What is the SI & why is it needed?

The International System of Units (Système International d'Unités) or SI is a global decimal system of measurement that in essence, means that any measurement made anywhere in the world can trace its accuracy back to seven ‘base’ units which in turn arise from fundamental, unchanging universal constants. The SI replaced the previously used metric system in 1960 when it was adopted at the 11th General Conference on Weights and Measures (see The History of Metrology). Since its introduction, the SI has continued to evolve, and on 20 May 2019, a major worldwide change was made to the way its base units were defined to ensure its suitability for the 21st century (link to SI redefinition/benefits).

Click on the units of the SI to get their definitions!

Base Unit: Ampere | Symbol: A

SI Unit: Electrical Current

Defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 x 10-19 when expressed in coulombs, which is equal to A s, where the second is defined in terms of ΔνCs.

Base Unit: Candela | Symbol: cd

SI Unit: Luminous intensity in a given direction

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.

Base Unit: Second | Symbol: s

SI Unit: Time

Defined bby taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s–1.

Base Unit: Kelvin | Symbol: K

SI Unit: Thermodynamic temperature

Defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10−23 when expressed in the unit J K−1, which is equal to kg m2s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.

Base Unit: Kilogram | Symbol: kg

SI Unit: Mass

Defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 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.

Base Unit: Metre | Symbol: m

SI Unit: Length

It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs.

Base Unit: Mole | Symbol: mol

SI Unit: Amount of substance

One mole contains exactly 6.022 140 76 x 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number.

Why do we need SI units?

Measurements affect everything around us; from our transportation systems, the heating in our homes or offices, to the very food we consume. 

When we buy a litre of water we do not pause to consider that the amount in the bottle is correct. The litre we buy from a store in Berlin we assume is the same volume as the litre we bought in Rome; we take it for granted that these measurements are universal. Yet, if we think about it, how is a litre defined? How accurate is the amount of water in the bottle and who checks this or sets its value? 

What are the SI units?

The principle of a common, standardised system of measurements underlies all worldwide trade and - as we measure and weigh at a cost equivalent to 6 % of our combined gross national product (GNP) in Europe - even small errors can lead to significant financial cost.

It is not just trade however that relies on standardised measurements; they underpin all of our scientific and technological advances too. In 1999, a $125 million-dollar satellite burned up in the atmosphere of Mars. The reason was a simple measurement inconsistency. One part of the software controlling the orbital entry thrusters was calculating force using a measurement system based on pounds whilst a different algorithm was performing calculations using newtons.

It is through the work of the metrology organisations, both at the national and international level, that we can be sure of things like the exact definition of a litre. It is through the legislation and the regulations implemented based on their recommendations that we can be sure of the consistency of international trade and the accuracy of our scientific instruments.

All of this is made possible by the International System of Units, our global measurement system (see table on left hand side), which is composed of seven base units – the metre, the second, the kilogram, the kelvin, the mole, the ampere and the candela. Most recently, in 2019, the system was entirely redefined to ensure a direct link between its base units & the fundamental constants of the universe – a change that has imparted a crucially-needed longevity in the SI. 

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