Fastener standards
Fasteners are ordered by standard number, and there are two systems in use for the same parts: the German DIN numbers, which are mostly withdrawn but still what everyone writes on a drawing, and the ISO numbers that replaced them. This maps one to the other and says where they disagree.
The disagreement worth knowing about
At M10, M12, M14 — DIN specifies 17 mm, 19 mm, 22 mm across the flats where ISO specifies 16, 18, 21 mm. Both bolts are the same thread and go in the same hole; only the head differs. A socket set built around the DIN sizes will meet bolts it cannot grip, which is why metric sets sold today usually include both 16 and 17 rather than one of them.
Hex head bolts
The two DIN numbers here differ only in whether the thread runs all the way to the head. Both map to ISO numbers that changed the head size at three of the thirteen sizes.
| DIN | ISO | What it is |
|---|---|---|
| DIN 933 | ISO 4017 | Hexagon head bolts threaded all the way to the head — DIN’s own title calls these “set screws”, which today reads as a grub screw and is not what this is |
| DIN 931 | ISO 4014 | Hexagon head bolts, partially threaded — a plain shank under the head |
Socket head cap screws
Driven by a hex key rather than a spanner. Here the DIN and ISO standards agree, and suppliers list them on one line.
| DIN | ISO | What it is |
|---|---|---|
| DIN 912 | ISO 4762 | Hexagon socket head cap screws — the cylindrical head driven by a hex key |
Hex nuts
The counterpart to the bolts above. These pages carry the standard and its equivalent, not a dimension table — see below for why.
| DIN | ISO | What it is |
|---|---|---|
| DIN 934 | ISO 4032 | Hexagon nuts — the plain hex nut |
Where the numbers on these pages come from
None of the dimensions here are transcribed from a catalogue. The standards themselves are paid documents and the free copies circulating are scans of supplier tables, which is the kind of source that produces a confident wrong figure. Every millimetre on these pages comes from the thread data this site already carries and checks, or is computed from it — across corners, for instance, is the across-flats divided by cos 30°, which the build verifies for every size.
Where that data does not reach, the pages say so rather than filling the gap. There is no nut dimension table here for that reason.
Metric thread sizes · Tap drill sizes · Spanner sizes · Screw sizes