Metric and imperial spanner sizes almost never match exactly. A few pairs are close enough to swap safely; most are not, and using a near-fit is the single most common way a hex head gets rounded. This chart gives the decimal inch equivalent of every common metric size, its nearest imperial spanner, and how far apart they actually are in thousandths of an inch.
Which sizes are genuinely interchangeable?
The working threshold is about five thousandths of an inch. Inside that, the spanner still bears on the flats of the fastener and the pairing is safe. Outside it, the jaws contact the corners instead, and the corners are what shear off — which is why a fastener that felt fine on the first turn suddenly rounds under load.
The notorious near-miss is 10 mm against 3/8 inch, which is roughly nineteen thousandths out — more than enough to round a tight fastener on the first pull. The table below marks every pairing so there is no guessing.
Metric to imperial spanner conversion
| Metric | Decimal inch | Nearest imperial | 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 imperial size minus the metric size, in thousandths of an inch, signed so a negative figure means the imperial spanner is the smaller of the two. "Safe to swap" is set where that difference is under five thousandths. Only sizes that exist in ordinary SAE spanner and socket sets are offered as matches — several 32nd sizes sit arithmetically closer to some metric sizes but are not stocked, and naming one would send you looking for a tool you do not own.
When the fastener is already damaged
Once the corners are gone, a correctly sized spanner will not grip either. That is the point to move to a gripping tool rather than a larger opening — a bolt extractor, or locking pliers on an exposed head. Going up one spanner size to get purchase makes the next attempt harder, not easier, because it removes what is left of the flats.
Full ranges are in spanners, wrenches and sockets and accessories.