3/8″ spanner
9.52 mm · 0.37 in
3/8″ across the flats
The nearest metric size
10 mm is 10.00 mm — 0.48 mm larger than 3/8″.
That is not close enough. Putting 10 mm on a 3/8″ head leaves 0.48 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 | 9.52 mm · 0.37 in |
| Nearest metric | 10 mm — 10.00 mm |
| Difference | 0.48 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 |
| 7/16″ | 11.11 | 11 mm | 0.11 mm — swap |
| 1/2″ | 12.70 | 13 mm | 0.30 mm — do not |
Frequently asked questions
What is 3/8″ in millimetres?
9.52 mm. The nearest metric spanner is 10 mm — 0.48 mm larger.
Can I use 10 mm instead of 3/8″?
Not safely. The gap is 0.48 mm. 10 mm is larger than 3/8″, 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 3/8″ used?
Imperial spanner sizes are "across flats" measurements and are still standard on American vehicles, plumbing and older machinery everywhere.