The International System of Units (SI)

The International System of Units, abbreviated SI from the French Système international d'unités, is the modern form of the metric system and is the world's most widely used system of measurement. It comprises seven base units from which all other SI units are derived.

SI Base Units

• Metre (m) - length: defined by the distance travelled by light in vacuum in 1/299,792,458 seconds
• Kilogram (kg) - mass: defined by the Planck constant (6.626 070 15 × 10⁻³⁴ J·s)
• Second (s) - time: defined by the caesium-133 atom's radiation frequency
• Ampere (A) - electric current: defined by the elementary charge (1.602 176 634 × 10⁻¹⁹ C)
• Kelvin (K) - thermodynamic temperature: defined by the Boltzmann constant (1.380 649 × 10⁻²³ J/K)
• Mole (mol) - amount of substance: defined by the Avogadro constant (6.022 140 76 × 10²³ mol⁻¹)
• Candela (cd) - luminous intensity: defined by the luminous efficacy of 540 THz radiation

Imperial and US Customary Units

Imperial units and US customary units are systems of measurement that originated in Britain and are still used in some countries, particularly the United States. These include units like inches, feet, yards, miles, ounces, pounds, and gallons. All imperial and US customary units used on this site have exact conversions to SI units as defined by international agreement.

Other Unit Systems

Various specialized fields use their own units, often for historical or practical reasons. Examples include astronomical units for cosmic distances, nautical miles for maritime navigation, and bars for pressure measurement. This site provides accurate conversions between these specialized units and their SI equivalents.