Metric thread pitch guide for bolts and set screws
Thread pitch is the distance between two adjacent threads, measured in millimetres. On a metric fastener it is the second number in the callout: an M10 × 1.5 bolt is 10 mm in diameter with a 1.5 mm pitch. Get the pitch wrong and the fastener either will not start or strips the thread - so when you are matching an existing bolt, set screw or tapped hole, the pitch has to be exact. This page has the standard reference table and a couple of ways to check a pitch you are unsure of.
ISO metric coarse thread pitch table
Coarse thread is the default across general engineering and construction. Each metric diameter has one standard coarse pitch:
| Diameter | Coarse pitch (mm) | Common fine pitch (mm) |
|---|---|---|
| M3 | 0.50 | - |
| M4 | 0.70 | - |
| M5 | 0.80 | - |
| M6 | 1.00 | 0.75 |
| M8 | 1.25 | 1.00 |
| M10 | 1.50 | 1.25 (or 1.00) |
| M12 | 1.75 | 1.50 (or 1.25) |
| M14 | 2.00 | 1.50 |
| M16 | 2.00 | 1.50 |
| M20 | 2.50 | 1.50 |
| M24 | 3.00 | 2.00 |
The coarse column is the ISO metric coarse standard and is what you will get if a size is quoted without a pitch - an "M12 bolt" means M12 × 1.75 unless stated otherwise. Fine pitches are also standardised but have to be called out explicitly.
Coarse or fine - which to use
- Coarse is more tolerant of dirt, minor thread damage and repeated assembly, runs down faster, and is less prone to cross-threading. It is the right default for site and general workshop work.
- Fine gives a marginally stronger joint, finer adjustment and better resistance to vibration loosening for a given diameter. It is common on precision, automotive and thin-wall components. The trade-off is that it is easier to cross-thread and less forgiving of damage.
The two are not interchangeable. An M10 × 1.5 nut will not run onto an M10 × 1.25 bolt, even though both are "M10".
How to check a pitch you are unsure of
- Thread pitch gauge. A set of leaves each cut to a standard pitch; you offer them up until one sits cleanly in the thread with no gaps. The fastest and most reliable method.
- Measure and count. With a rule or calliper, measure 10 mm along the thread and count the gaps between crests, then divide 10 by that count. Six gaps over 10 mm is roughly a 1.5 mm pitch, for example.
- Match a known nut. If a nut of known size runs on freely and without play, the bolt shares its diameter and pitch.
Diameter itself is measured across the outside of the thread crests with a calliper - an M12 measures a shade under 12 mm on the crests. Set screws and bolts use the same metric thread standard, so the table above applies to both.
Know the size - find the fastener
Once you have the diameter and pitch, browse bolts, sockets & set screws, nuts and threaded bar in the catalogue. Not sure of the grade or coating? The fastener guide covers property classes and finishes.