The Complete Unit Conversion Reference 2026: Every Factor, Formula, and Table You Need

The International System of Units (Systeme International), the modern metric system. Based on seven base units: meter, kilogram, second, ampere, kelvin, mole, and candela. Used by virtually all countries for science and most for daily life. Redefined in 2019 to base all units on fundamental physical constants.

A decimal measurement system originally developed in France in the 1790s. Uses base-10 prefixes (kilo = 1000, milli = 0.001, etc.) for easy conversion between units. The modern form is the SI system. Nearly every country in the world has officially adopted the metric system.

The measurement system used in the British Empire, formalized in 1824. Includes inches, feet, yards, miles (length); ounces, pounds, stones (mass); pints, quarts, gallons (volume). The US uses a variant called US Customary units with some differences (e.g., US gallon differs from Imperial gallon).

The measurement system used in the United States, derived from English units. Similar to Imperial but with key differences: US gallon = 3.785 L vs Imperial gallon = 4.546 L; US ton = 2000 lb vs Imperial ton = 2240 lb. Used for everyday measurements in the US.

A fundamental unit in a measurement system from which other units are derived. The SI has seven base units. All other SI units (newton, joule, watt, etc.) are derived from combinations of these base units through multiplication and division.

A unit defined as a combination of base units. For example, the Newton (force) = kg * m / s^2, the Joule (energy) = kg * m^2 / s^2, and the Watt (power) = kg * m^2 / s^3. Derived units often have special names for convenience.

A method of checking equations and converting units by treating units as algebraic quantities. If the units on both sides of an equation match, the equation is dimensionally consistent. Used to derive conversion formulas and catch errors in calculations.

A ratio that expresses one unit in terms of another. For example, 1 mile = 1.60934 km, so the conversion factor from miles to km is 1.60934. Multiply by the conversion factor to convert. Division by the factor converts in the reverse direction.

A modifier placed before a unit to indicate a power of 10. Common metric prefixes: tera (10^12), giga (10^9), mega (10^6), kilo (10^3), milli (10^-3), micro (10^-6), nano (10^-9), pico (10^-12). The binary prefixes (kibi, mebi, gibi) use powers of 1024.

Temperature measured from absolute zero, the lowest possible temperature. Kelvin (K) and Rankine (R) are absolute scales. 0 K = -273.15 C = -459.67 F. At absolute zero, all molecular motion ceases. Kelvin intervals equal Celsius intervals; Rankine intervals equal Fahrenheit intervals.

Temperature scale where water freezes at 0 degrees and boils at 100 degrees at standard atmospheric pressure. Defined relative to Kelvin: Celsius = Kelvin - 273.15. Used worldwide except in the US for daily temperature measurement.

Temperature scale where water freezes at 32 degrees and boils at 212 degrees. Used primarily in the US for weather and cooking. To convert to Celsius: C = (F - 32) * 5/9. Named after Daniel Gabriel Fahrenheit (1724).

The SI base unit for temperature. An absolute scale where 0 K = absolute zero. One kelvin equals one degree Celsius in magnitude. Used in science and engineering worldwide. Does not use the degree symbol (write 300 K, not 300 degrees K).

The SI unit of force. One newton is the force needed to accelerate one kilogram at one meter per second squared (1 N = 1 kg * m/s^2). Named after Sir Isaac Newton. Approximately the force of gravity on a small apple (about 100 grams).

The SI unit of energy. One joule equals the energy transferred by a force of one newton acting over one meter (1 J = 1 N * m = 1 kg * m^2/s^2). Named after James Prescott Joule. One food Calorie = 4184 joules.

The SI unit of power. One watt equals one joule per second (1 W = 1 J/s). Named after James Watt. A typical incandescent light bulb uses 60 watts; a human body at rest generates about 80 watts of heat.

The SI unit of pressure. One pascal equals one newton per square meter (1 Pa = 1 N/m^2). Named after Blaise Pascal. Standard atmospheric pressure is 101,325 Pa. The pascal is very small, so kilopascals (kPa) and megapascals (MPa) are more commonly used.

The SI unit of frequency. One hertz equals one cycle per second. Named after Heinrich Hertz. Human hearing range: 20 Hz to 20,000 Hz. CPU clock speeds are measured in gigahertz (GHz). Radio frequencies in megahertz (MHz).

A metric unit of pressure equal to 100,000 pascals. Close to standard atmospheric pressure (1 atm = 1.01325 bar). Used in meteorology, diving, and tire pressure. Not an SI unit but widely used.

Pounds per square inch. An Imperial unit of pressure used in the US for tire pressure, hydraulics, and HVAC. 1 PSI = 6894.76 Pa. Standard atmospheric pressure = 14.696 PSI.

British Thermal Unit. The amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. 1 BTU = 1055.06 joules. Used in the US for HVAC ratings, natural gas billing, and heating/cooling capacity.

A unit of energy. The small calorie (cal) is the energy to raise 1 gram of water by 1 degree Celsius. The food Calorie (Cal, kcal) = 1000 small calories. 1 kcal = 4.184 kJ. The calorie is not an SI unit but is widely used in nutrition.

A unit of power originally defined by James Watt to compare steam engine output to draft horses. 1 mechanical horsepower = 745.7 watts. Used for engine and motor ratings. Metric horsepower (PS) = 735.5 watts.

A unit of speed equal to one nautical mile per hour (1.852 km/h or 1.15078 mph). Used in maritime and aviation. Named for the knots tied in a rope on old sailing ships used to measure speed.

A unit of distance used in navigation. Defined as exactly 1852 meters. Approximately one minute of arc of latitude on the Earth surface. Different from a statute mile (1609.344 m).

A unit of area used primarily in the US and UK. 1 acre = 43,560 square feet = 4046.86 square meters. Originally defined as the area one man and one ox could plow in a day. 640 acres = 1 square mile.

A metric unit of area equal to 10,000 square meters (100m x 100m). 1 hectare = 2.471 acres. Used for measuring land area worldwide except in the US. Not an SI unit but accepted for use with SI.

A unit of mass used for precious metals (gold, silver, platinum). 1 troy ounce = 31.1035 grams. Heavier than the avoirdupois ounce (28.3495 g) used for other goods. Gold prices are quoted per troy ounce.

A unit of mass used for gemstones. 1 carat = 200 milligrams (0.2 g) exactly. Not to be confused with karat (gold purity). A 5-carat diamond weighs 1 gram.

A British unit of mass equal to 14 pounds (6.35 kg). Still commonly used in the UK and Ireland for body weight. A person weighing 150 lb would say they weigh 10 stone 10.

A unit of volume. The oil barrel = 42 US gallons (158.987 liters). The beer barrel = 31 US gallons. Used as the standard unit for crude oil pricing and production statistics.

A unit of volume. US fluid ounce = 29.5735 mL. Imperial fluid ounce = 28.4131 mL. Not to be confused with ounce (mass). 1 US cup = 8 US fluid ounces.

The smallest unit of digital information, representing a binary digit (0 or 1). 8 bits = 1 byte. Data transfer speeds are typically measured in bits per second (Mbps, Gbps).

A unit of digital information consisting of 8 bits. Can represent 256 distinct values (0-255). Storage capacity is measured in bytes: kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB).

A kilobyte (KB) = 1000 bytes (SI decimal). A kibibyte (KiB) = 1024 bytes (IEC binary). The confusion between these is why a "500 GB" hard drive shows about 465 GiB in your OS. Storage manufacturers use decimal (1000), operating systems use binary (1024).

The SI unit of angle. One radian is the angle subtended at the center of a circle by an arc equal in length to the radius. Full circle = 2*pi radians = 360 degrees. Used in mathematics, physics, and programming.

A unit of angle where a right angle equals 100 gradians and a full circle equals 400 gradians. Used in some surveying work. Also called gon. 1 gradian = 0.9 degrees.

The CGS unit of force. 1 dyne = 10^-5 newtons = 1 g * cm/s^2. Used in some older scientific literature. Replaced by the newton in modern usage.

A unit of energy used in particle physics. 1 eV = 1.602 x 10^-19 joules. The energy gained by an electron moving through a 1-volt potential difference. Particle masses are often expressed in eV/c^2.

A unit of astronomical distance. The distance light travels in one Julian year (365.25 days) in vacuum = 9.461 x 10^12 km. Used for interstellar and intergalactic distances. The nearest star (Proxima Centauri) is 4.24 light-years away.

An astronomical unit of distance defined as the distance at which one astronomical unit subtends an angle of one arcsecond. 1 parsec = 3.26156 light-years = 3.086 x 10^13 km. Used in professional astronomy.

The ratio of the speed of an object to the speed of sound in the surrounding medium. Mach 1 = speed of sound (343 m/s at sea level, 20C). Mach 2 = twice the speed of sound. The speed of sound varies with temperature and altitude.

The lowest possible temperature, at which all molecular motion ceases. 0 K = -273.15 C = -459.67 F. Cannot be reached experimentally, only approached. Quantum mechanics prevents perfect stillness even at 0 K (zero-point energy).

A reference pressure defined as exactly 101,325 pascals. Used as the standard for atmospheric pressure. At this pressure, water boils at 100 C. 1 atm = 1.01325 bar = 14.696 PSI = 760 mmHg.

The digits in a number that carry meaningful information about its precision. In unit conversions, the result should have no more significant figures than the least precise input. 2.54 cm/in (exact) times 5.0 inches = 12.7 cm (3 significant figures, matching the 5.0).

Precision is how close repeated measurements are to each other (reproducibility). Accuracy is how close a measurement is to the true value (correctness). A scale that consistently reads 2 grams too heavy is precise but not accurate.

Prefixes that multiply a unit by a power of 10. From largest to smallest commonly used: tera (T, 10^12), giga (G, 10^9), mega (M, 10^6), kilo (k, 10^3), milli (m, 10^-3), micro (u, 10^-6), nano (n, 10^-9), pico (p, 10^-12). Newer: quetta (Q, 10^30), ronna (R, 10^27), added in 2022.

Prefixes for powers of 1024 (2^10) used for digital storage: kibi (Ki, 1024), mebi (Mi, 1024^2), gibi (Gi, 1024^3), tebi (Ti, 1024^4), pebi (Pi, 1024^5). Standardized by IEC in 1998 to distinguish from SI decimal prefixes.

The centimeter-gram-second system, an older metric system. Used centimeter, gram, and second as base units instead of meter, kilogram, and second. Still used in some areas of physics (electromagnetism, astrophysics). Being replaced by SI.

The standard system of weights used in the US and UK for most goods (excluding precious metals and gemstones). 1 avoirdupois pound = 16 ounces = 453.592 grams. Distinguishes from troy weight (used for precious metals) and apothecary weight.

The process of adopting the metric system as the standard system of measurement. Most countries completed metrication by the mid-20th century. The US is the only major industrialized country that has not fully metricated, though metric is used in science, medicine, and the military.

Various non-standard measurement systems used for specific applications: wire gauge (AWG for wire diameter), sheet metal gauge, shotgun gauge (bore diameter). These are not part of SI or Imperial and are industry-specific legacy systems.

A unit of pressure approximately equal to 1 mmHg (millimeter of mercury). Named after Evangelista Torricelli. 1 Torr = 133.322 Pa. Used in vacuum technology and blood pressure measurement (though mmHg is the official medical unit).

An absolute temperature scale using Fahrenheit-sized degrees. 0 R = absolute zero = -459.67 F. Rankine = Fahrenheit + 459.67. Used in some US engineering applications, particularly thermodynamics.

An old English unit of length equal to 660 feet (201.168 meters). Originally the length of a furrow in a plowed field. Still used in horse racing. 8 furlongs = 1 mile.

A unit of depth used in nautical contexts, equal to 6 feet (1.8288 meters). Originally the span of outstretched arms. Used for measuring water depth. "I cannot fathom it" means "I cannot measure its depth."

An ancient unit of mass based on a grain of wheat. 1 grain = 64.8 mg. Used in ammunition (bullet weight), pharmacy (medication dosing), and precious metals. 7000 grains = 1 avoirdupois pound.

A unit of length equal to 10^-10 meters (0.1 nanometers). Used for atomic and molecular dimensions: bond lengths, crystal structures, X-ray wavelengths. Not an SI unit but widely used in chemistry and physics.