Metric Bolt Size Calculator
Pick a metric bolt size and see everything that follows from it: coarse and fine pitch, the wrench or socket size, head dimensions, the hex key for a socket head screw, and the tap drill you would use to tap the hole. Values follow ISO 4014, ISO 4762, ISO 724, ISO 898-1 and ISO 68-1.
What the numbers mean
- Across flats (s) — the distance across the head flats. This is the wrench or socket size, so it is the number people actually need.
- Across corners (e) — the distance across opposite corners. Always larger than s; it is what a twelve-point socket or a worn hex key has to clear.
- Tap drill — the drill used before cutting the thread. For metric coarse threads this is close to nominal diameter minus pitch (M8 × 1.25 → 6.75 mm), giving roughly a 75% thread. A smaller hole makes a stronger thread but is harder to tap and risks breaking the tap.
- Tensile stress area — the effective cross-section through the threaded portion, not the shank. Multiply it by the bolt's strength class to get its load capacity: an M12 class 8.8 bolt carries about 84.3 mm² × 800 MPa ≈ 67 kN.
Selected values
| Size | Coarse pitch (mm) | Across flats (mm) | Hex key (mm) | Tap drill (mm) | Stress area (mm²) |
|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 | 2.5 | 2.5 | 5.03 |
| M4 | 0.7 | 7.0 | 3.0 | 3.3 | 8.78 |
| M5 | 0.8 | 8.0 | 4.0 | 4.2 | 14.2 |
| M6 | 1.0 | 10.0 | 5.0 | 5.0 | 20.1 |
| M8 | 1.25 | 13.0 | 6.0 | 6.75 | 36.6 |
| M10 | 1.5 | 16.0 | 8.0 | 8.5 | 58.0 |
| M12 | 1.75 | 18.0 | 10 | 10.2 | 84.3 |
| M14 | 2.0 | 21.0 | 12 | 12.0 | 115 |
| M16 | 2.0 | 24.0 | 14 | 14.0 | 157 |
| M18 | 2.5 | 27.0 | 14 | 15.5 | 192 |
| M20 | 2.5 | 30.0 | 17 | 17.5 | 245 |
| M24 | 3.0 | 36.0 | 19 | 21.0 | 353 |
| M30 | 3.5 | 46.0 | 22 | 26.5 | 561 |
| M36 | 4.0 | 55.0 | 27 | 32.0 | 817 |
Standard: ISO 4014 / 4017 (hex head), ISO 4762 (socket head), ISO 724 / 898-1 (thread & strength), ISO 68-1 (tap drill) · Data checked: 2026-10-14
This is a dimension lookup, and nothing more. Which strength class you need, what torque to tighten to,
what preload is appropriate, and whether a given bolt can carry a given load all depend on the material, the load, the
joint, the environment and the code that applies. A size table cannot answer any of those, and guessing from one is how
joints fail. Work those out from the governing standard or with a qualified engineer.
Things people get wrong
- The old DIN head sizes. Withdrawn DIN 931 / 933 bolts used 17, 19 and 22 mm across flats at M10, M12 and M14, while ISO 4014 uses 16, 18 and 21 mm. A socket set that fits one will round the other. If a bolt has been in a drawer for thirty years, measure it.
- Coarse is the default. If a drawing says "M10" and nothing else, it almost always means M10 × 1.5 coarse. Fine pitch has to be written out, e.g. M10 × 1.25.
- Washers are not optional with high grades. A soft washer under a class 12.9 bolt collapses before the bolt reaches preload.
- Pitch affects more than fit. Fine threads have a larger minor diameter, so they are stronger in tension and resist vibration loosening better, but they cross-thread more easily and are less tolerant of dirt.