1″ spanner
25.40 mm · 1.00 in
1″ across the flats
The nearest metric size
24 mm is 24.00 mm — 1.40 mm smaller than 1″.
That is not close enough. 24 mm is smaller than the 1″ head, so it will not go on at all — which is the harmless failure of the two.
The numbers
| Across flats | 25.40 mm · 1.00 in |
| Nearest metric | 24 mm — 24.00 mm |
| Difference | 1.40 mm — not safe to substitute |
Nearby imperial sizes
| Size | mm | Nearest metric | Gap |
|---|---|---|---|
| 7/8″ | 22.22 | 22 mm | 0.22 mm |
| 15/16″ | 23.81 | 24 mm | 0.19 mm |
| 1″ | 25.40 | 24 mm | 1.40 mm — do not |
| 1-1/16″ | 26.99 | 27 mm | 0.01 mm — swap |
| 1-1/8″ | 28.57 | 30 mm | 1.43 mm — do not |
Frequently asked questions
What is 1″ in millimetres?
25.40 mm. The nearest metric spanner is 24 mm — 1.40 mm smaller.
Can I use 24 mm instead of 1″?
Not safely. The gap is 1.40 mm. 24 mm is smaller than 1″ and simply will not go on.
Where is 1″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.