5.5 mm spanner
5.50 mm · 0.22 in
5.5 mm across the flats · fits a M3 hex head
The nearest imperial size
1/4″ is 6.35 mm — 0.85 mm larger than 5.5 mm.
That is not close enough. Putting 1/4″ on a 5.5 mm head leaves 0.85 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 | 5.50 mm · 0.22 in |
| Nearest imperial | 1/4″ — 6.35 mm |
| Difference | 0.85 mm — not safe to substitute |
| Fits | M3 hex head |
Nearby metric sizes
| Size | mm | Nearest imperial | Gap |
|---|---|---|---|
| 5.5 mm | 5.50 | 1/4″ | 0.85 mm — do not |
| 6 mm | 6.00 | 1/4″ | 0.35 mm — do not |
| 7 mm | 7.00 | 1/4″ | 0.65 mm — do not |
| 8 mm | 8.00 | 5/16″ | 0.06 mm — swap |
Frequently asked questions
What is 5.5 mm in inches?
0.22 inches. The nearest imperial spanner is 1/4″ at 6.35 mm, 0.85 mm larger.
Can I use 1/4″ instead of 5.5 mm?
Not safely. The gap is 0.85 mm. 1/4″ is larger than 5.5 mm, so it sits on the corners of the head instead of the flats and rounds them off — this is how fasteners are destroyed.
What bolt does a 5.5 mm spanner fit?
A M3 hex head. The full metric thread chart gives the tapping drill and clearance hole for it too.