Unit Converter

Why is a US gallon different from a UK (Imperial) gallon?

Historical accident. Before 1824, both countries used a variety of gallon definitions. Britain's 1824 Weights and Measures Act standardized the Imperial gallon as 10 pounds of distilled water at 62°F, giving 4.546 liters. The United States kept the older Queen Anne's wine gallon of 231 cubic inches from 1707, giving 3.785 liters. The 20% difference matters enormously for fuel economy (US mpg to UK mpg is not a simple label change), recipe scaling, and any commercial agreement written in gallons without specifying the variant. Australia and New Zealand went metric decades ago; Canada officially went metric but retains Imperial measures in many commercial contexts. When in doubt, ask which variant is meant.

What is the difference between mass and weight?

Mass is how much matter an object contains (measured in kilograms, pounds-mass, grams); weight is the force gravity exerts on that mass (measured in newtons, pounds-force). On Earth's surface they are proportional — 1 kg of mass weighs about 9.81 newtons (about 2.2 pounds-force) — so in everyday use the terms are interchangeable. In precise engineering and physics contexts they must be distinguished, especially when dealing with variable gravity (spacecraft, Moon surface, deep-ocean pressure). A 1-pound bag of flour has the same mass on the Moon but weighs 1/6th as much; a kitchen scale calibrated for Earth would misread.

Why does Celsius to Fahrenheit use 9/5 + 32 instead of just multiplying?

Because the two scales have different zero points. Anders Celsius originally defined 0°C as the freezing point of water and 100°C as the boiling point at standard pressure. Daniel Fahrenheit used 0°F as the temperature of a particular brine solution and 100°F as roughly human body temperature (close but not exact). So 0°C does not equal 0°F — it equals 32°F. Any conversion must account for the offset, not just the scale ratio. The formula F = C × 9/5 + 32 captures both the scale ratio (180°F per 100°C = 9/5) and the offset (32°F at 0°C). Forgetting the +32 gives wrong answers everywhere except at exactly the scale's zero point.

What is the difference between kB and KiB in file sizes?

Decimal vs binary prefixes. The SI prefix kilo- strictly means 1000, so 1 kilobyte (kB) = 1000 bytes. But early computing adopted kB informally to mean 1024 bytes (2^10), because memory is addressed in powers of two. The IEC standardized KiB = 1024 bytes (and MiB = 1024² bytes, GiB = 1024³, etc.) in 1998 to disambiguate. In practice, disk manufacturers, network specs, and most hardware documentation use decimal (kB = 1000, MB = 1,000,000); most operating systems and software use binary (but often still write it as MB). The practical impact: a '1 TB' drive advertises 10^12 bytes but your OS reports it as 931 GiB. The data is the same; the unit definition differs.

Which conversion factors does this tool use for currency or live rates?

None — currency is not converted here because exchange rates fluctuate second by second and a static conversion factor would be stale within minutes. For live currency conversion use the currency-converter tool on this site, which fetches a fresh rate from a public foreign-exchange API on each query. Unit conversions in this tool (length, weight, volume, temperature, etc.) are fixed by international standards and do not change, so they are safe to compute offline with constants. If someone presents 'kilobucks' or a similar humorous unit, that is slang not physics; there is no accepted conversion factor we can use.