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Square Nut Dimensions (DIN 557, M5 to M16)

Guide · Updated October 28, 2026

DIN 557 covers plain square nuts M5 to M16, product grade C. Square width s, width across corners e max and nut height m agree across the fasteners.eu DIN 557 table, the Kotadia DIN 557 spec page, the Inoxmare DIN 557 data sheet, the JM Arfont DIN 557 sheet and the DIV Group DIN 557 catalog page. M10 and M12 each appear in two square sizes (16 or 17 mm, 18 or 19 mm); fasteners.eu lists both and each catalog lists one, so we show both. Not shown: bearing face dw, e min and m min, because only one source gave a full set and dw for the wider M10 conflicts (14.5 vs 15.5 mm). Check the current edition or your supplier for critical work.

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Step by step

Confirm the size: DIN 557 covers plain square nuts M5 to M16, product grade C, chamfered on one side.
Read the square: s is the square width across flats: M6 10 mm, M8 13 mm, M16 24 mm.
Check which M10 or M12: M10 comes in s = 16 or 17 mm and M12 in s = 18 or 19 mm. Measure an existing nut or ask the supplier which one they stock.
Read the height: m is the nut height: M6 5 mm, M8 6.5 mm, M10 8 mm, M12 10 mm.
Allow for corners: e max is the diagonal: M8 18.4 mm. A square nut in a slot or channel needs the slot wider than s but narrower than e so it cannot spin.
Pick the wrench: Use an open-end wrench equal to s; a 6-point socket will not fit a square nut.

DIN 557 square nut dimensions (mm)

Columns: thread size (coarse pitch), square width s, width across corners e max, nut height m. M10 and M12 come in two square widths.

Size | s | e max | m
M5 | 8 | 11.3 | 4
M6 | 10 | 14.1 | 5
M8 | 13 | 18.4 | 6.5
M10 (narrow) | 16 | 22.6 | 8
M10 (wide) | 17 | 24 | 8
M12 (narrow) | 18 | 25.4 | 10
M12 (wide) | 19 | 26.9 | 10
M16 | 24 | 33.9 | 13

Worked example: square nut in a T-slot or channel

An M8 DIN 557 nut is 13 mm square and 18.4 mm across corners. In a 14 mm wide slot it slides in flat, but if it tries to turn the corners hit the walls after a few degrees, so you can tighten the bolt from one side. Bolt length: 5 mm plate + 6.5 mm nut = 11.5 mm, so an M8 x 12 just fills the nut; M8 x 16 gives a little extra.

Common problems

Wrong M10/M12 square: a 17 mm nut will not drop into a slot cut for 16 mm.
Grade C tolerance: the square is not precise; do not use it as a locating feature.
Chamfer on one side only: put the chamfered face toward the part.
Rounding the corners with an adjustable wrench: use a correct open-end wrench.

FAQ

What is a DIN 557 nut?

A plain square nut, product grade C (coarse tolerance), used in wood, slots, channels and simple assemblies where the flat sides stop it turning.

Why are there two M10 and M12 sizes?

Sources list M10 with s = 16 or 17 mm and M12 with s = 18 or 19 mm. Older and newer wrench sizes both appear in catalogs, so check before you design a slot around it.

What wrench fits a DIN 557 M8 square nut?

13 mm, the same as the square width s.

How is DIN 557 different from DIN 562?

DIN 562 is a thin square nut with a lower height. DIN 557 is the standard height square nut.

How is DIN 557 different from DIN 928?

DIN 928 is a square weld nut with corner projections and a pilot for resistance welding. DIN 557 is a plain square nut with no weld features.

What slot width holds a DIN 557 nut from turning?

Wider than s but well under e: for M8 something like 13.5 to 14 mm traps a 13 mm square that is 18.4 mm across corners.

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