How to measure a bolt
Start with diameter, length and head shape. Three practical checks before you look for a replacement.
Using ordinary hex and coach bolts, with pitch explained at the end if you need it.
An old bolt, a ruler and a set of callipers are a useful starting point. Write down the diameter and length, then check the head shape. A 10mm spanner does not mean you have a 10mm bolt.
Need help identifying the part first? See our bolts, nuts and washers guide.
Measure the diameter
Measure across the outside of the threads, from crest to crest. Do not measure the head or the dips between the threads.
Keep the calliper jaws square to the bolt. Record the reading, then confirm the nominal size against the product specification. For example, M8 is a nominal 8mm metric thread, not an 8mm head.
Measure the length
For ordinary hex and coach bolts, measure from directly underneath the head to the tip. Leave the head height out.
Coach bolt
Start at the flat underside of the dome. The square neck is part of the measured length.
Flat countersunk head
Measure from the outer face of the head to the tip. The head is included because it sits flush with the material.
Check the head
Look at the head from above, then from the side. These six shapes help you describe what you have.
Top view above · Side profile below · Simplified drawings, not to scale
Hex head
Six outer flats. Turn the head with a spanner or socket.
Coach / carriage
Smooth dome, no drive recess. Look for the square neck underneath.
Socket cap
Tall cylindrical head. This example has an internal hex socket.
Button head
Low, rounded top. This example has an internal hex socket.
Pan head
Slightly rounded top with short upright sides. Often seen on machine screws.
Flat countersunk
Flat top with a tapered underside. Sits flush in a matching countersink.
Two domes can hide different fixings
A smooth dome alone is not enough to identify a coach bolt. Turn it over and check for the square neck directly below the head.
Also match the neck to the original fitting. The threaded diameter does not tell you the neck size.
Note the finish and any head markings too. A similar-looking bolt is not automatically the same grade or suitable for the same job.
Example: M8 × 60mm coach bolt
M8 × 60mm
M8 means a nominal 8mm metric thread diameter.
60mm is the length from under the domed head to the tip, including the square neck.
Our M8 stainless coach-bolt range has a 60mm option. Check the length and choose either bolts only or the kit with nuts and washers.
View the M8 × 60mm coach boltActual range photo, not a scale drawing. The linked option is bolts only; nuts and washers are included with the kit option.
Your replacement-bolt note
Keep the useful details together:
- Match the head and neck to the original fixing.
- Check the amount of thread, finish and any specified grade.
- Confirm the matching nut or threaded part.
Then compare the Bolts range. For a separate nut, check the thread specification before choosing from the Nuts range.
Thread pitch: only if you need it
Check pitch when the thread is uncertain, the part may have a fine or unusual thread, or the matching nut does not turn freely. Never force it.
Metric pitch is the distance from one thread crest to the next, measured along the bolt. A thread-pitch gauge is the clearest way to check. If you have no gauge, ask a fastener supplier to identify the thread.
Imperial threads commonly use threads per inch. Converting the diameter does not make an imperial thread match a metric one.
Measurement conventions: Bolt Depot and the Pro-Bolt measuring guide. Follow the specified fixing for structural, safety-critical or warranty-sensitive work.