Reference chart
Metric and Imperial Conversion Chart
By &Pixels, the studio that builds MyCalculator.to. The last column says whether a factor is exact by definition or a rounded decimal, which decides how safely you can shorten it.
Length, weight and volume in one place, with the factors that are exact marked as exact.
Which conversion factors are exact?
More than most people expect. An inch has been defined as exactly 2.54 centimetres since 1959, and everything derived from it inherits that: a foot is exactly 30.48 cm, a mile exactly 1.609344 km. A pound is exactly 0.45359237 kg. These are definitions, not measurements.
What is not exact is the reverse direction. A centimetre in inches is 0.3937007874 and onwards forever, because it is one divided by an exact number rather than an exact number itself. The last column below marks which is which, because it decides whether you can safely shorten a factor.
Length and distance
| From | To | Multiply by | Exact? |
|---|---|---|---|
| 1 inch | centimetres | 2.54 | exact |
| 1 foot | centimetres | 30.48 | exact |
| 1 yard | metres | 0.9144 | exact |
| 1 mile | kilometres | 1.609344 | exact |
| 1 centimetre | inches | 0.3937007874 | rounded |
| 1 metre | feet | 3.280839895 | rounded |
| 1 kilometre | miles | 0.6213711922 | rounded |
The calculators behind this
Weight and mass
| From | To | Multiply by | Exact? |
|---|---|---|---|
| 1 pound | kilograms | 0.45359237 | exact |
| 1 ounce | grams | 28.349523125 | exact |
| 1 stone | kilograms | 6.35029318 | exact |
| 1 kilogram | pounds | 2.204622622 | rounded |
| 1 gram | ounces | 0.03527396195 | rounded |
Volume
| From | To | Multiply by | Exact? |
|---|---|---|---|
| 1 US fluid ounce | millilitres | 29.5735295625 | exact |
| 1 US cup | millilitres | 236.5882365 | exact |
| 1 US pint | millilitres | 473.176473 | exact |
| 1 US gallon | litres | 3.785411784 | exact |
| 1 imperial pint | millilitres | 568.26125 | exact |
| 1 imperial gallon | litres | 4.54609 | exact |
The volume table is where the real trap is. A US gallon is 3.785 litres and an imperial gallon is 4.546, a difference of about twenty percent, and the same split runs through pints, quarts and fluid ounces. A British recipe and an American one using the same words are not using the same measures.
Converting without introducing error
- Check which gallon or pint you mean. A US gallon is 3.785 litres and an imperial gallon is 4.546. That is a twenty percent difference.
- Use the exact factor when it exists. Inches, pounds and miles are defined exactly. There is no reason to approximate them.
- Round at the end, not in the middle. Rounding 2.54 to 2.5 looks harmless and is nearly two percent off before you have finished.
- Watch fluid ounces against ounces. One is volume and one is weight. They are only interchangeable for water, and only roughly.
- Convert temperature with the offset first. Subtract 32 before multiplying by 5/9. Doing it the other way is the classic error.
For one conversion with the chart alongside it, the kilograms to pounds converter and the inches to centimetres converter each show the formula they used, so the number can be checked rather than trusted.
Formula & Methodology
Did you know? Minus 40 is the same temperature in Celsius and Fahrenheit. It is the single point where the two scales cross, which falls out of the fact that one is a linear transformation of the other with a different slope and offset.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units
- International Yard and Pound Agreement of 1959
- NIST Handbook 44, appendix on units and systems of measurement