Metric Bolt Dimension Chart — Coarse & Fine Pitch, Head & Wrench Sizes
Metric bolt sizes M3–M30: ISO 261/262 coarse and fine pitches, ISO 4014/4017 hex head across-flats and heights with DIN 931 splits, and ISO 4762 socket heads.
Updated August 18, 2026
Three separate ISO standards answer the three questions a metric bolt raises at the bench: ISO 261/262 fix the thread pitch (what nut fits), ISO 4014/4017 fix the hex head (what wrench fits), and ISO 4762 fixes the socket head (what counterbore and hex key fit). The pitch table below lists the coarse pitch for M3–M30 plus the fine pitches ISO 262 actually selects for fasteners — the fine selection only starts at M8, and it is deliberately narrower than the full ISO 261 pitch series, which offers extra-fine options (M6 × 0.75, M10 × 0.75) that standard bolts are not made in.
The hex-head table carries a trap worth a footnote: when ISO 4014/4017 superseded DIN 931/933, four across-flats sizes changed. An ISO M10 bolt takes a 16 mm wrench but the DIN legacy head — still common on older machines and in some suppliers’ stock — takes 17 mm; likewise M12 (18 vs 19), M14 (21 vs 22), and M22, where ISO went larger (34 vs 32). Head heights did not change. Socket head cap screws are simpler: the ISO 4762 head is exactly as tall as the nominal diameter at every size and about 1.5 × the diameter across, and the key sizes repeat at M18, M22, and M27, which take the same key as the size below.
Thread Pitch — ISO 261 Coarse & ISO 262 Fine Selection, M3–M30
| Size | Coarse pitch (mm) | ISO 262 fine pitch(es) (mm) |
|---|---|---|
| M3 | 0.5 | — |
| M4 | 0.7 | — |
| M5 | 0.8 | — |
| M6 | 1 | — |
| M8 | 1.25 | 1 |
| M10 | 1.5 | 1.25, 1 |
| M12 | 1.75 | 1.5, 1.25 |
| M14 | 2 | 1.5 |
| M16 | 2 | 1.5 |
| M18 | 2.5 | 2, 1.5 |
| M20 | 2.5 | 2, 1.5 |
| M22 | 2.5 | 2, 1.5 |
| M24 | 3 | 2 |
| M27 | 3 | 2 |
| M30 | 3.5 | 2 |
“—” = coarse only: the ISO 262 fine selection for screws, bolts and nuts starts at M8. ISO 261 lists further fine pitches (e.g. M6 × 0.75, M10 × 0.75) that ISO 262 does not select for standard fasteners. M14, M18, M22 and M27 are ISO 262 second-choice diameters.
Hex Bolt Heads — ISO 4014/4017, Product Grade A, with DIN 931/933 Legacy Splits
| Size | Across flats s (mm) | Head height k (mm) | DIN 931/933 legacy s (mm) |
|---|---|---|---|
| M3 | 5.5 | 2 | — |
| M4 | 7 | 2.8 | — |
| M5 | 8 | 3.5 | — |
| M6 | 10 | 4 | — |
| M8 | 13 | 5.3 | — |
| M10 | 16 | 6.4 | 17 |
| M12 | 18 | 7.5 | 19 |
| M14 | 21 | 8.8 | 22 |
| M16 | 24 | 10 | — |
| M18 | 27 | 11.5 | — |
| M20 | 30 | 12.5 | — |
| M22 | 34 | 14 | 32 |
| M24 | 36 | 15 | — |
| M27 | 41 | 17 | — |
| M30 | 46 | 18.7 | — |
s is the across-flats maximum (= wrench size), k the nominal head height — identical for ISO 4014 (part thread) and 4017 (full thread). “—” in the DIN column = identical to ISO; the standards differ ONLY at M10/M12/M14 (DIN one size larger) and M22 (DIN smaller, 32 vs 34). M27 and M30 are product grade B in ISO 4014 (same nominals, wider tolerances).
Socket Head Cap Screws — ISO 4762 (DIN 912)
| Size | Head Ø dk max (mm) | Head height k (mm) | Hex key s (mm) |
|---|---|---|---|
| M3 | 5.5 | 3 | 2.5 |
| M4 | 7 | 4 | 3 |
| M5 | 8.5 | 5 | 4 |
| M6 | 10 | 6 | 5 |
| M8 | 13 | 8 | 6 |
| M10 | 16 | 10 | 8 |
| M12 | 18 | 12 | 10 |
| M14 | 21 | 14 | 12 |
| M16 | 24 | 16 | 14 |
| M18 | 27 | 18 | 14 |
| M20 | 30 | 20 | 17 |
| M22 | 33 | 22 | 17 |
| M24 | 36 | 24 | 19 |
| M27 | 40 | 27 | 19 |
| M30 | 45 | 30 | 22 |
dk is the smooth-head maximum diameter and k the maximum (= nominal) head height. k = d at every size; dk runs about 1.5·d — exactly 1.5·d at M12–M24 and M30, slightly larger below M12 (M5 8.5, M6 10, M8 13, M10 16) and 40 at M27. Key sizes repeat at M18 (14, as M16), M22 (17, as M20) and M27 (19, as M24). ISO 4762 also tabulates slightly larger knurled-head diameters.
Sources & Further Reading
- ISO 261 / ISO 262 — metric screw thread general plan and selected sizes, per the Bossard Metric ISO Threads technical sheet (01-2025) and Brighton-Best PDS Thread Details Metric Series (cross-checked cell-for-cell)
- ISO 4014 / ISO 4017 — hex head bolts and screws, product grade A, per the Intafast ISO 4014 dimension tables, Bolting Specialist, and Westfield Fasteners; DIN 931/933 legacy splits per the Wermac DIN-vs-ISO comparison
- ISO 4762 (DIN 912) — hexagon socket head cap screws, per the Intafast ISO 4762 dimension tables, Engineering Hardware, and SSR Precision (cross-checked)
Frequently Asked Questions
Why does my M10 bolt take a 17 mm wrench when the chart says 16?
You have a DIN 931/933 legacy head. When the ISO standards took over, the across-flats shrank at M10 (17 → 16 mm), M12 (19 → 18) and M14 (22 → 21), and grew at M22 (32 → 34). European suppliers still stock both patterns, and older machines are full of DIN heads — so a metric wrench set needs both sizes either way.
When should I pick a fine-pitch thread over coarse?
Fine pitch buys a larger stress area (a stronger bolt from the same blank), finer adjustment, and better resistance to vibration loosening because the helix angle is shallower. Coarse wins for assembly speed, tolerance of nicks and dirt, and tapping into soft materials. That is why coarse is the default and ISO 262 only selects fine pitches from M8 up.
What hex key does an M8 socket head cap screw take?
A 6 mm key. The ISO 4762 pattern is easy to memorize: head height equal to the size at every diameter, head diameter about 1.5 × the size, and the key runs 4, 5, 6, 8, 10 mm for M5–M12. Above that the key repeats at some sizes — M16 and M18 both take 14 mm, M20 and M22 take 17 mm, M24 and M27 take 19 mm.
Are ISO 4014 and ISO 4017 heads different?
No — the head and wrench dimensions are identical. ISO 4014 is the partially threaded bolt (a plain shank under the head) and ISO 4017 the fully threaded screw. Pick 4014 when the shank crosses a shear plane or locates parts; pick 4017 when you need thread engagement right up to the head.
Try the Calculators
Metric Bolt Dimension Chart — Coarse & Fine Pitch, Head & Wrench Sizes — reuven.tools/reference/metric-bolt-dimensions — verified against: ISO 261 / ISO 262 — metric screw thread general plan and selected sizes, per the Bossard Metric ISO Threads technical sheet (01-2025) and Brighton-Best PDS Thread Details Metric Series (cross-checked cell-for-cell); ISO 4014 / ISO 4017 — hex head bolts and screws, product grade A, per the Intafast ISO 4014 dimension tables, Bolting Specialist, and Westfield Fasteners; DIN 931/933 legacy splits per the Wermac DIN-vs-ISO comparison; ISO 4762 (DIN 912) — hexagon socket head cap screws, per the Intafast ISO 4762 dimension tables, Engineering Hardware, and SSR Precision (cross-checked)