Length Converter

What each input means

The calculator asks for 3 pieces of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.

A worked example using the default inputs

With value to convert of 10, from of Metres (m), to of Feet (ft), the calculator returns 32.808 ft.

The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.

How this is calculated

Convert to metres first (value × factor), then divide by the target unit’s factor. 1 in = 2.54 cm exactly.

Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.

How these conversions stay exact

Conversions are multiplications by fixed factors, so accuracy depends entirely on using the right factor and the right definition. Where a factor is defined exactly by international agreement it is used exactly; where a unit is a measurement convention, the agreed value is used and shown to a precision the input can support.

Temperature is the one family where the conversion is not a simple ratio. Because the scales start in different places, you multiply and then offset, which is why 20 °C is not twice as warm as 10 °C in any physical sense: it is 293 K against 283 K.

Recipe cups are the classic trap. A US customary cup is 236.6 mL, a metric cup is 250 mL and an Australian cup is 250 mL, so a cup of flour measured in different countries is a different weight before density is even considered.

Three worked examples from this section

Ten kilometres in miles

One mile is exactly 1.609344 km, so 10 ÷ 1.609344 = 6.2137 miles. For a quick check, multiply by 5 and divide by 8: 10 km is about 6.25 miles, which is close enough to sanity-check the number.

180 pounds in kilograms

One pound is exactly 0.45359237 kg, so 180 × 0.45359237 = 81.65 kg. In the other direction, a 70 kg adult is 70 ÷ 0.45359237 ≈ 154.3 lb.

180 lb

81.65 kg

70 kg

154.3 lb

1 stone

6.35 kg

100 °F in Celsius and kelvin

Subtract 32, multiply by 5 and divide by 9: (100 − 32) × 5 ÷ 9 = 37.8 °C. Adding 273.15 gives 310.9 K. The same steps reversed turn 37.0 °C into 98.6 °F.

Conversion factors and significant figures

Conversions use the exact defined factors where they exist (an inch is exactly 25.4 mm, a pound exactly 0.45359237 kg, a US gallon exactly 3.785411784 litres) and the internationally agreed values where measurement is involved.

• Results are shown to a precision that reflects the input. Multiplying a two-figure measurement by an exact factor still leaves you with a two-figure answer.
• US and imperial units differ: a US gallon is about 3.79 litres, an imperial gallon about 4.55. Choose the right one for the recipe or the vehicle.
• Temperature conversions are affine, not multiplicative; the offset is why doubling a Celsius temperature does not double a Fahrenheit one.

Sources and further reading

• BIPM: SI measurement units
• NIST: unit conversion