mm per thread when you divide by ten. And the difference between two near pitches adds up with every thread, so a long measurement shows it clearly when one or two threads do not.
The near misses
Some metric and inch pitches are so close that the wrong pitch gauge leaf seems to fit, and the wrong nut will start and run a few turns before it binds. The table shows how far apart they drift - multiply the difference per thread by the number of threads:
| Metric pitch | Nearest inch pitch | Inch pitch in mm | Difference over 10 threads | Over 20 threads | Where you meet it |
|---|---|---|---|---|---|
| 0.8 mm | 32 TPI | 0.794 mm | 0.06 mm | 0.13 mm | M5 vs 10-32 UNF |
| 1.0 mm | 24 TPI | 1.058 mm | 0.58 mm | 1.17 mm | M10 x 1 fine vs 3/8" UNF |
| 1.25 mm | 20 TPI | 1.270 mm | 0.20 mm | 0.40 mm | M12 x 1.25 fine vs 1/2" UNF |
| 1.5 mm | 16 TPI | 1.588 mm | 0.88 mm | 1.75 mm | M10 vs 3/8" UNC |
| 1.75 mm | 14 TPI | 1.814 mm | 0.64 mm | 1.29 mm | M12 vs 7/16" UNC |
| 2.0 mm | 13 TPI | 1.954 mm | 0.46 mm | 0.92 mm | M14 vs 1/2" UNC |
| 2.5 mm | 10 TPI | 2.540 mm | 0.40 mm | 0.80 mm | M20 vs 3/4" UNC |
| 3.0 mm | 8 TPI | 3.175 mm | 1.75 mm | 3.50 mm | M24 vs 1" UNC |
3. Check the thread angle
Hold a screw-cutting gauge against the thread profile:
| Angle | Threads |
|---|---|
| 60° | Metric, Unified (UNC, UNF), NPT |
| 55° | Whitworth (BSW), BSF, BSP |
| 47.5° | BA (small instrument threads) |
Whitworth-family threads also have rounded tops and roots; metric and Unified are flat.
4. Parallel or taper?
Measure the diameter near the end of the thread and again further up. If it gets smaller towards the end, it is a taper pipe thread - BSPT (55°) or NPT (60°) - which seals on the thread itself. A parallel pipe thread (BSPP, also marked G) does not seal on the thread and needs a washer, O-ring or bonded seal. A taper male will screw a few turns into a parallel female, which is how leaks start.
5. Left or right hand?
Hold the bolt upright. If the threads slope up to the right, it is right hand - by far the most common. Sloping up to the left means a left-hand thread; these are often marked with a groove or notches on the nut or bolt head.
6. Look it up
With the diameter, pitch and angle, find the thread in the thread identification chart - it lists sizes by measured diameter, every pitch with its near misses, and the odd threads (UNEF, BSB, cycle, Trapezoidal, Acme). The tap drill chart then gives the tapping and clearance drill for it.
Still not sure? Bring it in or ask us - we are happy to help.