13/16″ spanner
20.64 mm · 0.81 in
13/16″ across the flats
The nearest metric size
21 mm is 21.00 mm — 0.36 mm larger than 13/16″.
That is not close enough. Putting 21 mm on a 13/16″ head leaves 0.36 mm of slack, so the tool bears on the corners rather than the flats. On anything tight the corners round off, and once they do the fastener needs cutting out. This is the most common way a bolt is destroyed in a home garage.
The numbers
| Across flats | 20.64 mm · 0.81 in |
| Nearest metric | 21 mm — 21.00 mm |
| Difference | 0.36 mm — not safe to substitute |
Nearby imperial sizes
| Size | mm | Nearest metric | Gap |
|---|---|---|---|
| 11/16″ | 17.46 | 17 mm | 0.46 mm — do not |
| 3/4″ | 19.05 | 19 mm | 0.05 mm — swap |
| 13/16″ | 20.64 | 21 mm | 0.36 mm — do not |
| 7/8″ | 22.22 | 22 mm | 0.22 mm |
| 15/16″ | 23.81 | 24 mm | 0.19 mm |
Frequently asked questions
What is 13/16″ in millimetres?
20.64 mm. The nearest metric spanner is 21 mm — 0.36 mm larger.
Can I use 21 mm instead of 13/16″?
Not safely. The gap is 0.36 mm. 21 mm is larger than 13/16″, so it sits on the corners of the head instead of the flats and rounds them off — this is how fasteners are destroyed.
Where is 13/16″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.