12 mm spanner
12.00 mm · 0.47 in
12 mm across the flats
The nearest imperial size
1/2″ is 12.70 mm — 0.70 mm larger than 12 mm.
That is not close enough. Putting 1/2″ on a 12 mm head leaves 0.70 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 | 12.00 mm · 0.47 in |
| Nearest imperial | 1/2″ — 12.70 mm |
| Difference | 0.70 mm — not safe to substitute |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 9 mm | 9.00 | 3/8″ | 0.52 mm — do not |
| 10 mm | 10.00 | 3/8″ | 0.48 mm — do not |
| 11 mm | 11.00 | 7/16″ | 0.11 mm — swap |
| 12 mm | 12.00 | 1/2″ | 0.70 mm — do not |
| 13 mm | 13.00 | 1/2″ | 0.30 mm — do not |
| 14 mm | 14.00 | 9/16″ | 0.29 mm — do not |
| 15 mm | 15.00 | 9/16″ | 0.71 mm — do not |
Frequently asked questions
What is 12 mm in inches?
0.47 inches. The nearest imperial spanner is 1/2″ at 12.70 mm, 0.70 mm larger.
Can I use 1/2″ instead of 12 mm?
Not safely. The gap is 0.70 mm. 1/2″ is larger than 12 mm, 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 12 mm used?
It sits between the standard hex head sizes rather than on one, so it turns up on proprietary fasteners, plumbing fittings and older machinery rather than on ordinary bolts.