Tools reference
What bolt or nut size fits this wrench?
A correct way to use an across-flats wrench measurement without pretending one head width proves one thread size.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Common metric regular-hex sizes
Current ISO regular hex dimensions differ from older DIN dimensions at several familiar sizes. Both remain common in real toolboxes, especially on machinery and vehicles. That is why M10 may appear under either 16 or 17 mm.
| Thread size | Current ISO | Older DIN |
|---|---|---|
| M3 | 5.5 mm | 5.5 mm |
| M4 | 7 mm | 7 mm |
| M5 | 8 mm | 8 mm |
| M6 | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm |
| M10 | 16 mm | 17 mm |
| M12 | 18 mm | 19 mm |
| M14 | 21 mm | 22 mm |
| M16 | 24 mm | 24 mm |
| M18 | 27 mm | 27 mm |
| M20 | 30 mm | 30 mm |
Common inch regular-hex sizes
Regular inch hex cap screws and finished hex nuts share many wrench sizes, but not every one. The table keeps the two roles separate where the published patterns differ.
| Thread size | Regular bolt head | Finished hex nut |
|---|---|---|
| 1/4 in | 7/16 in | 7/16 in |
| 5/16 in | 1/2 in | 1/2 in |
| 3/8 in | 9/16 in | 9/16 in |
| 7/16 in | 5/8 in | 11/16 in |
| 1/2 in | 3/4 in | 3/4 in |
| 9/16 in | 13/16 in | 7/8 in |
| 5/8 in | 15/16 in | 15/16 in |
| 3/4 in | 1 1/8 in | 1 1/8 in |
Measure the opening correctly
- Use the inside faces of an open-end wrench, not the outside of the jaws.
- For a six-point socket, use the working hex opening inside the round body.
- For a nut or bolt head, measure across two opposite flats—not corner to corner.
- Use the measured width to shortlist patterns, then measure the thread itself.
When the table should not win
- Flange bolts often use a smaller head than a regular bolt of the same thread.
- Heavy-hex nuts and structural bolts use larger flats.
- Nylon-insert and other locknuts can follow their own body dimensions.
- Reduced-head, automotive, and specialty fasteners may intentionally break the regular pattern.
- A rounded head or spread wrench jaw can produce a believable but wrong measurement.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.