Unit Converter

Background.

Since 1959 an inch has been exactly 2.54 centimetres and a pound exactly 0.45359237 kilograms. Those two figures, agreed between the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, turned most metric-to-customary conversions into matters of definition rather than measurement. A mile is exactly 1,609.344 metres, an acre exactly 4,046.8564224 square metres, a US gallon (231 cubic inches) exactly 3.785411784 litres, and 100 miles an hour exactly 160.9344 kilometres an hour. When two converters disagree in the later decimal places, one of them has rounded a factor.

Seven kinds of quantity are covered: length, mass, volume, temperature, time, area and speed. Choose the quantity first and the From and To menus list only its units, so a kilogram can never be paired with a metre. Switching the quantity moves both menus to that quantity's units on their own, so glance at them before reading the answer.

All but one of the quantities convert by a single scale factor, because zero means the same thing in every unit: zero metres and zero feet are the same length. Temperature is the exception. Zero degrees Celsius is the freezing point of water, zero Fahrenheit is colder, and zero kelvin is absolute zero, so converting a temperature needs an offset as well as a scale, and a temperature change converts differently from a temperature reading.

The volume units are US measures: the US gallon, quart, pint, cup, fluid ounce, tablespoon and teaspoon. British and other imperial measures share the names but not the sizes; an imperial pint or gallon is about 20% larger than its US namesake, which matters for recipes, fuel economy figures and beer alike. Litres and millilitres are the safe common ground between the two.

What is unit converter?

A unit conversion re-expresses the same physical amount against a different yardstick. Because two units of the same quantity measure the same thing, their ratio is a fixed number, and converting means multiplying by that ratio written so the old unit cancels: 26.2 mi × (1.609344 km ÷ 1 mi) = 42.1648 km. Each quantity has a base unit here, the metre, gram, litre, second, square metre or metre per second, and every other unit is stored as its size in that base. Converting between any two units of a quantity goes through the base, so only one factor per unit is needed rather than one for every possible pair. Temperature is handled separately through the kelvin, the SI unit, because its scales start from different zeros. The SI units themselves are fixed by constants of nature: the kilogram through the Planck constant (since 2019), the metre through the speed of light and the second through a frequency of the caesium atom, and because the customary units are defined as exact multiples of SI units, they inherit that stability. A conversion can add digits but not accuracy, though. If a walk was "about 5 miles", 8.0467 km claims more than you know; round the answer to the precision of the value you started with.

How to use this calculator.

1. 01Choose the quantity: length, mass, volume, temperature, time, area or speed. The From and To menus then offer only that quantity's units; switching quantity resets both menus to its first two units, Celsius to Fahrenheit for temperature, for example.
2. 02Type the value. Decimals are fine. Only temperature may be negative: the converter refuses a negative length, mass, volume, time, area or speed, and a temperature below absolute zero (0 K, −273.15 °C or −459.67 °F), because neither describes anything real.
3. 03Pick the unit the value is in and the unit you want. Volume uses US measures; for imperial pints or gallons, convert to litres and use 0.56826125 L per imperial pint or 4.54609 L per imperial gallon. Fluid ounces (volume) and ounces (mass) sit in different menus; a US fluid ounce of water weighs about 1.04 oz.
4. 04For mixed measures such as feet and inches or stones and pounds, turn them into one unit first: 5 ft 9 in is 69 in, and 11 st 4 lb is 158 lb.
5. 05Read the converted value, shown to up to four decimal places. Results smaller than 0.001 or of 10¹⁵ and above are shown in scientific notation with four significant figures, so 1 mg comes out as 3.527 × 10⁻⁵ oz. A few results worth remembering: 1 m is 3.2808 ft, 1 kg is 2.2046 lb, 1 ha is 2.4711 acres and 1 m³ is 264.1721 US gallons.

The formula.

For length, mass, volume, time, area and speed the conversion is result = V × f_from ÷ f_to, where V is your value and each f is the unit's size in the base unit. Multiplying by f_from turns the value into metres, grams, litres, seconds, square metres or metres per second; dividing by f_to turns that into the target unit.

Almost every factor is exact by definition. Length uses 0.0254 m for the inch, 0.3048 m for the foot, 0.9144 m for the yard and 1,609.344 m for the mile. Mass uses 453.59237 g for the pound and 6,350.29318 g for the 14-pound stone. The US gallon is 3.785411784 L, and the smaller US measures are exact fractions of it: a quart is a quarter, a pint an eighth, a cup a sixteenth, a fluid ounce 1/128, a tablespoon 1/256 and a teaspoon 1/768. Area uses 0.09290304 m² for the square foot, 4,046.8564224 m² for the acre and 640 acres for the square mile. Time uses the Julian year of 365.25 days, 31,557,600 seconds. Speed uses 0.44704 m/s for the mile per hour, which is 1,609.344 ÷ 3,600, 1 ÷ 3.6 m/s for the kilometre per hour and 1,852 ÷ 3,600 m/s for the knot.

The tables are consistent with one another because derived units are built from the same definitions: the square foot is 0.3048² = 0.09290304 m², the square inch 0.0254² = 0.00064516 m², and the foot per second 0.3048 m/s. That is why 60 mph converts to exactly 88 ft/s.

Two factors that are often rounded are kept exact here. The ounce is a sixteenth of a pound, 28.349523125 g, so 16 oz is exactly 1 lb. The knot is 1,852 m per hour, 0.514444… m/s recurring, carried to 40 digits, so 100 knots is exactly 185.2 km/h.

Temperatures go through kelvin: K = °C + 273.15, or K = (°F − 32) × 5/9 + 273.15, and then back out the same way to the target scale. The offsets are why zero does not convert to zero here: 0 °C is 32 °F and 273.15 K. Both offsets are exact, so −40 °C comes out as exactly −40 °F and 37 °C as 98.6 °F. The check is made on the kelvin value, so it applies whichever scale you type: 0 K, −273.15 °C and −459.67 °F are the same lowest point and are accepted, and anything below that point is refused with an error.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology. NIST Special Publication 811, 2008 Edition — Guide for the Use of the International System of Units (SI). The authoritative US reference for SI usage, derived-unit definitions, and exact conversion factors between SI and US customary units (foot, pound, gallon, acre, mile). Every linear-conversion factor in this calculator traces to SP 811.
2. [2]Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019. The treaty-level reference for the SI base units following the 2019 redefinition that fixed all seven base units via fundamental constants (Planck constant, Boltzmann constant, elementary charge, etc.). Source for the kelvin definition that underpins the calculator's temperature handling.
3. [3]International Organization for Standardization. ISO 80000 series — Quantities and units (Parts 1–14). The international standard for the names, symbols, and definitions of quantities and units across science and engineering. Reviewed periodically; the calculator's symbol conventions (kg, m, s, K, mol) follow ISO 80000-1 (general principles).
4. [4]National Institute of Standards and Technology. Handbook 44 (2024 edition) — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. The US legal-metrology reference adopted by every US state for commercial scales, fuel meters, and retail measuring devices. Source for the US gallon, US fluid ounce, and avoirdupois pound exact definitions.
5. [5]International Astronomical Union. Resolution B2 (2012) — Re-definition of the Astronomical Unit of Length, and adoption of the Julian year (365.25 days × 86 400 s = 31 557 600 s) as the conventional astronomical year. Source for the calculator's year-to-second conversion factor in the time category.
6. [6]International Union of Pure and Applied Chemistry (IUPAC). Compendium of Chemical Terminology, 'Gold Book', entry on Temperature (T). Confirms the kelvin as the SI base unit for temperature and the 273.15 K offset relating the Kelvin and Celsius scales; the conceptual basis for the calculator's Kelvin-anchored temperature algorithm.
7. [7]International Civil Aviation Organization (ICAO). Annex 5 to the Convention on International Civil Aviation — Units of Measurement to be Used in Air and Ground Operations. Source for the international knot (exactly 1.852 km/h) and the nautical mile (exactly 1852 m) used in the calculator's speed category.