Few things are more frustrating than a socket that almost fits. Metric (mm) and SAE (inch) wrenches overlap in size, and a few pairs are close enough to swap — but most are not, and using the wrong one rounds off bolt heads fast. This guide shows the common sizes in both systems (SAE 1/4 in to 1 in, metric 6 to 24 mm), the approximate difference, and when it is safe to substitute.
SAE Wrench Sizes in Millimeters
SAE sizes are fractions of an inch. The table converts each to millimeters (rounded to 0.01 mm) and shows the nearest metric wrench and the gap between them.
| SAE size | mm (rounded) | Nearest metric | Difference | Swap? |
|---|---|---|---|---|
| 1/4″ | 6.35 mm | 6 mm | 0.35 mm | No |
| 5/16″ | 7.94 mm | 8 mm | 0.06 mm | Usually OK |
| 3/8″ | 9.53 mm | 10 mm | 0.48 mm | No |
| 7/16″ | 11.11 mm | 11 mm | 0.11 mm | Only the larger tool fits, loosely — avoid |
| 1/2″ | 12.70 mm | 13 mm | 0.30 mm | No |
| 9/16″ | 14.29 mm | 14 mm | 0.29 mm | No |
| 5/8″ | 15.88 mm | 16 mm | 0.13 mm | Only the larger tool fits, loosely — avoid |
| 11/16″ | 17.46 mm | 17 mm | 0.46 mm | No |
| 3/4″ | 19.05 mm | 19 mm | 0.05 mm | Usually OK |
| 13/16″ | 20.64 mm | 21 mm | 0.36 mm | No |
| 7/8″ | 22.23 mm | 22 mm | 0.23 mm | No |
| 15/16″ | 23.81 mm | 24 mm | 0.19 mm | Only the larger tool fits, loosely — avoid |
| 1″ | 25.40 mm | 25 mm | 0.40 mm | No |
“Only the larger tool fits” means the bigger of the two sizes will slip over the smaller fastener with some play; the smaller tool will not fit the bigger fastener at all.
Metric Wrench Sizes in Inches
| Metric size | Inches (rounded) | Nearest SAE (1/32″) | Difference | Swap? |
|---|---|---|---|---|
| 6 mm | 0.236″ | 1/4″ | 0.35 mm | No |
| 7 mm | 0.276″ | 9/32″ | 0.14 mm | Only the larger tool fits, loosely — avoid |
| 8 mm | 0.315″ | 5/16″ | 0.06 mm | Usually OK |
| 9 mm | 0.354″ | 11/32″ | 0.27 mm | No |
| 10 mm | 0.394″ | 13/32″ | 0.32 mm | No |
| 11 mm | 0.433″ | 7/16″ | 0.11 mm | Only the larger tool fits, loosely — avoid |
| 12 mm | 0.472″ | 15/32″ | 0.09 mm | Usually OK |
| 13 mm | 0.512″ | 1/2″ | 0.30 mm | No |
| 14 mm | 0.551″ | 9/16″ | 0.29 mm | No |
| 15 mm | 0.591″ | 19/32″ | 0.08 mm | Usually OK |
| 16 mm | 0.630″ | 5/8″ | 0.13 mm | Only the larger tool fits, loosely — avoid |
| 17 mm | 0.669″ | 21/32″ | 0.33 mm | No |
| 18 mm | 0.709″ | 23/32″ | 0.26 mm | No |
| 19 mm | 0.748″ | 3/4″ | 0.05 mm | Usually OK |
| 20 mm | 0.787″ | 25/32″ | 0.16 mm | Only the larger tool fits, loosely — avoid |
| 21 mm | 0.827″ | 13/16″ | 0.36 mm | No |
| 22 mm | 0.866″ | 7/8″ | 0.23 mm | No |
| 24 mm | 0.945″ | 15/16″ | 0.19 mm | Only the larger tool fits, loosely — avoid |
Which Sizes Are Truly Interchangeable?
Only a handful of pairs are within about 0.1 mm, which is often within the clearance of a good-quality wrench:
- 3/4″ and 19 mm — 0.05 mm apart. The best-known swap.
- 5/16″ and 8 mm — 0.06 mm apart. The 5/16″ size (7.94 mm) is the smaller one: an 8 mm wrench fits a 5/16″ head, but a 5/16″ wrench is too small for an 8 mm head.
- 7/8″ and 22 mm are 0.23 mm apart and 1/2″ and 13 mm are 0.30 mm apart — both will feel loose and can slip under load.
Anything marked “No” in the tables will either not fit or will sit loose on the corners of the nut. On a tight or rusted fastener that is how heads get rounded.
How to Tell If a Bolt Is Metric or SAE
- Where it came from: most modern cars and bikes, and equipment imported from Europe and Asia, use mostly metric fasteners. Older US machinery and some US trucks and tools still use SAE.
- Head markings: metric bolts often show a strength number such as 8.8 or 10.9; SAE bolts use radial lines (3 lines = grade 5, 6 lines = grade 8).
- Measure it: measure the head across the flats with calipers. A reading near a whole millimeter (for example 13.0 mm) means metric; near a fraction of an inch (12.7 mm = 1/2″) means SAE.
6-Point vs 12-Point Sockets
A 6-point socket grips the flat sides of the nut and is less likely to round it — use it for tight, rusted or high-torque fasteners. A 12-point socket is easier to fit in tight spaces because it engages at more angles, but it grips closer to the corners. If you have to use a near-size socket, a 6-point is the safer choice.
Frequently Asked Questions
Is a 19 mm socket the same as 3/4 inch?
Almost. 3/4″ is 19.05 mm, only 0.05 mm larger, so they are usually interchangeable.
What SAE size is closest to 10 mm?
10 mm is 0.394″. The closest SAE sizes are 3/8″ (9.53 mm, too small) and 13/32″ (10.32 mm, rarely sold). Use a 10 mm wrench.
Can I use a 1/2 inch socket on a 13 mm bolt?
No. 1/2″ is 12.70 mm, which is 0.30 mm too small for a 13 mm head. The next size up, 9/16″ (14.29 mm), is far too loose.
Related: Metric bolt sizes in inches · Drill bit size chart · Small metric sizes in inches
Sources and Checking
- NIST Guide to the SI, Appendix B.8 (1 inch = 0.0254 m exactly)
- Differences calculated from the nominal sizes. Real tools have manufacturing tolerances, so test the fit before applying high torque.
Published by Johnson, who runs mmintoinches.com. Every value in this guide was recalculated from 1 inch = 25.4 mm and checked against the sources below on 9 October 2026. Found a mistake? Email support@mmintoinches.com with the page and the value.