The SI system

Three older systems were widely used: CGS (centimetre, gram, second), FPS or British (foot, pound, second) and MKS (metre, kilogram, second).

Today's internationally accepted standard is SI, short for the French Système Internationale d'Unités. The BIPM (International Bureau of Weights and Measures) set up its scheme of units and symbols in 1971, and the General Conference on Weights and Measures revised it in November 2018.

SI has seven base quantities and units: length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), thermodynamic temperature (kelvin, K), amount of substance (mole, mol) and luminous intensity (candela, cd).

Since the 2018 revision each base unit is defined by fixing the numerical value of a constant of nature: the caesium-133 hyperfine frequency for the second, the speed of light c for the metre, the Planck constant h for the kilogram, the elementary charge e for the ampere, the Boltzmann constant k for the kelvin, the Avogadro constant N_A for the mole, and a luminous efficacy of 683 lm W⁻¹ at 540 × 10¹² Hz for the candela.

The exact defining values are shown to indicate how precisely the units are fixed; NCERT states they need not be memorised.

When the mole is used, the elementary entities must be specified: atoms, molecules, ions, electrons or other particles.

Two supplementary units are defined as ratios: the plane angle dθ = ds/r in radian (rad), and the solid angle dΩ = dA/r² in steradian (sr). Both are dimensionless.

SI is a decimal system, so converting between multiples and sub-multiples only shifts powers of 10; some derived units carry special names, such as newton, joule and watt.