How to measure a screw and work out its metric size

1. Measure the diameter across the threads

Put a caliper across the outside of the threads, not the shank under the head. Whatever you read is close to the M number. A reading of 1.95 to 2.05 mm is an M2.

No caliper? Lay the screw on a steel ruler with millimetre marks and read the thread width under good light, or drop it through the holes of a printed screw gauge until it just fits.

• 1.2 mm reading → M1.2
• 1.4 mm reading → M1.4
• 1.6 mm reading → M1.6
• 2.0 mm reading → M2
• 2.5 mm reading → M2.5
• 3.0 mm reading → M3

2. Measure the length the right way

For a flat-topped screw (pan, cheese, button, hex) measure from under the head to the tip. The head does not count.

For a countersunk screw that sits flush, measure the whole screw including the head, because the head sinks into the material.

Round to the nearest half millimetre. Screws are sold in steps like 3, 4, 5, 6 mm.

3. Check the pitch if the screw will not start

Pitch is the distance between two threads. Most small screws use the standard coarse pitch, so you rarely need to check it — but if a screw of the right diameter binds after one turn, you have a pitch mismatch.

Count the threads over 5 mm and divide: 10 threads in 5 mm means a 0.5 mm pitch, which is standard M3.

4. Watch for the imperial trap

American #4-40 screws are 2.84 mm and look almost exactly like M2.5 or M3. They will start to thread and then jam or strip the hole permanently.

If a screw goes in a turn and a half and then fights you, stop. That is a different thread standard, not a tight hole.

Common questions

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