How to convert units without mistakes

Steps

1. Write the number with its unit, for example 5 mi, not just 5.
2. Check that the unit means what you think. A gallon, a ton and a bigha all change with the country or state.
3. Use a converter with exact factors, such as the length or weight converter, and read the answer with its unit.
4. Sanity check: is the answer the right size? A marathon of 42 km should come out near 26 miles.
5. Keep the original number and unit, and round only at the very end.

Units that share a name

A US gallon is 3.785 litres but a UK gallon is 4.546 litres, about 20 percent more. A US short ton is 2,000 pounds, a UK long ton is 2,240 pounds and a metric tonne is 1,000 kilograms.

In computing, 1 KB is 1,000 bytes for disk makers and phone plans, but 1,024 bytes in some operating systems. That is why a 1 TB drive shows about 931 GB. The names KiB, MiB and GiB always mean 1,024.

Temperature is different

Most conversions multiply by a factor. Temperature scales start at different zeros, so you must add or subtract as well: Fahrenheit equals Celsius times 9/5 plus 32. That is why doubling a Celsius temperature does not double the Fahrenheit one.

Local land units

A bigha is not one fixed size in India. It is about 27,000 square feet in Uttar Pradesh, about 17,424 in Gujarat and 14,400 in West Bengal and Assam, and it can differ by district. A gaj is a square yard (9 square feet). For anything legal, use the figure in the local revenue record.

Why it matters

In 1999 NASA lost the Mars Climate Orbiter because one team's software used pound-force seconds and another expected newton-seconds. A units mix-up is a small mistake with big consequences, so always keep the unit with the number.