Unit Conversion Traps: Gallons, Tons and Degrees

Same name, different size

Several common units mean different amounts depending on where they are used. A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres, about 20% more; pints follow, at 473 ml and 568 ml. A US fluid ounce is 29.57 ml and an imperial one 28.41 ml, so here the US unit is the larger. A ton can be the metric tonne of 1,000 kg, the US short ton of 907.2 kg or the British long ton of 1,016 kg.

Distance has its own pair: a statute mile is 1.609 km, while a nautical mile, used at sea and in aviation, is 1.852 km. Many countries also keep traditional units for particular uses, such as the pyeong (about 3.306 m²) for floor area in Korea. When a figure comes from another country or field, check which unit it means before converting.

Temperatures convert in two different ways

A temperature reading converts with an offset: °F = °C × 9/5 + 32, so 20 °C is 68 °F. A temperature difference converts without it: a rise of 10 °C is a rise of 18 °F, not 50 °F. Mixing the two is a common error in weather reports, cooking and lab notes, where "5 degrees warmer" is converted as if it were a reading.

Kelvin uses the same step size as Celsius, offset by 273.15, so a difference of 10 °C is also 10 K. Because Kelvin starts at absolute zero, it is the only one of the three in which "twice as hot" means anything physically; 20 °C is not twice as warm as 10 °C.

Area and volume scale faster than length

A foot is 0.3048 m, but a square foot is 0.3048² = 0.0929 m², so one square metre is about 10.76 square feet, not 3.28. Volumes go further: a cubic foot is about 28.3 litres. Converting an area by the length factor is one of the most frequent mistakes in floor plans and property listings.

The same rule explains why doubling the size of something does more than double other quantities. Double a room's width and length and its floor area is four times larger; double a box in every direction and it holds eight times as much. A 400 m² floor is about 121 pyeong, while multiplying the area by a length factor would give a meaningless answer.

Fuel economy runs in opposite directions

Miles per gallon measures distance per fuel, so higher is better, while litres per 100 km measures fuel per distance, so lower is better. Converting between them is a division, not a multiplication: 30 US mpg is 235.2 ÷ 30 ≈ 7.8 L/100 km. Because the gallons differ, 30 UK mpg is about 9.4 L/100 km, and 30 US mpg equals about 36 UK mpg.

The inverse relationship also changes how savings look. Going from 10 to 20 mpg saves far more fuel over the same distance than going from 30 to 40 mpg, even though both are a 10 mpg improvement. Per-distance units such as L/100 km show the real saving directly.

Keep the precision you started with

A converted number cannot be more precise than the original. "About 5 miles" is about 8 km, not 8.04672 km; a recipe's "1 cup" does not become 236.588 ml of meaningful precision. Long decimals suggest a measurement accuracy that never existed, and they make numbers harder to read.

For exact definitions, though, the long figure matters: an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg by international agreement since 1959. Use full precision in intermediate steps and round once at the end, to about as many significant figures as the input had.

• Check whether gallons, pints, ounces or tons are US, imperial or metric
• Convert temperature readings with the offset and differences without it
• Square the factor for areas and cube it for volumes
• Divide, do not multiply, between mpg and L/100 km
• Round once at the end, to the precision of the original
• Know the local units used in the country the figure comes from