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International System of Units (SI)

Also called: SI, metric system, SI units

Definition: The International System of Units (SI) is the modern metric system: seven base units, including the meter, kilogram, second and kelvin, with prefixes that scale by powers of ten.

Updated October 28, 2026 · How we check facts

The International System of Units (SI) is the international standard for measurement, the modern form of the metric system. NIST lists seven base units: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity). Other units, such as the newton and the pascal, are derived from them. Prefixes scale units by powers of ten, from 10³⁰ down to 10⁻³⁰.

Example

Prefixes make conversion a matter of moving the decimal point: 2.5 km = 2,500 m, and 2,500 m = 250,000 cm. Compare US customary units, where 2.5 miles = 13,200 feet = 158,400 inches.

The metric vs imperial guide compares the two systems. The inches to cm tool and the Celsius to Fahrenheit tool connect them. The United States was among the seventeen countries that signed the Treaty of the Meter in 1875.

Calculate it

Free toolInches to CM ConverterInches to cm, cm to inches and height in feet.Free toolCelsius to Fahrenheit Converter°C to °F and back, with the formula shown.

Guides that explain it

Related terms

Kelvin (K)The kelvin is the SI base unit of temperature. It starts at absolute zero (0 K), and one kelvin is the same size as one degree Celsius, with 0 °C equal to 273.15 K.US Customary UnitsUS customary units are the inch, foot, mile, ounce, pound, cup and gallon used for everyday measures in the United States; each is defined by an exact or fixed value in SI units.Absolute ZeroAbsolute zero is 0 K, the lowest point of the kelvin temperature scale. On the Celsius scale it equals −273.15 °C, and on the Fahrenheit scale about −459.67 °F.Greatest Common Factor (GCF)The greatest common factor (GCF) of two or more positive integers is the largest whole number that divides each of them with no remainder; it is also called the GCD or HCF.Least Common Multiple (LCM)The least common multiple (LCM) of two or more positive integers is the smallest positive number that each of them divides into evenly; it gives the lowest common denominator.Mixed NumberA mixed number combines a whole number and a proper fraction, such as 4 9/100; it equals an improper fraction, here 409/100, and is what a decimal with a nonzero whole part converts to.Percent ChangePercent change is the difference between a new and an original value, divided by the original value and expressed as a percentage; a positive result is an increase and a negative one a decrease.Repeating DecimalA repeating decimal is a decimal whose digits eventually repeat in a fixed block forever, such as 0.333… or 0.4545…; every repeating decimal is a rational number and can be written as a fraction.

Sources

  1. NIST — SI Units