Metric fasteners
Metric - spanner and hex key sizes, mm
| Thread | Hex head / nut (ISO) | Old DIN 931/934 | Socket cap (ISO 4762) | Button head (ISO 7380) | Countersunk (ISO 10642) | Grub screw (ISO 4026) |
|---|
| M3 | 5.5 | same | 2.5 | 2 | 2 | 1.5 |
| M4 | 7 | same | 3 | 2.5 | 2.5 | 2 |
| M5 | 8 | same | 4 | 3 | 3 | 2.5 |
| M6 | 10 | same | 5 | 4 | 4 | 3 |
| M8 | 13 | same | 6 | 5 | 5 | 4 |
| M10 | 16 | 17 | 8 | 6 | 6 | 5 |
| M12 | 18 | 19 | 10 | 8 | 8 | 6 |
| M14 | 21 | 22 | 12 | - | - | - |
| M16 | 24 | same | 14 | 10 | 10 | 8 |
| M18 | 27 | same | 14 | - | - | - |
| M20 | 30 | same | 17 | - | 12 | 10 |
| M22 | 34 | 32 | 17 | - | - | - |
| M24 | 36 | same | 19 | - | - | - |
| M27 | 41 | same | 19 | - | - | - |
| M30 | 46 | same | 22 | - | - | - |
| M33 | 50 | same | - | - | - | - |
| M36 | 55 | same | 27 | - | - | - |
| M42 | 65 | same | - | - | - | - |
| M48 | 75 | same | - | - | - | - |
Since ISO replaced DIN, four sizes changed across-flats: M10 17 → 16, M12 19 → 18, M14 22 → 21 and M22 32 → 34 mm. Older machines may still use the DIN sizes, so keep both spanners.
Inch fasteners
Unified (UNC/UNF) - spanner and hex key sizes
| Bolt | Hex A/F | A/F inch | A/F mm | Socket cap hex key |
|---|
| #4" | - | - | - | 3/32" |
| #6" | - | - | - | 7/64" |
| #8" | - | - | - | 9/64" |
| #10" | - | - | - | 5/32" |
| 1/4" | 7/16" | 0.4375 | 11.1 | 3/16" |
| 5/16" | 1/2" | 0.5000 | 12.7 | 1/4" |
| 3/8" | 9/16" | 0.5625 | 14.3 | 5/16" |
| 7/16" | 5/8" | 0.6250 | 15.9 | 3/8" |
| 1/2" | 3/4" | 0.7500 | 19.0 | 3/8" |
| 9/16" | 13/16" | 0.8125 | 20.6 | - |
| 5/8" | 15/16" | 0.9375 | 23.8 | 1/2" |
| 3/4" | 1-1/8" | 1.1250 | 28.6 | 5/8" |
| 7/8" | 1-5/16" | 1.3125 | 33.3 | 3/4" |
| 1" | 1-1/2" | 1.5000 | 38.1 | 3/4" |
Whitworth (BSW) spanners
| Whitworth spanner | A/F inch | A/F mm |
|---|
| 1/4" BSW | 0.445 | 11.3 |
| 5/16" BSW | 0.525 | 13.3 |
| 3/8" BSW | 0.600 | 15.2 |
| 7/16" BSW | 0.710 | 18.0 |
| 1/2" BSW | 0.820 | 20.8 |
| 5/8" BSW | 1.010 | 25.7 |
| 3/4" BSW | 1.200 | 30.5 |
| 7/8" BSW | 1.300 | 33.0 |
| 1" BSW | 1.480 | 37.6 |
Whitworth spanners are marked with the bolt size, not the across-flats size, and older BSF spanners are marked one size smaller than the BSW spanner that fits the same head.
Spanner equivalents
Inch spanner sizes and the metric spanner that fits
| Inch A/F | = mm | Smallest metric that fits | Play mm | Use metric spanner? |
|---|
| 1/4" | 6.35 | 7 | 0.65 | No - use inch spanner |
| 5/16" | 7.94 | 8 | 0.06 | Yes - close fit |
| 11/32" | 8.73 | 9 | 0.27 | Only if loose nut - may round it |
| 3/8" | 9.52 | 10 | 0.48 | Only if loose nut - may round it |
| 7/16" | 11.11 | 12 | 0.89 | No - use inch spanner |
| 1/2" | 12.70 | 13 | 0.30 | Only if loose nut - may round it |
| 9/16" | 14.29 | 15 | 0.71 | No - use inch spanner |
| 5/8" | 15.88 | 16 | 0.12 | Yes - close fit |
| 11/16" | 17.46 | 18 | 0.54 | No - use inch spanner |
| 3/4" | 19.05 | 20 | 0.95 | No - use inch spanner |
| 13/16" | 20.64 | 21 | 0.36 | Only if loose nut - may round it |
| 7/8" | 22.22 | 23 | 0.78 | No - use inch spanner |
| 15/16" | 23.81 | 24 | 0.19 | Yes - close fit |
| 1" | 25.40 | 26 | 0.60 | No - use inch spanner |
| 1-1/16" | 26.99 | 27 | 0.01 | Yes - close fit |
| 1-1/8" | 28.57 | 29 | 0.43 | Only if loose nut - may round it |
| 1-1/4" | 31.75 | 32 | 0.25 | Only if loose nut - may round it |
| 1-5/16" | 33.34 | 34 | 0.66 | No - use inch spanner |
| 1-3/8" | 34.92 | 35 | 0.08 | Yes - close fit |
| 1-7/16" | 36.51 | 37 | 0.49 | Only if loose nut - may round it |
| 1-1/2" | 38.10 | 39 | 0.90 | No - use inch spanner |
| 1-5/8" | 41.27 | 42 | 0.73 | No - use inch spanner |
| 1-11/16" | 42.86 | 43 | 0.14 | Yes - close fit |
| 1-3/4" | 44.45 | 45 | 0.55 | No - use inch spanner |
| 1-7/8" | 47.62 | 48 | 0.38 | Only if loose nut - may round it |
| 2" | 50.80 | 51 | 0.20 | Only if loose nut - may round it |
Only use a metric spanner on an inch nut (or the reverse) when the difference is very small. A loose spanner bears on the corners and rounds the nut, especially when it's tight or rusted.
