Guide to Fundamental SI Units

The foundation of modern metrology is the International System of Units (SI), which is used internationally to define the fundamental units of measurement. These are used to derive other units and are used to define other measurement units, such as may be used locally.

Base Units

Parameter

SI Unit

Abbreviation

Definition

Time

second

s

The duration of 9,192,631,770 cycles of radiation corresponding to a particular transition of the cesium-133 atom.

Mass

kilogram

kg

The mass of a cylinder of platinum iridium alloy kept in Paris, France.

Electric Current

Ampere

A

The current producing a force of 2*10-7 newtons per meter of length between two long wires 1 m apart in space.

Thermodynamic Temperature

Kelvin

K

1/273.16 of the thermodynamic triple point of water.

Luminous Intensity

candela

cd

The luminous intensity in a given direction of a source that emits monochromatic radiation at a frequency of 540*1012 Hertz, with a radiant intensity in that direction of 1/683 watts per steradian.

Amount of Substance

mole

m

The amount of substance in a system that contains as many elementary items as there are atoms in 0.012 kg of carbon 12.

Length

metre

m

The distance traveled by light in a vacuum in 1/299,792,458 s.

 

Derivative Units

Parameter

SI Unit

Abbreviation

Definition

Electric Potential

volt

V

W/A

Frequency

hertz

Hz

1/s

Force

newton

N

kg*m/s

Pressure

pascal

Pa

N/m

Power

watt

W

J/s

Dose Equivalent

sievert

Sv

m/s

Electric Resistance

ohm

ohm

V/A

Quantity of Charge

coulomb

C

A*s

Electric Capacitance

farad

F

C/V

Conductance

siemens

S

A/V

Magnetic Flux

weber

Wb

V*s

Magnetic Flux Density

tesla

T

Wb/

Inductance

henry

H

Wb/A

Celsius Temperature

degree

C

K

Luminous Flux

lumen

lm

cd*sr

Illuminance

lux

lx

lm/m

Activity

becquerel

Bq

1/s

Absorbed Dose

gray

Gy

J/kg

Work or Energy

joule

J

N*m

 
Supplementary Units

Parameter

SI Unit

Abbreviation

Definition

Solid Angles

steradian

sr

The solid angle with vertex at the center of a sphere that is subtended by an area of the spherical circle equal to that of a square with sides equal in length to the radius.

Plane Angles

radian

rad

A plane angle with vertex at the center of a circle that is subtended by an arc equal in length to the radius.