Unit Conversion: Exact Definitions and Where Conversions Go Wrong

Since the international yard and pound agreement of 1959, one inch is defined as exactly 25.4 millimetres and one pound as exactly 0.45359237 kilograms. These are definitions, not measurements, so 2.54 cm per inch carries no error at all. A foot is exactly 0.3048 m and a mile exactly 1609.344 m. When a conversion looks slightly off, the cause is display rounding somewhere in the chain rather than the factor itself.

A kilogram measures mass; a pound is used for both mass and force depending on context, which is why pound-force and pound-mass exist as separate units. On Earth the distinction rarely bites, because the ratio is effectively constant, but it matters in engineering where a load in pounds-force converts to newtons and not to kilograms. If a spec sheet mixes kg and lbf, check which one the calculation actually needs before converting.

A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres, a difference of about 20 percent. Pints, quarts and fluid ounces diverge the same way, and a US fluid ounce is not even a consistent fraction of the imperial one. Recipes, fuel figures and shipping volumes crossing between the two systems need the origin specified; converting a UK recipe with US factors is roughly a fifth off on every liquid.

Celsius and Fahrenheit do not share a zero, so conversion needs both a factor and an offset: F = C × 9/5 + 32. This means a temperature cannot be scaled the way a length can. It also means a temperature difference converts differently from a temperature: a rise of 10 °C is a rise of 18 °F, not 50 °F. Kelvin shares Celsius degree size but starts at absolute zero, so K = C + 273.15 with no scaling at all.

Miles per gallon measures distance per unit of fuel; litres per 100 km measures fuel per unit of distance. They run in opposite directions, so the conversion is a division rather than a multiplication: L/100 km = 235.215 ÷ mpg for US gallons. Because it is reciprocal, equal steps in one are not equal steps in the other. Going from 15 to 20 mpg saves far more fuel over the same distance than going from 40 to 45 mpg, which is the whole reason regulators prefer the L/100 km form.

How many digits to keep depends on what the number is for. A room measurement rounded to the centimetre is fine; a medication dose converted from pounds to kilograms is not, because a rounding error scales with the dose. Round once at the end rather than at every step, since repeated intermediate rounding accumulates. And where a value feeds a regulated calculation, keep the exact factor in the computation and round only for display.

Confirm the source unit and the destination unit by name, not by abbreviation alone, because oz, gal and ton all have more than one definition. Prefer the exact factor over a rounded one when the result will feed another calculation. Round once at the end. For temperature and fuel economy, use the specialised formulas rather than a plain multiply. If the value is a regulated dose, dimension, or invoice quantity, keep more digits than the display needs until the final step.