Nut Size Chart — Metric & SAE Nut Sizes

Metric nut sizes are named for the thread, so an M10 nut takes an M10 bolt — but the tool is a 17 mm wrench. That gap between thread size and wrench size is where most mixed-toolbox mistakes happen. The rule of thumb across the whole range is that across the flats runs about 1.5 to 1.8 times the thread diameter, tightening toward 1.5 as the sizes get larger.

Small sizes matter for panel and enclosure hardware. M2 nut dimensions are 4 mm across the flats and 1.6 mm thick on a 0.4 mm coarse pitch; M2.5 is 5 mm across the flats and 2.0 mm thick. Below M3 a nut driver or a small ratcheting wrench does far less damage than an adjustable spanner, because there is almost no corner material to grip.

Thread sizes come in coarse and fine pitch for the same nominal diameter. The pitch column above is coarse, which is what is stocked by default: M6 × 1.0, M8 × 1.25, M10 × 1.5, M12 × 1.75. A fine-pitch nut of the same nominal size — M10 × 1.25, for example — looks identical and takes the same wrench but will not thread onto a coarse bolt past a turn or two. If a nut starts and then binds, check the pitch before forcing it.

Nut size and grade are separate specifications. A plain hex nut is roughly 0.8 to 0.9 times the bolt diameter thick and is intended to develop the full strength of a property class 8.8 bolt. Jam nuts are about half that thickness and are for locking, not load. Nylon-insert lock nuts are taller than the chart value, so allow extra bolt projection when you swap one in.