Instant answers for the exact conversions people search most — each with the formula, a reference table and related values. Grouped by category.
Almost every conversion on this site is the same two-step operation. Each unit carries a factor relating it to one base unit for its dimension, so any conversion is: multiply by the source factor to reach the base, then divide by the destination factor to leave it. Five miles in kilometres is 5 × 1.609344 = 8.04672. Nothing about the pair matters except those two numbers, which is why one engine can serve length, mass, pressure, energy and data alike.
The first exception is scales that do not share a zero. Temperature is affine, not proportional: 100 °F is not "some multiple" of a degree Celsius, because the two scales start in different places. The conversion needs an offset as well as a ratio — (100 − 32) × 5/9 = 37.78 °C — and any tool that treats temperature as a plain factor will be wrong everywhere except absolute zero.
The second is scales that run backwards. Fuel economy in miles per gallon rises as consumption improves, while litres per 100 km falls, so converting between them means dividing rather than multiplying: 235.214583 ÷ 30 mpg = 7.84 L/100 km. Averaging mpg figures directly is wrong for the same reason.
The third is precision. Some conversions are exact by definition — one inch is precisely 2.54 cm, so twelve inches is exactly 30.48 cm — while others are measured and carry uncertainty. A result is never more precise than the value you started from, so converting a rounded figure and quoting eight decimals invents accuracy that was never there.