Unit Converters

Exact factors, not approximations

Since the international yard and pound agreement of 1959, the customary units used in the United States and the imperial units used in the UK have been defined in terms of the metric system. One inch is exactly 2.54 centimetres. One pound is exactly 0.45359237 kilograms. A foot is therefore exactly 0.3048 metres and a mile exactly 1,609.344 metres. Because these are definitions rather than measurements, a converter that uses them has no conversion error at all; the only rounding is in how many decimal places are displayed. The converters here store every factor at full precision and trim only the tiny binary floating-point noise that computers introduce, showing up to six decimal places.

Temperature is different because the scales do not share a zero. Converting Fahrenheit to Celsius needs an offset as well as a ratio: subtract 32, then multiply by 5/9. Kelvin shifts Celsius by exactly 273.15, and Rankine is the Fahrenheit-sized absolute scale used in some US engineering work. The temperature converter handles all four and refuses values below absolute zero.

General converters or pair pages?

The Length, Weight, Temperature, Volume, Area, Speed and Time converters let you pick any source and target unit and then list the value in every unit of that kind at once — handy when you are not sure which unit a spec sheet wants. The pair pages, cm to inches, kg to lbs and Fahrenheit to Celsius, are tuned to a single conversion in both directions. Each has a printable reference table, mental-math shortcuts and extra outputs that matter for that pair, such as feet-and-inches for heights or pounds-and-ounces for baby weights.

Volume, area, speed and time

Volume is where US and UK units part ways: a US gallon is exactly 231 cubic inches (3.785411784 L) while an imperial gallon is 4.54609 L, so the volume converter lists them separately along with cups, spoons and fluid ounces. Area units are squares of length units, so a square foot is exactly 0.09290304 m² and an acre exactly 4,046.8564224 m². Speeds convert through metres per second, with a knot defined as one nautical mile per hour, and the speed converter also gives running pace. Time units are exact up to weeks; months and years use Gregorian averages, which the time converter labels clearly.

Where unit mix-ups cause real trouble

Conversions are not just homework. Recipes from another country list oven temperatures in the other scale, luggage limits are posted in kilograms while bathroom scales read pounds, and hardware in the US is sized in fractions of an inch while plans may be metric. Medicine dosing is a setting where a slip between pounds and kilograms is genuinely dangerous; always use the units your clinician or the product label specifies. In engineering, a famous unit mismatch between pound-force seconds and newton seconds contributed to the loss of NASA's Mars Climate Orbiter in 1999.

Tips for accurate conversions

Convert once, from the original measurement, rather than chaining several rounded conversions. Keep more decimal places than you need until the final step. Match the precision of the answer to the precision of the measurement: a person measured as 170 cm tall is about 66.9 inches, not 66.92913 inches. And remember that "ton" and "ounce" have several meanings — the weight converter lists metric tonnes, US short tons and UK long tons separately, and uses the avoirdupois ounce rather than the troy ounce used for precious metals or the fluid ounce used for volume.