Unit conversion without the rounding traps

Converting units looks like multiplication, and most of the time it is. The errors that survive review come from three other places: a unit name that means two different things, a scale with an offset rather than a pure ratio, and precision that was invented somewhere in the arithmetic.

Ambiguous unit names

A US gallon is about 3.785 litres and an imperial gallon is about 4.546 litres. Tons, fluid ounces, and even miles have regional variants. Always convert from a named unit rather than a remembered factor, and state the variant in any document a supplier will read.

Offsets are not ratios

Temperature is the classic trap: Fahrenheit and Celsius have different zero points, so conversion needs the offset as well as the ratio. Multiplying by 9/5 and forgetting to add 32 produces a number that is plausible and wrong.

Decimal versus binary prefixes

Digital sizes carry two families of prefixes. A megabyte is one million bytes; a mebibyte is 1,048,576. The gap compounds with each step, which is why a "1 TB" drive reports roughly 931 GB. Mixing them silently in one calculation produces a 7% error per prefix.

Rounding is a claim about accuracy

Converting a rounded 2 m into 6.5617 ft implies millimetre precision the source never had. Round the output to the precision the input justified, and keep the raw value if a later step needs it.

Convert locally

Use Length Unit Converter and Temperature Converter for physical measurements, Data Size Converter for digital ones, and read MB vs MiB when a size limit does not add up.