8 mm spanner
8.00 mm · 0.31 in
8 mm across the flats · fits a M5 hex head
The nearest imperial size
5/16″ is 7.94 mm — 0.06 mm smaller than 8 mm.
That is close enough to be interchangeable. 0.06 mm is well inside the manufacturing tolerance of the fastener, and the two will feel identical in use.
The numbers
| Across flats | 8.00 mm · 0.31 in |
| Nearest imperial | 5/16″ — 7.94 mm |
| Difference | 0.06 mm — interchangeable |
| Fits | M5 hex head |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 5.5 mm | 5.50 | 1/4″ | 0.85 mm — do not |
| 6 mm | 6.00 | 1/4″ | 0.35 mm — do not |
| 7 mm | 7.00 | 1/4″ | 0.65 mm — do not |
| 8 mm | 8.00 | 5/16″ | 0.06 mm — swap |
| 9 mm | 9.00 | 3/8″ | 0.52 mm — do not |
| 10 mm | 10.00 | 3/8″ | 0.48 mm — do not |
| 11 mm | 11.00 | 7/16″ | 0.11 mm — swap |
Frequently asked questions
What is 8 mm in inches?
0.31 inches. The nearest imperial spanner is 5/16″ at 7.94 mm, 0.06 mm smaller.
Can I use 5/16″ instead of 8 mm?
Yes. They are 0.06 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.
What bolt does a 8 mm spanner fit?
A M5 hex head. The full metric thread chart gives the tapping drill and clearance hole for it too.