Unit conversion reference
- Length: 1 mile = 1.609 km | 1 inch = 2.54 cm | 1 foot = 30.48 cm
- Weight: 1 lb = 0.4536 kg | 1 oz = 28.35 g | 1 stone = 6.35 kg
- Temperature: °F = (°C × 9/5) + 32 | °C = (°F − 32) × 5/9
- Speed: 1 mph = 1.609 km/h | 1 knot = 1.852 km/h
A quick sanity check for any conversion
Converting from a larger unit to a smaller one should always produce a bigger number — a mile is bigger than a kilometer, so miles to km should increase the number, not shrink it. If a result looks smaller than the original when it should have grown (or the other way around), the conversion has likely been run backwards — multiplied where it should have divided, or vice versa. Checking that the number moved in the expected direction catches most conversion errors before they matter.
Why temperature conversion works differently
Length, weight and speed conversions are all a straight multiplication by a fixed factor — double the input, double the output. Temperature isn't, because Celsius and Fahrenheit don't share a zero point: 0°C is 32°F, not 0°F. That's why the temperature formula has a multiply and an add or subtract step, while every other conversion here is multiplication alone.
Common conversion mistakes
A metric ton (1,000 kg) and a US ton (2,000 lb, roughly 907 kg) are not the same unit despite the shared name — always check which one a source is using before converting. Nautical miles and statute miles also differ (a nautical mile is about 1.15 statute miles), which matters for anything related to aviation or marine navigation.
When a unit mix-up actually costs money
This isn't just a classroom problem. NASA's Mars Climate Orbiter was lost in 1999 after one team's software used pound-force seconds while ground systems expected newton-seconds — a single unmarked unit mismatch that ended a $327 million mission. On a smaller scale, a US company once quoted a shipment of rice at 39¢ a pound; the overseas buyer read it as 39¢ a kilogram, more than doubling the price they expected to pay. The lesson scales down to everyday use too: always check which unit a price, recipe, or spec sheet is actually quoting before converting it, not just what looks familiar.
Area and volume have their own traps
Real estate listings switch between square feet and square meters depending on the country, and the two aren't close enough to eyeball — 1 square meter is about 10.76 square feet, so a like-for-like comparison across listings from different countries needs an actual conversion, not a guess. Volume has a quieter trap: a US gallon and a UK (imperial) gallon are not the same size — the UK gallon is about 20% larger. Recipes, fuel economy figures, and fluid specs can all be quoted in either, so it's worth checking which one a source means before assuming.
Speed units aren't interchangeable either
Aviation and marine navigation use knots, road speeds use mph or km/h depending on the country, and mixing them up isn't a rounding error — a knot is a nautical mile per hour, which is already a different distance unit from a statute mile, so converting "knots to mph" involves two unit changes stacked on top of each other, not one. Worth double-checking which speed unit a source actually means, especially for anything travel or navigation related.