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SI Units

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Base Units
Dimensionless Units
Derived Units
Other

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SI Units

SI stands for Système international. The International System of Units (French Système international d'unités is, as its name implies, the international standard for metric units. It's based on the metre, kilogram and second – and is sometimes referred to by the abbreviation MKS, to distinguish it from the earlier CGS system which was based on the centimetre, gram and second.

The most common type of question in this category is "What is the SI unit of ... ?"

This page provides a little more background information.

Base Units

These are the seven basic SI units from which all the others are derived.

Quantity Unit Abbr Definition
Length Click to show or hide the answer Click to show or hide the answer The distance travelled by light in a vacuum in a specified time. Historically, 10-7 times the distance from the North Pole to the Equator, measured through Paris; in other words, that distance was exactly 10,000 kilometres
Mass Click to show or hide the answer Click to show or hide the answer The weight of a specific cylinder of platinum–iridium, kept at the International Bureau of Weights and Measures in Paris. Historically, the weight of one litre of water
Time Click to show or hide the answer Click to show or hide the answer Defined according to the radiation period of a Caesium–133 atom at 0oC
Electrical current Click to show or hide the answer Click to show or hide the answer Defined according to the force (10-7 newtons) produced between two conductors one metre apart by this amount of current flowing in each conductor
Thermodynamic temperature Click to show or hide the answer Click to show or hide the answer 1/273.16 of the thermodynamic (absolute) temperature at the triple point of water. Historically, 1/100th of the difference between the freezing and boiling points of water
Quantity of matter (mass) Click to show or hide the answer Click to show or hide the answer The quantity of substance that contains the same number of elementary entities (atoms, molecules, ions, electrons or particles, depending on the substance) as there are atoms in 0.012 kilograms of pure carbon–12; approximately equal to 6 x 1023; historically, one gram per atomic mass unit
Luminous intensity Click to show or hide the answer Click to show or hide the answer The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Based on the amount of light emitted by a standard candle (1 candlepower)

Dimensionless Derived Units

Quantity   Unit   Abbr Definition
Angle Click to show or hide the answer Click to show or hide the answer The angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are 2π radians in a circle; a radian is equal to approximately 57.2°
Solid angle Click to show or hide the answer Click to show or hide the answer The solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2. There are 4π steradians on a sphere.

Derived Units

Quantity Unit Abbr Expressed as
Frequency Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Force, weight Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Energy, work, heat Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Power, radiant flux Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Pressure, stress Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Luminous flux Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Illuminance Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Electrical charge or flux Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Electrical potential difference, electromotive force Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Electrical resistance, impedance, reactance Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Electrical capacitance Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Magnetic flux Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Magnetic flux density, magnetic induction Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Inductance Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Electrical conductance Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Radioactivity Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Absorbed dose of radiation Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Equivalent dose of radiation Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Catalytic activity Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer
Thermodynamic temperature Click to show or hide the answer Click to show or hide the answer Click to show or hide the answer

Other quantities and units (selected)

Quantity   Expressed as   Abbr
Area Click to show or hide the answer Click to show or hide the answer
Volume Click to show or hide the answer Click to show or hide the answer
Speed, velocity Click to show or hide the answer Click to show or hide the answer
Acceleration Click to show or hide the answer Click to show or hide the answer
Jerk Click to show or hide the answer Click to show or hide the answer
Angular velocity Click to show or hide the answer Click to show or hide the answer
Momentum, impulse Click to show or hide the answer Click to show or hide the answer
Angular momentum Click to show or hide the answer Click to show or hide the answer
Torque, moment of force Click to show or hide the answer Click to show or hide the answer
Density, mass density Click to show or hide the answer Click to show or hide the answer
Specific volume Click to show or hide the answer Click to show or hide the answer
Heat capacity, entropy Click to show or hide the answer Click to show or hide the answer
Surface tension Click to show or hide the answer Click to show or hide the answer
Thermal conductivity Click to show or hide the answer Click to show or hide the answer
Kinematic viscosity Click to show or hide the answer Click to show or hide the answer
Dynamic viscosity Click to show or hide the answer Click to show or hide the answer
Conductivity Click to show or hide the answer Click to show or hide the answer
Luminance Click to show or hide the answer Click to show or hide the answer
Exposure (to X and gamma rays) Click to show or hide the answer Click to show or hide the answer
Absorbed dose rate Click to show or hide the answer Click to show or hide the answer

© Haydn Thompson 2017