Bolt head to spanner size
Picking the right spanner means knowing a bolt’s across-flats head size, which is entirely separate from the thread diameter. An M10 bolt has a 10 mm shank, but its head takes a 17 mm spanner — confusing the two is one of the most common workshop mistakes. This reference maps standard metric and imperial bolts to their correct across-flats wrench size, plus the nearest cross-equivalent in the other system.
What “across-flats” means
A hexagonal bolt head has six flat faces. Across-flats (AF) is the distance between two opposite flat faces — the dimension a spanner or socket grips. The spanner’s nominal size equals the AF dimension of the fastener it fits. A bolt’s thread diameter (the shank) is always smaller than the AF head size: the head exists to provide grip and torque, not to indicate thread size.
Standard metric bolt head sizes
| Thread (M) | Across-flats (AF) | Notes |
|---|---|---|
| M4 | 7 mm | Smaller screws, electronics |
| M5 | 8 mm | Common in light assemblies |
| M6 | 10 mm | Very common; bicycles, furniture fittings |
| M8 | 13 mm | Automotive and general engineering |
| M10 | 17 mm | Widely used across industries |
| M12 | 19 mm | Structural and heavy machinery |
| M14 | 22 mm | Automotive wheel bolts |
| M16 | 24 mm | Large structural connections |
| M20 | 30 mm | Heavy engineering |
Some manufacturers — particularly for high-tensile bolts and flange bolts — use non-standard head sizes one step smaller, so the figures above reflect the ISO standard. Always measure the actual head if in doubt.
Standard imperial (SAE) sizes
Imperial spanners are sized in fractions of an inch by their AF dimension:
| Common SAE AF | Approx. metric equivalent | Typical application |
|---|---|---|
| 1/4 in (6.35 mm) | — | Instrument and fine work |
| 3/8 in (9.52 mm) | 10 mm (close, not exact) | Light automotive |
| 7/16 in (11.11 mm) | 11 mm | American cars |
| 1/2 in (12.7 mm) | 13 mm (loose) | Common American bolt |
| 9/16 in (14.29 mm) | 14 mm (loose) | American automotive |
| 5/8 in (15.88 mm) | 16 mm (loose) | Common pairing |
| 3/4 in (19.05 mm) | 19 mm (nearly exact) | Often interchangeable |
“Loose” means the metric spanner is slightly too large and risks rounding the head under torque. Never use a substitute that is too small — it will not fit. Always use the correctly matched system when available.
Coarse vs fine thread: does it change the spanner?
No. The across-flats head dimension depends only on the bolt’s nominal thread diameter (M-size), not the thread pitch. An M10 coarse (1.5 mm pitch) and an M10 fine (1.25 mm pitch) bolt both take the same 17 mm spanner. Thread pitch only matters when selecting a matching nut or tapped hole.
Common mistakes
- Confusing M-size with spanner size — “I need a 10 mm spanner for an M10 bolt” is wrong. M10 → 17 mm spanner.
- Using a metric substitute on imperial — a 13 mm spanner on a 1/2 in (12.7 mm) AF head is 0.3 mm too large and will round the corners on a stubborn fastener.
- Ignoring flange bolts — some flange bolt heads sit one size smaller than the equivalent non-flange head.
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