3/4″ spanner
19.05 mm · 0.75 in
3/4″ across the flats
The nearest metric size
19 mm is 19.00 mm — 0.05 mm smaller than 3/4″.
That is close enough to be interchangeable. 0.05 mm is well inside the manufacturing tolerance of the fastener, and the two will feel identical in use.
The numbers
| Across flats | 19.05 mm · 0.75 in |
| Nearest metric | 19 mm — 19.00 mm |
| Difference | 0.05 mm — interchangeable |
Nearby imperial sizes
| Size | mm | Nearest metric | Gap |
|---|---|---|---|
| 5/8″ | 15.88 | 16 mm | 0.13 mm — swap |
| 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 |
Frequently asked questions
What is 3/4″ in millimetres?
19.05 mm. The nearest metric spanner is 19 mm — 0.05 mm smaller.
Can I use 19 mm instead of 3/4″?
Yes. They are 0.05 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 3/4″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.