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Spanner and hex-key sizes for metric hexagon bolts, nuts and socket cap screws, M1.6 to M64. The spanner size is the across-flats (A/F) width of the bolt head, set by ISO 4014 and ISO 4017 for hex bolts and ISO 4032 for nuts. An M10 hex bolt takes a 16 mm spanner under the current ISO standard, an M12 takes 18 mm, an M16 takes 24 mm and an M36 takes 55 mm. A hex nut takes the same spanner as the bolt it fits.
| Bolt size |
Spanner A/F ISO (mm) |
Older DIN A/F (mm) |
Hex key (mm) |
| M1.6 | 3.2 | - | 1.5 |
| M2 | 4 | - | 1.5 |
| M2.5 | 5 | - | 2 |
| M3 | 5.5 | - | 2.5 |
| M3.5 | 6 | - | - |
| M4 | 7 | - | 3 |
| M5 | 8 | - | 4 |
| M6 | 10 | - | 5 |
| M8 | 13 | - | 6 |
| M10 | 16 | 17 | 8 |
| M12 | 18 | 19 | 10 |
| M14 | 21 | 22 | 12 |
| M16 | 24 | - | 14 |
| M18 | 27 | - | 14 |
| M20 | 30 | - | 17 |
| M22 | 34 | 32 | 17 |
| M24 | 36 | - | 19 |
| M27 | 41 | - | 19 |
| M30 | 46 | - | 22 |
| M33 | 50 | - | - |
| M36 | 55 | - | - |
| M39 | 60 | - | - |
| M42 | 65 | - | - |
| M45 | 70 | - | - |
| M48 | 75 | - | - |
| M52 | 80 | - | - |
| M56 | 85 | - | - |
| M60 | 90 | - | - |
| M64 | 95 | - | - |
What size bolt fits my spanner?
Read this table when you know the spanner and need the thread. Measure across the flats of the head or nut, find that figure in the left column and read off the bolt size. Sizes marked older DIN are the pre-ISO head widths still found on older machinery and on a lot of current stock.
| Spanner size (mm) |
Fits bolt and nut size |
| 3.2 | M1.6 |
| 4 | M2 |
| 5 | M2.5 |
| 5.5 | M3 |
| 6 | M3.5 |
| 7 | M4 |
| 8 | M5 |
| 10 | M6 |
| 13 | M8 |
| 16 | M10 |
| 17 | M10 (older DIN bolts) |
| 18 | M12 |
| 19 | M12 (older DIN bolts) |
| 21 | M14 |
| 22 | M14 (older DIN bolts) |
| 24 | M16 |
| 27 | M18 |
| 30 | M20 |
| 32 | M22 (older DIN bolts) |
| 34 | M22 |
| 36 | M24 |
| 41 | M27 |
| 46 | M30 |
| 50 | M33 |
| 55 | M36 |
| 60 | M39 |
| 65 | M42 |
| 70 | M45 |
| 75 | M48 |
| 80 | M52 |
| 85 | M56 |
| 90 | M60 |
| 95 | M64 |
A 13 mm spanner fits an M8 bolt, a 17 mm spanner fits an M10 bolt made to the older DIN standard, a 19 mm spanner fits a DIN M12 and a 24 mm spanner fits an M16.
Large bolts: M33 to M64
Above M30 the spanner sizes climb quickly: M36 takes 55 mm, M42 takes 65 mm, M48 takes 75 mm, M56 takes 85 mm and M64 takes 95 mm. M33, M39, M45, M52 and M60 are second-choice sizes in the ISO series, so they are less often stocked but turn up on older plant and structural work.
Imperial spanner sizes: AF, Whitworth, BSF and BA
Imperial fasteners use three unrelated spanner systems, and a spanner from one will not fit another. AF spanners fit Unified (UNC and UNF) bolts and are marked with the across-flats width. Whitworth and BSF spanners are marked with the bolt diameter, not the head width. BA spanners are marked with the BA number.
AF spanner sizes for UNC and UNF bolts
Across-flats widths for standard hex bolts and hex nuts to ASME B18.2.1 and B18.2.2. The nut takes the same spanner as the bolt head on every size except 7/16 inch, where the nut is 11/16 inch and the bolt head is 5/8 inch and 9/16 inch, where the nut is 7/8 inch and the bolt head is 13/16 inch.
| Bolt diameter (UNC/UNF) |
Bolt head A/F |
Bolt head A/F (mm) |
Hex nut A/F |
Hex nut A/F (mm) |
| 1/4" | 7/16" | 11.11 | 7/16" | 11.11 |
| 5/16" | 1/2" | 12.70 | 1/2" | 12.70 |
| 3/8" | 9/16" | 14.29 | 9/16" | 14.29 |
| 7/16" | 5/8" | 15.88 | 11/16" | 17.46 |
| 1/2" | 3/4" | 19.05 | 3/4" | 19.05 |
| 9/16" | 13/16" | 20.64 | 7/8" | 22.23 |
| 5/8" | 15/16" | 23.81 | 15/16" | 23.81 |
| 3/4" | 1-1/8" | 28.58 | 1-1/8" | 28.58 |
| 7/8" | 1-5/16" | 33.34 | 1-5/16" | 33.34 |
| 1" | 1-1/2" | 38.10 | 1-1/2" | 38.10 |
| 1-1/8" | 1-11/16" | 42.86 | 1-11/16" | 42.86 |
| 1-1/4" | 1-7/8" | 47.63 | 1-7/8" | 47.63 |
| 1-3/8" | 2-1/16" | 52.39 | 2-1/16" | 52.39 |
| 1-1/2" | 2-1/4" | 57.15 | 2-1/4" | 57.15 |
Whitworth and BSF spanner sizes
A Whitworth spanner is marked with the diameter of the Whitworth bolt it was originally made for, so a spanner marked 3/8 W measures 0.710 inch (18.03 mm) across the jaws. BSF bolts use a head one size smaller than Whitworth bolts of the same diameter, so the same spanner fits the BSF bolt one size up: the 3/8 W spanner fits a 7/16 inch BSF bolt. Spanners are often marked with both sizes for that reason.
| Spanner marking |
A/F (in) |
A/F (mm) |
Fits BSW bolt (original hexagon) |
Fits BSF bolt |
| 3/16 W | 0.445 | 11.30 | 3/16" | 1/4" |
| 1/4 W | 0.525 | 13.34 | 1/4" | 5/16" |
| 5/16 W | 0.600 | 15.24 | 5/16" | 3/8" |
| 3/8 W | 0.710 | 18.03 | 3/8" | 7/16" |
| 7/16 W | 0.820 | 20.83 | 7/16" | 1/2" |
| 1/2 W | 0.920 | 23.37 | 1/2" | 9/16" |
| 9/16 W | 1.010 | 25.65 | 9/16" | 5/8" |
| 5/8 W | 1.100 | 27.94 | 5/8" | 11/16" |
| 11/16 W | 1.200 | 30.48 | 11/16" | 3/4" |
| 3/4 W | 1.300 | 33.02 | 3/4" | 7/8" |
| 7/8 W | 1.480 | 37.59 | 7/8" | 1" |
| 1 W | 1.670 | 42.42 | 1" | 1-1/8" |
| 1-1/8 W | 1.860 | 47.24 | 1-1/8" | 1-1/4" |
BA spanner sizes
British Association (BA) threads run from 0BA, the largest, to 10BA and smaller. They are found on older British electrical and instrument work.
| BA size |
A/F (in) |
A/F (mm) |
| 0BA | 0.413 | 10.5 |
| 1BA | 0.365 | 9.275 |
| 2BA | 0.324 | 8.225 |
| 3BA | 0.282 | 7.175 |
| 4BA | 0.248 | 6.3 |
| 5BA | 0.220 | 5.6 |
| 6BA | 0.193 | 4.9 |
| 7BA | 0.172 | 4.375 |
| 8BA | 0.152 | 3.85 |
| 9BA | 0.131 | 3.325 |
| 10BA | 0.117 | 2.975 |
Socket sizes for metric bolts and nuts
A socket is sized by the same across-flats (A/F) width as a spanner, so the socket for a bolt or nut is the spanner size in the chart above. An M8 bolt takes a 13 mm socket, an M10 takes 16 mm, an M12 takes 18 mm and an M16 takes 24 mm. The most searched sizes are repeated here for quick reference, with the older DIN figure beside the ISO one because some bolts and nuts, mostly older or imported ones, still use it.
| Bolt or nut |
Socket size, ISO (mm) |
Socket size, older DIN (mm) |
| M4 | 7 | 7 |
| M5 | 8 | 8 |
| M6 | 10 | 10 |
| M8 | 13 | 13 |
| M10 | 16 | 17 (differs) |
| M12 | 18 | 19 (differs) |
| M14 | 21 | 22 (differs) |
| M16 | 24 | 24 |
| M18 | 27 | 27 |
| M20 | 30 | 30 |
| M22 | 34 | 32 (differs) |
| M24 | 36 | 36 |
| M27 | 41 | 41 |
| M30 | 46 | 46 |
Three sizes need care. An M10 bolt or nut can be 16 mm (ISO) or 17 mm (DIN), an M12 can be 18 or 19 mm, and an M14 can be 21 or 22 mm. M22 runs the other way, 34 mm in ISO against 32 mm in DIN. If the socket feels loose, try the neighbouring size before forcing it, because a socket that is slightly too large rounds the corners of the fastener.
A 6-point socket grips the flats of the head and is the better choice for tight or corroded fasteners. A 12-point socket is easier to fit in a confined space because it engages at twice as many angles. Socket cap screws take a hex key, not a hex socket, and their key sizes are in the table at the top of this page and the hex key sizes chart.
How to find the right spanner size
Match the bolt thread size in the first column to the across-flats spanner size. The thread size (M10) is the diameter of the thread, not the spanner size - an M10 bolt doesn't take a 10 mm spanner. If you don't know the thread size, measure the across-flats width of the head with calipers and read the chart backwards.
Two width standards exist. Current ISO 4014/4017 sets M10 at 16 mm and M12 at 18 mm. Older DIN 931/933 bolts, still common in machinery built before the 1990s, used 17 mm for M10 and 19 mm for M12. The DIN column lists every size that differs: M10 (17 mm DIN against 16 mm ISO), M12 (19 mm DIN against 18 mm ISO), M14 (22 mm DIN against 21 mm ISO), M22 (32 mm DIN against 34 mm ISO). Carry both spanners for those sizes if you work on mixed-age equipment.
Socket cap screws (ISO 4762) use a hex key, not a spanner, and the key size is much smaller than the head: an M10 cap screw takes an 8 mm hex key. See the metric bolt torque chart for the tightening torque once you have the right tool engaged.
Related standards
- ISO 4014 / ISO 4017 - Hexagon head bolts and set screws (across-flats widths)
- ISO 4032 - Hexagon regular nuts (same across-flats widths)
- ISO 4762 - Hexagon socket head cap screws (hex-key sizes)
- DIN 931 / DIN 933 - Older hex bolt standard (17 mm for M10, 19 mm for M12)
Per-size nut cards: M6 · M8 · M10 · M12 · M16 · M20 (A/F, thickness and DIN 934 differences per size).
Related reference pages: washer dimensions chart.
Load figures for each bolt size and class are on the bolt proof load chart.
Every MLC reference tool in one place: engineering calculators and charts.