Most of the world uses the metric system (meters, kilograms, liters, Celsius), while the US still primarily uses imperial/US customary units (feet, pounds, gallons, Fahrenheit) — which is why unit conversion comes up so often for travel, recipes, and international communication.
Length, weight, and volume conversions are all simple multiplication — every unit has a fixed ratio to a base unit (like meters or kilograms), so converting is just scaling that ratio. Temperature is the exception: Celsius, Fahrenheit, and Kelvin don't share a common zero point, so converting between them requires an offset, not just a multiplier. That's why "double the number" tricks for temperature (a common rule of thumb) are only ever approximate.
A US cup is legally defined as 240 ml, while a metric cup (used in Australia and some other countries) is 250 ml — a small but real difference that matters in precise baking. This converter uses the US cup definition.
They're genuinely different units with the same name — a US gallon is about 3.785 liters, while a UK imperial gallon is about 4.546 liters, roughly 20% larger. This converter uses the US gallon, which is standard in American recipes, fuel economy figures, and everyday use.
No — this is the most common temperature conversion mistake. 0°C (freezing point of water) equals 32°F, and 0°F is a much colder −17.8°C. The two scales only intersect at −40, where both scales read the same number.
A stone equals 14 pounds (about 6.35 kg) and is still commonly used for body weight in the UK and Ireland — "12 stone 4" is a normal way to state weight there. The US and most other countries dropped stone in favor of pounds or kilograms, which is why it can look unfamiliar.