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Unit of Measure conversions
SI base units defined
SI Base Units
Quantity
Unit Name
Symbol
Wikipedia link
Current definition
Original Defintion
1
length
meter
m
Length of the path travelled by light in a vacuum in 1⁄299,792,458 of a second (17th CGPM)
The meter equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris
2
mass
kilogram
kg
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
The grave (the precursor of the kilogram) was defined as the mass of 1 litre (dm3) of water, which was determined to be 18841 grains.
3
time
second
s
The second is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.
The second was historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each (24 × 60 × 60 = 86400).
4
electric current
ampere
A
The ampere is defined by fixing the elementary charge e to be exactly 1.602176634×10−19 C (coulomb),
One ampere is equal to 6.241509074×1018 electrons worth of charge moving past a point in a second
5
thermodynamic temperature
kelvin
K
The scale is defined by fixing the Boltzmann constant k to be exactly 1.380649×10−23 J⋅K−1
The Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the melting point of water to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius
6
amount of substance
mole
mol
The mole is defined as containing exactly 6.02214076×1023 elementary entities
The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.
7
luminous intensity
candela
cd
The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
A common wax candle emits light with a luminous intensity of roughly one candela.
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