5/16″ spanner
7.94 mm · 0.31 in
5/16″ across the flats
The nearest metric size
8 mm is 8.00 mm — 0.06 mm larger than 5/16″.
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 | 7.94 mm · 0.31 in |
| Nearest metric | 8 mm — 8.00 mm |
| Difference | 0.06 mm — interchangeable |
Nearby imperial sizes
| Size | mm | Nearest metric | Gap |
|---|---|---|---|
| 1/4″ | 6.35 | 6 mm | 0.35 mm — do not |
| 5/16″ | 7.94 | 8 mm | 0.06 mm — swap |
| 3/8″ | 9.52 | 10 mm | 0.48 mm — do not |
| 7/16″ | 11.11 | 11 mm | 0.11 mm — swap |
Frequently asked questions
What is 5/16″ in millimetres?
7.94 mm. The nearest metric spanner is 8 mm — 0.06 mm larger.
Can I use 8 mm instead of 5/16″?
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.
Where is 5/16″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.