How to Measure a Bolt: Length, Diameter & Thread Pitch | WhatSizeBolt
Master the essential skills of bolt measurement with this comprehensive guide to tools and techniques.
Get a bolt’s size wrong and the replacement won’t thread, won’t seat, or won’t hold. Measuring one properly takes four numbers and a couple of cheap tools — whether you’re matching a fastener on a 2020 Toyota Corolla or buying hardware for a new build. Here’s how to nail each one.
The four measurements
Identify these four and you can order the exact bolt anywhere in the world.
1. Length
Where “length” starts depends on how the head sits in the work.
- Hex, cap and carriage bolts — measure from under the head to the tip. The head is not part of the length.
- Countersunk / flat heads — measure the total length including the head, because the head sits flush in the work.
- Round / pan heads — like hex bolts: under the head to the tip.
2. Diameter
Always the major diameter — the widest point, across the thread crests.
- Clean the threads so the caliper jaws sit on bare metal.
- Close the jaws across the crests at the widest part of the thread.
- Take a few readings along the bolt and keep the largest — that’s the major diameter.
Common diameters
| Metric | Major Ø | Imperial | Major Ø |
|---|---|---|---|
| M3 | 3.0 mm | #6 | 0.138" |
| M4 | 4.0 mm | #8 | 0.164" |
| M5 | 5.0 mm | #10 | 0.190" |
| M6 | 6.0 mm | 1/4" | 0.250" |
| M8 | 8.0 mm | 5/16" | 0.3125" |
| M10 | 10.0 mm | 3/8" | 0.375" |
| M12 | 12.0 mm | 1/2" | 0.500" |
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Digital calipers
Reads diameter, head size and length to a hundredth of a millimetre, and doubles as your thread-spacing measurer. If you buy one fastener tool, buy this.
3. Thread pitch
Metric pitch is the millimetres between crests; imperial pitch is threads per inch (TPI).
The fastest way is a thread pitch gauge: press leaves against the threads until one seats flush with no rocking. No gauge? Count the crests across a known length — crests per inch is the TPI, or divide 10 mm by the crests in that span for metric pitch. Our thread pitch guide walks through both methods in detail.
| Size | Coarse pitch | Size | Coarse pitch |
|---|---|---|---|
| M5 | 0.8 mm | 1/4" | 20 TPI |
| M6 | 1.0 mm | 5/16" | 18 TPI |
| M8 | 1.25 mm | 3/8" | 16 TPI |
| M10 | 1.5 mm | 7/16" | 14 TPI |
| M12 | 1.75 mm | 1/2" | 13 TPI |
Thread pitch gauge set (metric + SAE)
A fan of stamped leaves that takes the guesswork out of pitch. Get one that covers both metric and SAE so you’re never caught between systems.
4. Head type and size
The drive tells you which tool fits; the size tells you which one.
- Hex head — measure across the flats (not corner to corner); that’s your wrench/socket size.
- Socket / Allen head — measure the internal hex.
- Other drives — Phillips, slotted, and Torx are sized by their own gauges.
Bolt & nut thread gauge / checker
A go/no-go board with threaded holes and studs for common metric and SAE sizes — drop a fastener in and read the size and pitch straight off. A bench-top time-saver next to your calipers.
Write it down like a pro
Record all four (plus grade and finish) so the counter hands you the exact part.
- Imperial — diameter-TPI × length, e.g. 1/2"-13 × 2" grade 5, zinc.
- Metric — M-diameter × pitch × length, e.g. M8 × 1.25 × 40 mm class 8.8.
Frequently asked questions
Where do I measure bolt length from?
From under the head to the tip for hex, cap, carriage and round-head bolts. Countersunk and other flush-mount heads are the exception — measure their total length including the head, because the head sits in the work.
Is the bolt size the major or minor diameter?
The major diameter — the widest point across the thread crests. That’s the nominal size (the “3/8"” or “M8”). The minor diameter, down in the thread valleys, is always smaller and isn’t how bolts are sized.
Why does my caliper read just under the nominal size?
Major diameters run a few thou under nominal by design, so the bolt clears its nut. A 3/8" (0.375") bolt typically mics around 0.370"; round up to the nearest standard size.
How do I measure hex head size?
Across the flats — the straight-line distance between two opposite flat faces. That number is your wrench or socket size. Don’t measure across the corners; it reads high.
What do 1/2"-13 and M8 × 1.25 mean?
They pack diameter and pitch into one label. 1/2"-13 is a half-inch bolt with 13 threads per inch; M8 × 1.25 is an 8 mm bolt with 1.25 mm between threads. Add the length to fully specify it.
Keep going
Bottom line: four numbers identify any bolt — length under the head, major diameter across the crests, thread pitch, and head size across the flats. A caliper and a pitch gauge get all four in under a minute, so measure once, write it down, and order with confidence.