by: Keith Fenske
https://kwfenske.github.io/
November 2022
Copyright © 2022 by Keith Fenske. Apache License 2.0 or GNU GPLv2 or later.
In general, you should use the smallest socket or wrench (spanner) that fits over a nut or bolt without force. Some metric sizes (millimeter, mm) are almost the same as SAE sizes (fractional, imperial, inch, standard). Metric tools are normally for metric fasteners and SAE tools are for SAE fasteners.
| Original | Converted | Nearest | Ratio | Comment |
| 5/32 in | 3.969 mm | 4 mm | 1.00787 | good |
| 4 mm | 5.039 / 32 in | 5/32 in | 0.99219 | good |
| 4.5 mm | 5.669 / 32 in | 3/16 in | 1.05833 | |
| 3/16 in | 4.762 mm | 5 mm | 1.04987 | |
| 5 mm | 6.299 / 32 in | 3/16 in | 0.95250 | |
| 5.5 mm | 6.929 / 32 in | 7/32 in | 1.01023 | close |
| 7/32 in | 5.556 mm | 5.5 mm | 0.98988 | close |
| 6 mm | 7.559 / 32 in | 1/4 in | 1.05833 | |
| 1/4 in | 6.350 mm | 6 mm | 0.94488 | |
| 7 mm | 8.819 / 32 in | 9/32 in | 1.02054 | |
| 9/32 in | 7.144 mm | 7 mm | 0.97988 | |
| 5/16 in | 7.938 mm | 8 mm | 1.00787 | good |
| 8 mm | 10.079 / 32 in | 5/16 in | 0.99219 | good |
| 11/32 in | 8.731 mm | 9 mm | 1.03078 | |
| 9 mm | 11.339 / 32 in | 11/32 in | 0.97014 | |
| 3/8 in | 9.525 mm | 10 mm | 1.04987 | |
| 10 mm | 6.299 / 16 in | 3/8 in | 0.95250 | |
| 11 mm | 6.929 / 16 in | 7/16 in | 1.01023 | close |
| 7/16 in | 11.112 mm | 11 mm | 0.98988 | close |
| 12 mm | 7.559 / 16 in | 1/2 in | 1.05833 | |
| 1/2 in | 12.700 mm | 13 mm | 1.02362 | |
| 13 mm | 8.189 / 16 in | 1/2 in | 0.97692 | |
| 14 mm | 8.819 / 16 in | 9/16 in | 1.02054 | |
| 9/16 in | 14.288 mm | 14 mm | 0.97988 | |
| 15 mm | 9.449 / 16 in | 9/16 in | 0.95250 | |
| 5/8 in | 15.875 mm | 16 mm | 1.00787 | good |
| 16 mm | 10.079 / 16 in | 5/8 in | 0.99219 | good |
| 17 mm | 10.709 / 16 in | 11/16 in | 1.02721 | |
| 11/16 in | 17.462 mm | 17 mm | 0.97351 | |
| 18 mm | 11.339 / 16 in | 11/16 in | 0.97014 | |
| 19 mm | 11.969 / 16 in | 3/4 in | 1.00263 | very good |
| 3/4 in | 19.050 mm | 19 mm | 0.99738 | very good |
| 20 mm | 12.598 / 16 in | 13/16 in | 1.03187 | |
| 13/16 in | 20.638 mm | 21 mm | 1.01757 | |
| 21 mm | 13.228 / 16 in | 13/16 in | 0.98274 | |
| 22 mm | 13.858 / 16 in | 7/8 in | 1.01023 | close |
| 7/8 in | 22.225 mm | 22 mm | 0.98988 | close |
| 23 mm | 14.488 / 16 in | 7/8 in | 0.96630 | |
| 15/16 in | 23.812 mm | 24 mm | 1.00787 | good |
| 24 mm | 15.118 / 16 in | 15/16 in | 0.99219 | good |
| 25 mm | 15.748 / 16 in | 1 in | 1.01600 | |
| 1 in | 25.400 mm | 25 mm | 0.98425 | |
| 26 mm | 16.378 / 16 in | 1 in | 0.97692 | |
| 1-1/16 in | 26.987 mm | 27 mm | 1.00046 | very good |
| 27 mm | 17.008 / 16 in | 1-1/16 in | 0.99954 | very good |
| 28 mm | 17.638 / 16 in | 1-1/8 in | 1.02054 | |
| 1-1/8 in | 28.575 mm | 29 mm | 1.01487 | |
| 29 mm | 18.268 / 16 in | 1-1/8 in | 0.98534 | |
| 30 mm | 18.898 / 16 in | 1-3/16 in | 1.00542 | good |
| 1-3/16 in | 30.162 mm | 30 mm | 0.99461 | good |
| 31 mm | 19.528 / 16 in | 1-1/4 in | 1.02419 | |
| 1-1/4 in | 31.750 mm | 32 mm | 1.00787 | good |
| 32 mm | 20.157 / 16 in | 1-1/4 in | 0.99219 | good |
The comments "good" and "very good" are subjective. Here, "good" means less than one percent error and "very good" means less than 0.5% error. Depending upon what you are working with, you may have more or less tolerance for error.
Sockets and wrenches are manufactured to standards: ANSI, ASME, DIN, ISO, JIS, etc. There is no guarantee that specified dimensions will be the same in metric and SAE even when a size might be equivalent.
Sizes larger than 1-1/4" or 32mm are industrial and do not have a regular progression in their size numbers.
file: SocketWrenchSizes.doc 2022-11-27