Metric screw sizes, spanners and drill sizes
An M8 bolt is not 8 mm across the head and not 8 mm across the thread valleys. The number is the nominal outside diameter of the thread, and almost everything else you need — which spanner fits, which drill to use before tapping, which hole to drill to let the bolt through — follows from it by convention rather than by calculation.
Reading the designation
M8×1.25 means an eight-millimetre nominal diameter with 1.25 mm between adjacent thread crests. That 1.25 is the coarse pitch, the default when no pitch is written. Fine pitches exist for the same diameter — M8×1 for example — and they will start into a coarse thread and then bind, which is the most common way of ruining a good hole.
Measuring an unknown screw
Measure across the outside of the thread with a caliper and round up to the nearest standard size; threads are always slightly under nominal because of manufacturing tolerance, so 7.8 mm reads as M8. For the pitch, lay the screw against a scale, count ten crests and divide the distance by ten — a single crest-to-crest gap is too small to judge by eye.
Tapping drill versus clearance drill
Two different holes get confused constantly. The tapping drill is smaller than the bolt and leaves material for the tap to cut a thread into — the rule of thumb is diameter minus pitch. The clearance drill is larger than the bolt so it passes through freely. Drill a tapping hole where you needed clearance and the bolt jams halfway; drill clearance where you needed a thread and there is nothing left to cut.
| Thread | Coarse pitch | Spanner | Tapping drill | Clearance drill |
|---|---|---|---|---|
| M3 | 0.50 mm | 5.5 mm | 2.5 mm | 3.2 mm |
| M4 | 0.70 mm | 7 mm | 3.3 mm | 4.3 mm |
| M5 | 0.80 mm | 8 mm | 4.2 mm | 5.3 mm |
| M6 | 1.00 mm | 10 mm | 5.0 mm | 6.4 mm |
| M8 | 1.25 mm | 13 mm | 6.8 mm | 8.4 mm |
| M10 | 1.50 mm | 17 mm | 8.5 mm | 10.5 mm |
| M12 | 1.75 mm | 19 mm | 10.2 mm | 13 mm |
| M14 | 2.00 mm | 22 mm | 12.0 mm | 15 mm |
| M16 | 2.00 mm | 24 mm | 14.0 mm | 17 mm |
Frequently asked questions
Why do some M10 bolts take a 16 mm spanner?
Head sizes were reduced in the newer DIN/ISO ranges to save material. Older stock takes 17 mm, newer takes 16 mm, and both are correct for their standard.
Can I measure a screw with my phone?
Yes, if the screen is calibrated. Lay the screw against an on-screen caliper and match the jaws to the thread's outside diameter; that gets you close enough to name the size.
What is the tapping drill rule again?
Nominal diameter minus the pitch. M6 with a 1.0 mm pitch wants a 5.0 mm hole; M8 with 1.25 wants 6.75, and 6.8 is the drill that exists.
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