Every conversion on this page is exact. Not accurate to several decimal places — exact, because the imperial units have been legally defined in terms of metric ones since 1959, so an inch is 25.4 mm by definition rather than by measurement. This is the quick reference: the exact factor for each common pair, short tables at everyday values, and a route to the full chart wherever you need more rows than fit here.
The exact factors
The figures below are definitions, not measurements. That is worth knowing because it means there is no accuracy to be gained by carrying more decimal places than you need — the only error in any of these conversions is the one you introduce by rounding the answer.
| Convert | To | Multiply by |
|---|---|---|
| Inches | Millimetres | 25.4 |
| Feet | Metres | 0.3048 |
| Yards | Metres | 0.9144 |
| Miles | Kilometres | 1.609344 |
| Ounces | Grams | 28.349523125 |
| Pounds | Kilograms | 0.45359237 |
| Short tons | Tonnes | 0.90718474 |
| US fluid ounces | Millilitres | 29.5735295625 |
| US quarts | Litres | 0.946352946 |
| US gallons | Litres | 3.785411784 |
| Cubic inches | Litres | 0.016387064 |
| Pound-feet | Newton metres | 1.3558179483314003 |
Length
| Inches | mm | Feet | Metres | Miles | km |
|---|---|---|---|---|---|
| 1 | 25.4 | 1 | 0.3048 | 1 | 1.609 |
| 2 | 50.8 | 2 | 0.6096 | 2 | 3.219 |
| 3 | 76.2 | 3 | 0.9144 | 3 | 4.828 |
| 5 | 127 | 5 | 1.524 | 5 | 8.047 |
| 10 | 254 | 10 | 3.048 | 10 | 16.093 |
| 12 | 304.8 | 12 | 3.6576 | 12 | 19.312 |
| 20 | 508 | 20 | 6.096 | 20 | 32.187 |
| 25 | 635 | 25 | 7.62 | 25 | 40.234 |
| 50 | 1270 | 50 | 15.24 | 50 | 80.467 |
| 100 | 2540 | 100 | 30.48 | 100 | 160.934 |
Mass
| Ounces | Grams | Pounds | Kilograms | Short tons | Tonnes |
|---|---|---|---|---|---|
| 1 | 28.35 | 1 | 0.4536 | 1 | 0.9072 |
| 2 | 56.7 | 2 | 0.9072 | 2 | 1.8144 |
| 5 | 141.75 | 5 | 2.268 | 5 | 4.5359 |
| 10 | 283.5 | 10 | 4.5359 | 10 | 9.0718 |
| 20 | 566.99 | 20 | 9.0718 | 20 | 18.1437 |
| 25 | 708.74 | 25 | 11.3398 | 25 | 22.6796 |
| 50 | 1417.48 | 50 | 22.6796 | 50 | 45.3592 |
| 100 | 2834.95 | 100 | 45.3592 | 100 | 90.7185 |
Volume
| US fl oz | mL | US quarts | Litres | US gallons | Litres |
|---|---|---|---|---|---|
| 1 | 29.6 | 1 | 0.9464 | 1 | 3.785 |
| 2 | 59.1 | 2 | 1.8927 | 2 | 7.571 |
| 4 | 118.3 | 4 | 3.7854 | 4 | 15.142 |
| 5 | 147.9 | 5 | 4.7318 | 5 | 18.927 |
| 8 | 236.6 | 8 | 7.5708 | 8 | 30.283 |
| 10 | 295.7 | 10 | 9.4635 | 10 | 37.854 |
| 16 | 473.2 | 16 | 15.1416 | 16 | 60.567 |
| 20 | 591.5 | 20 | 18.9271 | 20 | 75.708 |
Method and precision
The inch (25.4 mm), the yard (0.9144 m), the foot (0.3048 m), the pound (0.45359237 kg) and the avoirdupois ounce are all defined exactly in metric terms by international agreement, and every other factor above follows from them arithmetically: the mile is 5,280 feet, the cubic inch is 2.54 cubed, and the US fluid ounce and quart are the US liquid gallon divided by 128 and 4. Nothing here is measured or approximated. Table values are rounded for display only; the converter above uses the exact factor.
Where the traps are
Three pairs share a name and do not share a value. A US gallon is 3.785 litres and an imperial gallon is 4.546, about twenty per cent apart. A short ton is 2,000 lb, a long ton is 2,240 lb and a tonne is 1,000 kg, all three in current use. And an ounce is a weight, a troy ounce is a different weight, and a fluid ounce is a volume. Where a figure matters, pin down which one is meant before converting it, not after.
The full charts
Each of these goes considerably deeper than the summary above. Fractions and drill sizes in particular need every row, because the values are a discrete standard rather than a formula.
- Fractions to decimals to millimetres — all 64ths of an inch
- Drill size chart — number, letter, inch and mm
- Metric to standard spanner sizes
- Pound-feet to newton metres
- US gallons to litres
- Grams to ounces
- Short tons to tonnes
- Yards to metres
Rules, calipers and depth gauges are in measuring equipment.