Quick answer: UK bolts are sized by thread diameter and length in millimetres, written like M8 × 40. Measure across the threads with a caliper for the diameter (M8 is 8 mm), measure from under the head to the tip for the length, and check the thread pitch with a gauge. Countersunk bolts are measured overall, including the head.
Almost every bolt sold in the UK today uses the ISO metric thread, but older British machinery, classic cars and American equipment still turn up with inch threads that look very similar. This guide shows how to read a metric bolt size, how to measure an unknown bolt and how to spot an imperial one. It belongs to our plumbing, electrical and hardware sizes section.
What does a bolt size like M8 × 40 mean?
The M stands for ISO metric thread, the 8 is the nominal thread diameter in millimetres and the 40 is the length in millimetres. If no pitch is given, the bolt has the standard coarse pitch for its size, which for M8 is 1.25 mm. A fine-pitch bolt has the pitch written in, such as M8 × 1 × 40 or M8 × 1.0 fine.
Metric screw threads are set out in ISO 261, and the coarse series is what you will find in DIY shops, flat-pack furniture fixings and most machinery. Fine threads are mainly used where vibration or thin walls make them useful, for example on some vehicle parts.
What do you need to measure a bolt?
- A vernier or digital caliper reading to 0.1 mm
- A thread pitch gauge with both metric and imperial leaves, if you have one
- A steel rule for length, as a backup to the caliper
- A known nut, or a nut and bolt gauge board, to test-fit unknown threads
No caliper? A steel rule works for length, and our guide to reading a tape measure covers millimetre scales, but a rule is not accurate enough to separate close diameters such as M8 and 5/16 inch.
How do you measure a bolt step by step?
Measure diameter, then length, then pitch, and only then decide whether the bolt is metric or imperial.
- Clean the thread. Wire-brush off rust, paint and thread-lock so the caliper sits on bare metal.
- Measure the diameter. Close the caliper’s outer jaws across the thread crests, not in the grooves. A bolt usually measures slightly under its nominal size: an M8 bolt will typically read a little under 8 mm, perhaps 7.8 to 7.9 mm.
- Measure the length. For hex, socket, button and coach bolts, measure from the underside of the head to the tip. For countersunk bolts, measure the overall length including the head.
- Find the pitch. Hold thread gauge leaves against the thread until one sits with no light showing. Without a gauge, measure across ten crests: the distance divided by 10 is the pitch in millimetres.
- Measure the head. Measure across the flats of the hex head to find the spanner size, which also helps identify the thread.
- Check against the chart. Match diameter and pitch to the metric table below. If neither fits, try the imperial table.
Metric bolt size chart
This chart lists common ISO metric sizes with their coarse pitch, a common fine pitch, spanner size and drill sizes. The tapping drill is the drill for cutting a thread with a tap; the clearance hole is the medium-fit hole for a bolt to pass through freely.
| Size | Coarse pitch | Common fine pitch | Spanner (across flats) | Tapping drill | Clearance hole |
|---|---|---|---|---|---|
| M3 | 0.5 mm | – | 5.5 mm | 2.5 mm | 3.4 mm |
| M4 | 0.7 mm | – | 7 mm | 3.3 mm | 4.5 mm |
| M5 | 0.8 mm | – | 8 mm | 4.2 mm | 5.5 mm |
| M6 | 1.0 mm | – | 10 mm | 5.0 mm | 6.6 mm |
| M8 | 1.25 mm | 1.0 mm | 13 mm | 6.8 mm | 9 mm |
| M10 | 1.5 mm | 1.25 mm | 16 mm (17 mm older DIN) | 8.5 mm | 11 mm |
| M12 | 1.75 mm | 1.5 mm or 1.25 mm | 18 mm (19 mm older DIN) | 10.2 mm | 13.5 mm |
| M16 | 2.0 mm | 1.5 mm | 24 mm | 14 mm | 17.5 mm |
| M20 | 2.5 mm | 1.5 mm | 30 mm | 17.5 mm | 22 mm |
The ten-crest method is easy to check against this table. Ten pitches on an M8 coarse bolt span 12.5 mm, on M10 they span 15 mm and on M6 exactly 10 mm. For more on spanner sizes, see our nut size guide.
Where do you measure bolt length?
Measure from the surface the head sits on to the tip. For most heads that is from under the head; for countersunk heads, which sit flush with the surface, the head is part of the length.
| Head type | Where length is measured from | Typical use |
|---|---|---|
| Hex head bolt or set screw | Under the head | General engineering, brackets, machinery |
| Socket cap screw | Under the head | Machinery, bikes, furniture |
| Button head | Under the head | Panels and trims with a low profile |
| Countersunk | Overall, including the head | Hinges and fittings that must sit flush |
| Coach (carriage) bolt | Under the domed head | Timber gates, sheds and structures |
In UK usage, a “bolt” usually has a plain, unthreaded section under the head, while a “set screw” is threaded all the way to the head. They are made to different standards, ISO 4014 and ISO 4017 respectively, but are measured the same way. Wood screws and self-tappers are covered in our screw size guide.
How can you tell if a bolt is imperial?
If the diameter is close to a fraction of an inch and the pitch matches a threads-per-inch figure rather than a metric pitch, the bolt is imperial. Inch bolts are measured in inches and threads per inch (TPI). The trap is that some sizes are very close: a 5/16 inch bolt is 7.94 mm across, almost the same as M8, but its UNC pitch of 18 TPI works out at 1.41 mm, against 1.25 mm for M8. Forcing one into the other strips the thread.
| Imperial size | Diameter | UNC (TPI) | BSW (TPI) | Nearest metric |
|---|---|---|---|---|
| ¼ in | 6.35 mm | 20 | 20 | M6 |
| 5/16 in | 7.94 mm | 18 | 18 | M8 |
| ⅜ in | 9.53 mm | 16 | 16 | M10 |
| ½ in | 12.70 mm | 13 | 12 | M12 |
UNC and UNF are the American unified threads found on US-made equipment and many classic vehicles. BSW (Whitworth) and BSF are older British threads, and Whitworth has a different thread angle from UNC, so even sizes with the same TPI are not a reliable match. The inches to cm converter turns any fractional inch size into millimetres.
What do the numbers on a bolt head mean?
Numbers such as 8.8 or 10.9 on a steel bolt head are the property class, which tells you its strength under ISO 898-1. The first number is one hundredth of the nominal tensile strength in megapascals, so 8.8 means 800 MPa. The second number gives the yield strength as a fraction of that, so 8.8 yields at about 80% of 800, or 640 MPa. Stainless bolts are marked differently, for example A2-70 or A4-80, where A4 is the more corrosion-resistant grade often chosen for coastal and marine use.
Always replace a bolt with one of at least the same property class, and follow the maker’s specification for anything safety-critical, such as brakes, suspension, lifting gear or structural steelwork.
Common mistakes to avoid
- Measuring in the thread grooves. That gives the smaller root diameter, so an M10 can look like an M8.
- Including the head in the length of a hex bolt. Only countersunk lengths include the head.
- Assuming coarse pitch. Fine-pitch bolts look similar but will not take a coarse nut. Check with a gauge if a nut feels tight after a turn or two.
- Mixing imperial and metric. 5/16 inch and M8, or ½ inch and M12, are close enough to start but wrong enough to strip.
- Downgrading strength. A plain, unmarked bolt is not a substitute for a marked 10.9 one.
Frequently asked questions
How is bolt length measured in the UK?
From the underside of the head to the tip for hex, socket, button and coach bolts. Countersunk bolts are measured overall, including the head, because the head sits flush with the surface.
What size spanner fits an M8 bolt?
A standard M8 hex bolt or nut takes a 13 mm spanner. M6 takes 10 mm, and M10 takes 16 mm or 17 mm depending on the standard it was made to.
How do I measure thread pitch without a gauge?
Measure the distance across ten thread crests with a caliper and divide by 10. For example, 12.5 mm across ten crests means a 1.25 mm pitch, the coarse pitch for M8.
Why does my M8 bolt measure less than 8 mm?
The nominal size is the basic thread diameter, and manufacturing tolerances put the actual crests slightly under it. A reading a few tenths of a millimetre under 8 mm is normal for an M8 bolt.
What does 8.8 on a bolt mean?
It is the property class under ISO 898-1. An 8.8 bolt has a nominal tensile strength of 800 MPa and a yield strength of about 80% of that, 640 MPa.
Sources
- ISO 261: ISO general purpose metric screw threads, general plan, ISO
- ISO 898-1: Mechanical properties of fasteners, bolts, screws and studs, ISO
- National Physical Laboratory, the UK’s national measurement institute