21 mm spanner
21.00 mm · 0.83 in
21 mm across the flats · fits a M14 (ISO) hex head
The nearest imperial size
13/16″ is 20.64 mm — 0.36 mm smaller than 21 mm.
That is not close enough. 13/16″ is smaller than the 21 mm head, so it will not go on at all — which is the harmless failure of the two.
The numbers
| Across flats | 21.00 mm · 0.83 in |
| Nearest imperial | 13/16″ — 20.64 mm |
| Difference | 0.36 mm — not safe to substitute |
| Fits | M14 (ISO) hex head |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 18 mm | 18.00 | 11/16″ | 0.54 mm — do not |
| 19 mm | 19.00 | 3/4″ | 0.05 mm — swap |
| 21 mm | 21.00 | 13/16″ | 0.36 mm — do not |
| 22 mm | 22.00 | 7/8″ | 0.22 mm |
| 24 mm | 24.00 | 15/16″ | 0.19 mm |
Frequently asked questions
What is 21 mm in inches?
0.83 inches. The nearest imperial spanner is 13/16″ at 20.64 mm, 0.36 mm smaller.
Can I use 13/16″ instead of 21 mm?
Not safely. The gap is 0.36 mm. 13/16″ is smaller than 21 mm and simply will not go on.
What bolt does a 21 mm spanner fit?
A M14 (ISO) hex head. The full metric thread chart gives the tapping drill and clearance hole for it too.