Explore the International System of Units (SI), the modern metric system used worldwide. Learn about base and derived units, conversion factors, and the history of standardized measurements. No Signup Required.
The International System of Units (SI) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world.
The International System of Units (SI) was established in 1960 and has been adopted by most countries worldwide. It consists of seven base units from which all other units are derived.
Measurement systems are more than just practical tools—they reflect human history, scientific advancement, and international cooperation. The SI system represents a global standard that enables scientific collaboration across borders.
The metric system was first proposed during the French Revolution as a rational, universal system based on nature. The original definition of the meter was one ten-millionth of the distance from the North Pole to the Equator along a meridian passing through Paris.
The International System of Units (SI) is the modern form of the metric system and the world's most widely used system of measurement. It consists of seven base units that define fundamental physical quantities, along with derived units formed by combining base units, and a system of prefixes for indicating multiples and fractions.
The seven base SI units are: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.
SI prefixes denote multiples or fractions of units in powers of 10. For example, 'kilo' means 1,000 times (10³), so a kilometer is 1,000 meters. 'Milli' means one-thousandth (10⁻³), so a millimeter is one-thousandth of a meter. Prefixes range from yocto (10⁻²⁴) to yotta (10²⁴).
The United States actually does use the metric system in many scientific, medical, and industrial applications. However, for everyday measurements, it continues to use the US customary system (similar to the Imperial system) due to historical precedent, the cost of conversion, and cultural resistance to change.
Since 2019, all SI base units are defined in terms of fundamental physical constants. For example, the meter is defined by the speed of light in vacuum, and the kilogram is defined using the Planck constant. This approach ensures that the definitions are stable and can be realized anywhere with sufficient technical capability.
Mass is a measure of the amount of matter in an object and is measured in kilograms (SI unit). Weight is the force exerted on an object due to gravity and is measured in newtons (SI unit). Your mass stays the same everywhere, but your weight would be different on the Moon compared to Earth due to different gravitational forces.
You can use conversion factors to switch between systems. For example, 1 inch = 2.54 centimeters (exactly), 1 pound ≈ 0.454 kilograms, and 1 gallon (US) ≈ 3.785 liters. Our tool provides these conversions automatically.