SI units and measurement

Scientists all over the world measure in the SI system (Système International). It has seven base units:

All other units are derived from these: speed in m s⁻¹, density in kg m⁻³, and so on.

Prefixes make a unit bigger or smaller by a power of ten. The common ones: femto 10⁻¹⁵, pico 10⁻¹², nano 10⁻⁹, micro 10⁻⁶, milli 10⁻³, centi 10⁻², deci 10⁻¹; and deca 10, kilo 10³, mega 10⁶, giga 10⁹. To convert, simply replace the prefix by its power of ten: 5 km = 5 × 10³ m, and 3 ns = 3 × 10⁻⁹ s.

Mass is the amount of matter in an object, and stays the same everywhere. Weight is the pull of gravity on that mass, and changes from place to place: you would weigh less on the Moon. Mass is measured with a balance.

Volume has the SI unit m³, but in the laboratory we use smaller units. 1 L = 1 dm³ = 1000 cm³, and one millilitre is the same as one cubic centimetre.

Density = mass ÷ volume. Its SI unit is kg m⁻³, but in the laboratory it is often given in g cm⁻³.

Temperature is measured on three scales. °F = (9/5)°C + 32, and K = °C + 273.15. Since the kelvin scale starts at the lowest possible temperature, kelvin temperatures are never negative. For example, 25 °C is 298.15 K.