In 1999 NASA lost the Mars Climate Orbiter, a spacecraft worth well over a hundred million dollars, because one team supplied thrust figures in pound-seconds while the software expected newton-seconds. Nobody made an arithmetic error. The numbers were simply in different units.
Temperature is not a simple multiplication
Most conversions are one multiplication: metres to feet, kilograms to pounds. Temperature is not, because Celsius and Fahrenheit do not share a zero point. Water freezes at 0°C and 32°F, so you must scale and then shift.
Fahrenheit to Celsius means subtracting 32 first, then multiplying by 5/9. Forgetting the offset gives answers that look plausible and are badly wrong: 100°F becomes 55.6°C instead of 37.8°C.
There is one exception worth knowing: a temperature difference converts by scale alone, since the offsets cancel. A rise of 18°F is a rise of 10°C, not 7.8°C.
A gallon is not a gallon
The US gallon is about 3.785 litres. The imperial gallon, still used in the UK and parts of the Caribbean, is about 4.546 litres. That is a 20% difference hiding behind an identical word.
The same trap catches pints and fluid ounces. A US pint is 16 US fluid ounces; an imperial pint is 20 imperial fluid ounces. Recipes crossing the Atlantic go wrong for exactly this reason, and fuel-economy figures in miles per gallon are not comparable between countries.
Mass and weight
In everyday speech they are interchangeable. In physics they are not: mass is how much matter there is, weight is the force gravity exerts on it. Kilograms measure mass; newtons measure force.
For everyday conversions on Earth this rarely matters, which is why converters treat kilograms and pounds as equivalent. It matters in engineering, and it is the reason the pound appears as both a unit of mass and a unit of force depending on context.
Precision and rounding
- Rounding too early compounds. Convert once at full precision and round only the final answer.
- Do not invent precision. A measurement of "about 5 feet" is not 1.524 metres; it is roughly 1.5 metres.
- Watch repeated conversions. Going metric to imperial and back through rounded intermediate values will not return the number you started with.
- For anything regulated, use the exact factor: one inch is exactly 25.4 millimetres by definition, not 25.4001 or 25.39.
The habit that prevents all of this
Write the unit next to every number, in spreadsheets, code and notes alike. A column headed "Weight" invites disaster; "Weight (kg)" does not. Most serious unit errors begin as a number with no label that someone later assumed they understood.
- Label every value with its unit at the point you record it.
- For temperature, check whether you need a value or a difference.
- For volume, establish whether a figure is US or imperial before converting.
- Sanity-check the result: 100 km/h should be about 62 mph, not 6.2 or 620.