1/4″ spanner
6.35 mm · 0.25 in
1/4″ across the flats
The nearest metric size
6 mm is 6.00 mm — 0.35 mm smaller than 1/4″.
That is not close enough. 6 mm is smaller than the 1/4″ head, so it will not go on at all — which is the harmless failure of the two.
The numbers
| Across flats | 6.35 mm · 0.25 in |
| Nearest metric | 6 mm — 6.00 mm |
| Difference | 0.35 mm — not safe to substitute |
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 |
Frequently asked questions
What is 1/4″ in millimetres?
6.35 mm. The nearest metric spanner is 6 mm — 0.35 mm smaller.
Can I use 6 mm instead of 1/4″?
Not safely. The gap is 0.35 mm. 6 mm is smaller than 1/4″ and simply will not go on.
Where is 1/4″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.