Where the factors come from
Most imperial units are defined exactly in terms of metric ones. The yard is defined as exactly 0.9144 of a metre and the pound as exactly 0.45359237 of a kilogram, so the foot, inch, mile, ounce and stone follow exactly from them. The imperial gallon is defined as 4.54609 litres. These definitions come from the Weights and Measures Act 1985, Schedule 1 (definitions of imperial units) (UK legislation (legislation.gov.uk)). US units such as the US gallon, cup and fluid ounce, and the formulas for temperature, come from the US National Institute of Standards and Technology. Where a source gives a rounded factor rather than an exact definition, the table below says so.
| Unit | In base units | Factor |
|---|---|---|
| 1 inche | 0.0254 m | exact |
| 1 feet | 0.3048 m | exact |
| 1 yard | 0.9144 m | exact |
| 1 mile | 1,609.34 m | exact |
| 1 hectare | 10,000 m² | exact |
| 1 square inche | 0.00064516 m² | exact |
| 1 square feet | 0.092903 m² | exact |
| 1 square yard | 0.836127 m² | exact |
| 1 acre | 4,046.86 m² | exact |
| 1 square mile | 2.58999e+6 m² | exact |
| 1 tonne | 1,000 kg | exact |
| 1 ounce | 0.0283495 kg | exact |
| 1 pound | 0.453592 kg | exact |
| 1 stone | 6.35029 kg | exact |
| 1 teaspoon (US) | 0.00492892 L | rounded |
| 1 tablespoon (US) | 0.0147868 L | rounded |
| 1 fluid ounce (US) | 0.0295735 L | rounded |
| 1 cup (US) | 0.236588 L | rounded |
| 1 gallon (US) | 3.78541 L | rounded |
| 1 gallon (UK, imperial) | 4.54609 L | exact |
| 1 kilometres per hour | 0.277778 m/s | exact |
| 1 miles per hour | 0.44704 m/s | exact |
| 1 knot | 0.514444 m/s | rounded |
Worked examples
- A 5-acre lifestyle block is 2.02343 hectares, or 20,234.3 square metres.
- A newborn weighing 7 pounds 8 ounces is 3.40194 kg.
- Someone who is 11 stone weighs 69.8532 kg.
- A 6-foot doorway is 182.88 cm.
- A summer day of 77 °F is 25 °C.
- A car doing 40 miles per imperial gallon uses 7.06202 litres per 100 km; at 40 miles per US gallon it uses 5.88036.
All of these are worked out by the same code the converter uses.
Mistakes to watch for
- Mixing up UK and US gallons. A UK (imperial) gallon is about a fifth bigger than a US gallon. A British car magazine quoting miles per gallon means the imperial gallon; an American one means the US gallon. The same mpg figure therefore means different fuel use.
- Reading fuel economy the wrong way round. Litres per 100 km goes down as a car gets more efficient; miles per gallon and kilometres per litre go up. The converter handles the flip for you.
- Treating US cups as metric cups. Recipes from the United States use the US cup, listed here as 236.588 mL. Many New Zealand and Australian recipes use a metric cup instead, which this converter doesn't include because it isn't defined in the sources used.
- Converting temperatures with a single factor. Celsius and Fahrenheit have different zero points, so you can't just multiply. A rise of 10 °C is a rise of 18 °F, but 10 °C is 50 °F.
- Stones and pounds. "11 stone 4" means 11 stone and 4 pounds, not 11.4 stone. Convert the pounds separately and add them.
Limits
Results are shown to six significant figures, which is more precision than everyday measuring needs. Rounded factors (marked above) carry about seven significant figures. Currency isn't included, because exchange rates change by the minute. Everything runs in your browser; the number you type isn't stored or sent anywhere, and only your choice of units is kept in the page address after the # sign. For money calculations, see the NZ calculators.
Questions people ask
How many hectares is an acre?
One acre is 0.404686 hectares, and one hectare is 2.47105 acres.
Why does my mpg figure give two different answers?
Because there are two gallons. The imperial gallon used in the UK is larger than the US gallon, so the same number of miles per gallon means a different amount of fuel per 100 km. Pick the gallon that matches where the figure came from.
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