UnitCharts

Wrench & Spanner Size Chart (Metric)

Wrench and socket sizes — the across-flats dimension of the fastener is the spanner size that fits it.

FastenerSizeAcross flats (mm)inch equivalentNotes
The across-flats dimension of a hex fastener is the wrench size — measure across the flats, not the corners. The classic metric mix-up: old DIN 931 / 933 bolts use 17 / 19 / 22 mm where ISO 4014 uses 16 / 18 / 21 mm, so a set that fits one will not fit the other. On a rusted or damaged fastener, use the size that fits snugly rather than forcing the next one up. Sources: ISO 4014, ISO 272, ISO 4762.

Standard: ISO 4014, ISO 272, ISO 4762  ·  Data checked: 2026-10-14

Frequently asked questions

Why do I need both metric and imperial spanners?

Because the two systems do not line up exactly, and the near-misses are the dangerous ones. A 5/16 spanner is 7.94 mm and often fits an M5 head at 8 mm, but 11/16 is 17.46 mm where the old DIN M10 head is 17 mm — close enough to go on, loose enough to round the corners.

What is the difference between a 6-point and a 12-point socket?

A 6-point socket contacts the flats and grips better, which matters on tight or damaged fasteners. A 12-point socket is easier to get onto a fastener in a confined space because it engages at twice as many angles. For a rusty or rounded bolt, use 6-point.

Should I use the size in the middle of the range or the closest fit?

The closest fit that goes on without force. If a spanner needs to be tapped on, it is the wrong size — you are deforming the head. If it rocks, it will round the corners under load. Both destroy fasteners on the way out.

Why does my socket set skip sizes?

Because sets are built around the common fastener sizes, not a continuous range. The gaps are usually in the sizes that only appear on old DIN fasteners or on specialist equipment, which is exactly when people discover the gap.

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