You have an inch-marked spanner in your hand and the fastener is metric. This chart runs the other way from the usual one: imperial size first, then the metric size it is closest to, and how far apart the two actually are. Most pairs are not close enough to use, and the difference column is what tells you which.
Reading it imperial-first
Imperial spanners are sized in fractions of an inch and metric ones in whole millimetres, so the two scales interleave without ever really meeting. Working from the imperial side, most sizes land between two metric sizes rather than on one. The difference column below is the metric size minus the imperial size in thousandths of an inch, so a negative number means the metric spanner is the smaller of the two and will not seat at all.
Imperial to metric spanner conversion
| Imperial | Decimal inch | Nearest metric | Difference (thou) | Safe to swap |
|---|---|---|---|---|
| 5/32" | 0.1575 | 4 mm | -1.2 | Yes |
| 3/16" | 0.1772 | 4.5 mm | +10.3 | No |
| 3/16" | 0.1969 | 5 mm | -9.4 | No |
| 7/32" | 0.2165 | 5.5 mm | +2.2 | Yes |
| 1/4" | 0.2362 | 6 mm | +13.8 | No |
| 9/32" | 0.2756 | 7 mm | +5.7 | No |
| 5/16" | 0.3150 | 8 mm | -2.5 | Yes |
| 11/32" | 0.3543 | 9 mm | -10.6 | No |
| 3/8" | 0.3937 | 10 mm | -18.7 | No |
| 7/16" | 0.4331 | 11 mm | +4.4 | Yes |
| 1/2" | 0.4724 | 12 mm | +27.6 | No |
| 1/2" | 0.5118 | 13 mm | -11.8 | No |
| 9/16" | 0.5512 | 14 mm | +11.3 | No |
| 9/16" | 0.5906 | 15 mm | -28.1 | No |
| 5/8" | 0.6299 | 16 mm | -4.9 | Yes |
| 11/16" | 0.6693 | 17 mm | +18.2 | No |
| 11/16" | 0.7087 | 18 mm | -21.2 | No |
| 3/4" | 0.7480 | 19 mm | +2.0 | Yes |
| 25/32" | 0.7874 | 20 mm | -6.2 | No |
| 13/16" | 0.8268 | 21 mm | -14.3 | No |
| 7/8" | 0.8661 | 22 mm | +8.9 | No |
| 7/8" | 0.9055 | 23 mm | -30.5 | No |
| 15/16" | 0.9449 | 24 mm | -7.4 | No |
| 1" | 0.9843 | 25 mm | +15.7 | No |
| 1" | 1.0236 | 26 mm | -23.6 | No |
| 1-1/16" | 1.0630 | 27 mm | -0.5 | Yes |
| 1-1/8" | 1.1024 | 28 mm | +22.6 | No |
| 1-3/16" | 1.1811 | 30 mm | +6.4 | No |
| 1-1/4" | 1.2598 | 32 mm | -9.8 | No |
| 1-5/16" | 1.3386 | 34 mm | -26.1 | No |
| 1-7/16" | 1.4173 | 36 mm | +20.2 | No |
| 1-1/2" | 1.4961 | 38 mm | +3.9 | Yes |
| 1-5/8" | 1.6142 | 41 mm | +10.8 | No |
| 1-13/16" | 1.8110 | 46 mm | +1.5 | Yes |
| 2" | 1.9685 | 50 mm | +31.5 | No |
Method and precision
Decimal inch values are exact at 25.4 mm to the inch. "Safe to swap" is set where the two sizes differ by under five thousandths of an inch, the point at which a spanner stops bearing on the flats of the fastener and starts bearing on its corners. Only spanner sizes that exist in ordinary SAE sets appear here.
The pairs worth memorising
Three imperial sizes are effectively identical to a metric one and can be used without thinking: 3/4 inch for 19 mm, 5/16 inch for 8 mm, and 1-1/4 inch for 32 mm. Everything else is a judgement call, and the one to be most careful with is 3/8 inch against 10 mm — it feels like it fits and it is nearly nineteen thousandths undersize.
Full ranges are in spanners, wrenches and sockets and accessories.