18 mm spanner
18.00 mm · 0.71 in
18 mm across the flats · fits a M12 (ISO) hex head
The nearest imperial size
11/16″ is 17.46 mm — 0.54 mm smaller than 18 mm.
That is not close enough. 11/16″ is smaller than the 18 mm head, so it will not go on at all — which is the harmless failure of the two.
The numbers
| Across flats | 18.00 mm · 0.71 in |
| Nearest imperial | 11/16″ — 17.46 mm |
| Difference | 0.54 mm — not safe to substitute |
| Fits | M12 (ISO) hex head |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 15 mm | 15.00 | 9/16″ | 0.71 mm — do not |
| 16 mm | 16.00 | 5/8″ | 0.13 mm — swap |
| 17 mm | 17.00 | 11/16″ | 0.46 mm — do not |
| 18 mm | 18.00 | 11/16″ | 0.54 mm — do not |
| 19 mm | 19.00 | 3/4″ | 0.05 mm — swap |
| 21 mm | 21.00 | 13/16″ | 0.36 mm — do not |
Frequently asked questions
What is 18 mm in inches?
0.71 inches. The nearest imperial spanner is 11/16″ at 17.46 mm, 0.54 mm smaller.
Can I use 11/16″ instead of 18 mm?
Not safely. The gap is 0.54 mm. 11/16″ is smaller than 18 mm and simply will not go on.
What bolt does a 18 mm spanner fit?
A M12 (ISO) hex head. The full metric thread chart gives the tapping drill and clearance hole for it too.