How do I work out what size a bolt or machine screw is?

Machine screws and bolts differ from wood screws in a fundamental way: their thread mates with a matching thread in a nut or a tapped hole, so it has to be exactly right rather than approximately right. Two threads that are almost the same will start, feel wrong, and destroy each other, which is why identifying what you have matters before you buy a replacement.

What you'll need

• Vernier or digital callipers
• A thread pitch gauge (optional)
• A steel rule with millimetre markings
• A nut and bolt gauge board, if you have one (optional)

Step by step

1. Measure the outside diameter of the thread.Callipers across the crests of the thread, not across the head. That figure is the nominal size: 6.0mm means M6. Threads measure very slightly under nominal, so 5.9mm is still M6.
2. Measure the pitch.The distance from one thread crest to the next. A pitch gauge is the easy way; otherwise measure across ten threads with callipers and divide by ten. M6 is normally 1.0mm coarse; M8 is 1.25mm coarse.
3. Check whether it is a coarse or fine thread.Most sizes exist in both. An M8 x 1.25 and an M8 x 1.0 will start in each other and jam. If a nut goes on two turns and then binds, this is almost always why.
4. Measure the length correctly for the head type.Length is measured from under the head for hex, pan, button and socket cap heads, and from the top of the head for countersunk ones — because a countersunk head sits inside the material.
5. Recognise an imperial thread.If the diameter lands on something like 6.35mm (quarter inch), 7.94mm (five sixteenths) or 9.53mm (three eighths), you have an imperial fastener. UNC and UNF are the common ones, and older British equipment may use BSW, BSF or BA.
6. Watch for pipe threads, which are a different family entirely.BSP threads on plumbing fittings are named by the bore of the pipe, not the outside diameter of the thread, so a half-inch BSP thread measures about 21mm across. They are also often tapered.
7. Take the old fastener to the merchant if you can.Every decent supplier has a gauge board with tapped holes and threaded posts. Thirty seconds there is more reliable than any amount of measuring at home.
8. Never force a nut that binds.A nut that goes on freely for a turn and then needs a spanner is the wrong thread. Forcing it destroys both parts and leaves you with a fastener you cannot now match.

Tips

• A machine screw is simply a small bolt with a screwdriver drive rather than a hex head — the thread system is the same, and the same M-sizes apply.
• Bicycles, appliances and flat-pack furniture are almost entirely metric in the UK. Older cars, American equipment and some plumbing are where imperial turns up.
• Keep a small assortment of common M4, M5, M6 and M8 bolts and nuts. Between them they cover a large share of household repairs.

Common mistakes

• Measuring the head across the flats and calling that the size — The spanner size is not the thread size. An M8 bolt commonly takes a 13mm spanner, and quoting 13 will get you the wrong bolt.
• Ignoring pitch and buying by diameter alone — Coarse and fine threads of the same diameter are different fasteners. They will engage far enough to feel plausible and then ruin each other.