15 mm spanner
15.00 mm · 0.59 in
15 mm across the flats
The nearest imperial size
9/16″ is 14.29 mm — 0.71 mm smaller than 15 mm.
That is not close enough. 9/16″ is smaller than the 15 mm head, so it will not go on at all — which is the harmless failure of the two.
The numbers
| Across flats | 15.00 mm · 0.59 in |
| Nearest imperial | 9/16″ — 14.29 mm |
| Difference | 0.71 mm — not safe to substitute |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 12 mm | 12.00 | 1/2″ | 0.70 mm — do not |
| 13 mm | 13.00 | 1/2″ | 0.30 mm — do not |
| 14 mm | 14.00 | 9/16″ | 0.29 mm — do not |
| 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 |
Frequently asked questions
What is 15 mm in inches?
0.59 inches. The nearest imperial spanner is 9/16″ at 14.29 mm, 0.71 mm smaller.
Can I use 9/16″ instead of 15 mm?
Not safely. The gap is 0.71 mm. 9/16″ is smaller than 15 mm and simply will not go on.
Where is 15 mm used?
This size sits between the standard hex head sizes rather than on one, so it turns up on proprietary fasteners, plumbing fittings and older machinery rather than on ordinary bolts.