Everyday maths
Unit conversion: choose the units before you do the maths
A practical guide to selecting the right measurement category, units, and level of precision.
Updated July 21, 2026 · 3 min read
Make sure the units measure the same thing
Before converting, identify what the number represents. Metres and miles are both distance units, while kilograms and pounds are both weight units. A conversion only makes sense inside the same measurement category.
The source unit is what you have. The destination unit is what you need. Keeping that distinction clear avoids a common error of reversing the conversion.
Temperature is not a straight multiplier
Length and weight units can usually be compared through a fixed base unit. Temperature scales are different because their zero points are not the same.
Use a proper temperature conversion for Celsius, Fahrenheit, and Kelvin. A shortcut based only on multiplication can give a result that looks plausible but is wrong.
Use enough decimal places, but no more
A rough travel conversion may only need one decimal place. A recipe or technical task may need more precision, depending on its instructions.
Keep the source value and converted unit in your notes. This gives you a simple way to check the calculation when you return to it later.