International System of Units (SI) Browser

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.

International System of Units (SI)

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.

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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.

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Understanding Measurement Systems

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.

Major Measurement Systems

metric:
The metric system is the foundation of the SI and is used by most countries worldwide. It's based on powers of 10, making conversions simple.
imperial:
Used primarily in the United States, the Imperial system includes units like inches, feet, pounds, and gallons. It evolved from British units of measurement.
us Customary:
Similar to Imperial but with some differences in volume measurements. The US is one of only three countries (along with Liberia and Myanmar) that hasn't fully adopted the metric system.
cgs:
The centimeter-gram-second system was an earlier metric system that used the centimeter, gram, and second as base units before SI was established.
natural Units:
Used in theoretical physics, these units are based on universal physical constants, where values like the speed of light and Planck's constant are set to 1.
planck:
The smallest meaningful units of measurement, Planck units are based on fundamental physical constants and are used in theoretical physics.
nautical:
Used in marine and air navigation, with units like the nautical mile (1.852 km) and knot (one nautical mile per hour).
astronomical:
Used for cosmic distances, including the astronomical unit (AU, Earth-Sun distance), light-year, and parsec.

Fascinating Unit Facts

  • The International System of Units (SI) was established in 1960 but has roots dating back to the French Revolution.
  • The kilogram was the last SI base unit defined by a physical artifact until 2019, when it was redefined based on fundamental physical constants.
  • Light travels exactly 299,792,458 meters per second - the meter is now defined based on this constant speed of light.
  • The second is defined by the radiation of a cesium-133 atom, specifically 9,192,631,770 cycles of radiation.
  • There are seven base SI units: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).
  • SI prefixes range from yocto (10^-24) to yotta (10^24), allowing measurements across 48 orders of magnitude.
  • In 2019, all seven SI base units became defined by fundamental physical constants rather than physical artifacts.

Did You Know?

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.

Frequently Asked Questions