The nearest standard (inch) socket for every metric size — and whether it's a safe swap or close enough to round off the bolt. Numbers computed live, not copied off an old PDF.
A swap is "safe" only when the gap is under ~0.15 mm (0.006 in). Anything looser can round the corners off a tight or high-torque fastener — use the exact size.
| Metric | Nearest SAE | mm (dec in) | SAE (dec in) | Gap | Fit |
|---|
Decimal inches shown to 4 places. "Gap" is the absolute difference between the metric size and the closest common SAE socket.
13/32" (0.4063") is the closest, but it sits about 0.32 mm loose — snug enough for light work, not for anything tight or torqued. There is no clean inch equivalent to 10mm, which is exactly why the 10mm is the socket everyone loses.
Only when the gap is tiny. 19mm ≈ 3/4" (0.05 mm) is the one genuinely safe swap; 8mm ≈ 5/16" is another. 13mm to 1/2" is close but ~0.30 mm loose — usable on low-torque bolts, risky on a tight one. Most other pairings will slip and round the head, especially on rusted fasteners.
Almost. 1/2" is 12.70 mm and 13mm is 0.5118", a gap of about 0.30 mm. It works in a pinch on low-torque bolts but is slightly loose — fine for grip, not ideal for breaking a tight one loose.
The two systems were designed independently — metric sockets step in whole millimetres, SAE in fractions of an inch. They only coincide by accident, which is why you need both sets in the box.