What does across-flats mean?

Across-flats (AF) is the distance between two opposite flat faces of a hexagonal nut or bolt head — the dimension a spanner or socket grips. It is the same as the spanner's nominal size. For example, a standard M10 bolt has a 17 mm across-flats head, so it needs a 17 mm spanner.

Why doesn't the bolt diameter match the spanner size?

The thread diameter (M10 = 10 mm shank) is much smaller than the hex head it drives. The head is sized for grip and torque, so an M10 takes a 17 mm spanner, an M12 a 19 mm, and so on. Always size the spanner to the head, never the thread.

Can I use a metric spanner on an imperial bolt?

Sometimes, with care. Some sizes are very close — a 13 mm spanner nearly fits a 1/2-inch AF head (12.7 mm) but is slightly loose and can round corners. Use the correctly matched system whenever possible; the cross-equivalents here are emergency substitutes, not exact fits.

What is the difference between coarse and fine thread for spanner size?

The spanner size depends only on the hex head's across-flats dimension, not the thread pitch. An M10 coarse and M10 fine bolt share the same 17 mm head, so the same spanner fits both. Thread pitch only matters for the matching nut, not the wrench.

Is this reference processed online?

No. All size data and the lookup run locally in your browser. Nothing is uploaded.

Nut & Spanner Size Reference

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Bolt head to spanner size

Picking the right spanner means knowing a bolt’s across-flats head size, which is unrelated to its thread diameter. This reference maps standard metric and imperial bolts and nuts to their across-flats wrench size, and shows the nearest size in the other system for when your toolkit doesn’t match the fastener.

How it works

The number that matters for tools is across-flats (AF) — the distance between opposite faces of the hex head, which equals the spanner’s nominal size. A bolt’s thread (e.g. M10 = a 10 mm shank) is much smaller than its head. Standard heads follow fixed conventions: M6→10 mm, M8→13 mm, M10→17 mm, M12→19 mm, and so on. Imperial AF spanners are sized in inches to fit SAE bolt heads. The cross-equivalent column gives the closest size in the other system; small differences (fractions of a millimetre) mean a substitute may be loose and risk rounding the head.

Tips and examples

Always size the spanner to the head, not the thread — fitting a “10 mm spanner” to an M10 bolt is the classic mistake; an M10 takes a 17 mm spanner. Coarse and fine threads of the same M-size share a head, so one spanner fits both. When metric and imperial mix in a garage, prefer the matched system: a 13 mm spanner on a 1/2″ AF head (12.7 mm) is slightly loose and rounds corners under torque. Newer cars and older equipment with flange or low-profile heads may use one size smaller than these standards, so always check the actual head before reaching for a tool.