Unit Converters

Unit converters translate measurements between different systems and scales — metric to imperial, Celsius to Fahrenheit, kilograms to pounds, liters to gallons, and hundreds of other conversions. They are among the most universally needed calculation tools, used daily by students, cooks, scientists, engineers, travelers, and tradespeople worldwide.

The world uses two major measurement systems: the International System of Units (SI, or metric system), used by the vast majority of countries and by all scientific disciplines globally, and the United States Customary System (USCS), used colloquially in the United States and informally in a few other countries. The United Kingdom officially uses SI but retains miles for road distances and pints for beer. This mixed landscape means unit conversion is a daily necessity for anyone working across borders or disciplines.

Conversion errors have historically caused serious problems. The Mars Climate Orbiter was lost in 1999 because one engineering team used metric units and another used US customary units without converting. Medication dosing errors involving unit confusion have caused patient deaths. The Gimli Glider incident in 1983 resulted from a fuel quantity error caused by pounds-vs.-kilograms confusion. Reliable unit converters matter.

Our converters cover all major measurement categories: length, area, volume, mass/weight, temperature, speed, pressure, energy, power, digital storage, time, and more. Each converter is based on exact or highly precise conversion factors from NIST (National Institute of Standards and Technology) and the Bureau International des Poids et Mesures (BIPM).

Length is one of the most fundamental physical quantities. The SI base unit is the meter (m), defined since 1983 as the distance light travels in vacuum in exactly 1/299,792,458 of a second. Common metric multiples include the kilometer (1,000 m), centimeter (0.01 m), and millimeter (0.001 m).

US customary length units include the inch (exactly 25.4 mm), foot (12 inches), yard (3 feet), and mile (5,280 feet = 1,609.344 m). Nautical miles (1 NM = 1,852 m) are used in aviation and maritime navigation. Astronomical units (AU), light-years, and parsecs measure cosmic distances.

Temperature is measured on several scales: Celsius (°C), Fahrenheit (°F), Kelvin (K), and Rankine (°R). Celsius and Fahrenheit are the most commonly encountered in everyday life; Kelvin is the SI base unit used in science and is an absolute scale where 0 K (absolute zero) is the lowest possible temperature (−273.15°C).

The Celsius and Fahrenheit scales differ in both their zero points and their degree sizes. Water freezes at 0°C = 32°F and boils at 100°C = 212°F at standard atmospheric pressure. A 1°C change equals a 1.8°F (or 9/5°F) change. The conversion formulas are: °F = (°C × 9/5) + 32 and °C = (°F − 32) × 5/9.

Kelvin and Celsius differ only in their zero points: K = °C + 273.15. The degree size is identical. Rankine is the absolute scale based on Fahrenheit degrees: °R = °F + 459.67. Rankine is used in some US engineering applications such as thermodynamics of gas turbines.

Mass and weight are often used interchangeably in everyday language, but they are physically distinct. Mass is an intrinsic property of matter measured in kilograms; weight is the gravitational force acting on a mass, measured in Newtons (SI) or pounds-force. At Earth's surface, 1 kg of mass has a weight of approximately 9.81 N or 2.205 lbf.

The metric system uses kilograms (kg) as the base unit, with grams (0.001 kg), milligrams (0.000001 kg), and metric tons (1,000 kg) as common subdivisions and multiples. US customary units include ounces (oz), pounds (lb = 16 oz), and US tons (short ton = 2,000 lb). The UK uses long tons (2,240 lb) and metric tons (2,204.6 lb) in different contexts.

Volume conversions are especially confusing because the US and UK use different definitions of liquid volume units with the same names. A US gallon is 3.785 liters; a UK (Imperial) gallon is 4.546 liters. Similarly, a US fluid ounce is 29.57 mL while a UK fluid ounce is 28.41 mL. A US cup is 236.6 mL; a metric cup used in Australia and Canada is 250 mL.

The liter (L) is the SI unit for everyday volume; the cubic meter (m³) is the SI base unit. One liter = 0.001 m³ = 1,000 mL = 1,000 cm³. For dry goods in the US, the bushel, peck, and dry gallon are used in agriculture; these differ from liquid volume units of the same name.