SAE and metric sockets look interchangeable in a tray and are not. A socket that is a few thousandths oversize contacts the corners of the fastener rather than its flats, and corners are what shear off under a breaker bar. This chart gives every common size in both standards with the exact gap between them, so the near-misses are visible before they cost you a bolt head.
Why sockets are less forgiving than spanners
An open-ended spanner grips two flats and will usually protest before it rounds anything. A six-point socket grips all six and a twelve-point grips the corners as much as the flats, so an imperfect fit is loaded evenly and quietly right up to the moment the head deforms. Add an impact wrench and there is no warning at all. That is why the same few thousandths that are tolerable on a spanner are not on a socket.
SAE to metric socket and spanner sizes
| Metric | Decimal inch | Nearest SAE | Difference (thou) | Safe to swap |
|---|---|---|---|---|
| 4 mm | 0.1575 | 5/32" | -1.2 | Yes |
| 4.5 mm | 0.1772 | 3/16" | +10.3 | No |
| 5 mm | 0.1969 | 3/16" | -9.4 | No |
| 5.5 mm | 0.2165 | 7/32" | +2.2 | Yes |
| 6 mm | 0.2362 | 1/4" | +13.8 | No |
| 7 mm | 0.2756 | 9/32" | +5.7 | No |
| 8 mm | 0.3150 | 5/16" | -2.5 | Yes |
| 9 mm | 0.3543 | 11/32" | -10.6 | No |
| 10 mm | 0.3937 | 3/8" | -18.7 | No |
| 11 mm | 0.4331 | 7/16" | +4.4 | Yes |
| 12 mm | 0.4724 | 1/2" | +27.6 | No |
| 13 mm | 0.5118 | 1/2" | -11.8 | No |
| 14 mm | 0.5512 | 9/16" | +11.3 | No |
| 15 mm | 0.5906 | 9/16" | -28.1 | No |
| 16 mm | 0.6299 | 5/8" | -4.9 | Yes |
| 17 mm | 0.6693 | 11/16" | +18.2 | No |
| 18 mm | 0.7087 | 11/16" | -21.2 | No |
| 19 mm | 0.7480 | 3/4" | +2.0 | Yes |
| 20 mm | 0.7874 | 25/32" | -6.2 | No |
| 21 mm | 0.8268 | 13/16" | -14.3 | No |
| 22 mm | 0.8661 | 7/8" | +8.9 | No |
| 23 mm | 0.9055 | 7/8" | -30.5 | No |
| 24 mm | 0.9449 | 15/16" | -7.4 | No |
| 25 mm | 0.9843 | 1" | +15.7 | No |
| 26 mm | 1.0236 | 1" | -23.6 | No |
| 27 mm | 1.0630 | 1-1/16" | -0.5 | Yes |
| 28 mm | 1.1024 | 1-1/8" | +22.6 | No |
| 30 mm | 1.1811 | 1-3/16" | +6.4 | No |
| 32 mm | 1.2598 | 1-1/4" | -9.8 | No |
| 34 mm | 1.3386 | 1-5/16" | -26.1 | No |
| 36 mm | 1.4173 | 1-7/16" | +20.2 | No |
| 38 mm | 1.4961 | 1-1/2" | +3.9 | Yes |
| 41 mm | 1.6142 | 1-5/8" | +10.8 | No |
| 46 mm | 1.8110 | 1-13/16" | +1.5 | Yes |
| 50 mm | 1.9685 | 2" | +31.5 | No |
Method and precision
Decimal inch values are exact at 25.4 mm to the inch. The difference column is the SAE size minus the metric size in thousandths of an inch. "Safe to swap" is set below five thousandths. Only sizes stocked in ordinary SAE sets are offered as matches.
If the head is already rounded
Going up a size to get purchase makes it worse, because it removes what is left of the flats. The tools that still work at that point grip rather than surround: a bolt extractor socket with a reverse-spiral flute, or locking pliers where the head is exposed. Both work better cold than after heat has been applied.
Ranges are in sockets and accessories, socket sets and spanners.