5/8″ spanner
15.88 mm · 0.63 in
5/8″ across the flats
The nearest metric size
16 mm is 16.00 mm — 0.13 mm larger than 5/8″.
That is close enough to be interchangeable. 0.13 mm is well inside the manufacturing tolerance of the fastener, and the two will feel identical in use.
The numbers
| Across flats | 15.88 mm · 0.63 in |
| Nearest metric | 16 mm — 16.00 mm |
| Difference | 0.13 mm — interchangeable |
Nearby imperial sizes
| Size | mm | Nearest metric | Gap |
|---|---|---|---|
| 1/2″ | 12.70 | 13 mm | 0.30 mm — do not |
| 9/16″ | 14.29 | 14 mm | 0.29 mm — do not |
| 5/8″ | 15.88 | 16 mm | 0.13 mm — swap |
| 11/16″ | 17.46 | 17 mm | 0.46 mm — do not |
| 3/4″ | 19.05 | 19 mm | 0.05 mm — swap |
Frequently asked questions
What is 5/8″ in millimetres?
15.88 mm. The nearest metric spanner is 16 mm — 0.13 mm larger.
Can I use 16 mm instead of 5/8″?
Yes. They are 0.13 mm apart, which is inside the tolerance of the fastener itself — these two are interchangeable in practice and most mechanics never notice they own both.
Where is 5/8″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.