Metric and Imperial Conversions: The Mistakes That Matter

Converting between units looks like arithmetic, and for length and weight it mostly is. Problems appear when units share a name but not a size, when the conversion includes an offset, or when the result is rounded too early. Knowing the exceptions prevents the mistakes that actually matter.

Length and distance

The common factors are worth memorising: one inch is 2.54 centimetres exactly, one foot is 30.48 centimetres, one mile is 1.609344 kilometres, and one yard is 0.9144 metres. Because the inch-to-centimetre figure is exact, conversions derived from it are exact as well; any difference you see comes from rounding.

Round only at the end. Converting feet to metres, rounding, then converting again compounds the error. Keep full precision through intermediate steps and round to the precision the situation justifies. A room measurement rarely needs more than two decimal places; a machined part may need four.

Weight and mass

One pound is 0.45359237 kilograms and one ounce is 28.349523125 grams. In everyday use, "weight" usually means mass, but the two differ in physics. For cooking and shipping, the mass conversion is the one you want.

Watch the difference between the avoirdupois ounce used for food and the fluid ounce used for liquids. They share a name and nothing else. A recipe calling for eight ounces of flour means weight; a bottle labelled eight fluid ounces means volume.

Temperature is not a multiplication

Celsius and Fahrenheit have different zero points, so a scale factor alone is wrong. The conversion is °F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9. A useful sanity check: −40 is the same in both scales, 0 °C is 32 °F, and 100 °C is 212 °F. If a converted temperature does not fall between those anchors, the arithmetic or the formula is wrong.

Kelvin is offset differently again: add 273.15 to a Celsius value. It is never written with a degree symbol.

Volume: US and imperial are different systems

This is the most common source of error in recipes and fuel calculations:

• One US gallon is about 3.785 litres.
• One imperial gallon is about 4.546 litres.
• One US cup is about 236.6 millilitres; a metric cup is 250 millilitres.
• One US fluid ounce is about 29.57 millilitres; an imperial fluid ounce is about 28.41 millilitres.

A recipe from the United States and a recipe from the United Kingdom using "cups" are not describing the same quantity. When precision matters, convert to millilitres and note which system the source used.

Speed, area and data size

Speed conversions follow from length: one mile per hour is 1.609344 kilometres per hour, and one metre per second is 3.6 kilometres per hour. A quick approximation for mph to km/h is to multiply by 1.6, which is fine for estimating travel time but not for a formal calculation.

Data size has its own trap. Storage manufacturers usually use decimal units, where one kilobyte is 1,000 bytes, while some operating systems report binary units, where one kibibyte is 1,024 bytes. That gap explains why a "500 GB" drive shows less capacity in the file manager. The difference grows with each step: 1,024 versus 1,000 repeated across gigabytes produces a visible discrepancy.

Rounding, significant figures and context

Ask what the number will be used for. A converted distance for a hiking estimate can be rounded generously; a converted dose or a structural measurement cannot. Where a result feeds a safety, medical or financial decision, verify it independently rather than relying on a single conversion, and never strip the unit from the number in your notes.

Finally, label units explicitly. Most conversion errors are not arithmetic errors at all; they are cases where someone assumed the wrong unit at the start.