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The mass of a substance is the amount we measure when we weigh the substance. Mass is a measure of the quantity of a substance. In science, we use the term mass as opposed to weight of a substance to describe the amount because the weight of a substance is related to the gravitational force acting on the substance. We often use the words ‘mass’ and ‘weight’ interchangeably, but they are not really the same thing scientifically.

The SI unit of mass is the kilogram. However, in the laboratory, chemicals and other objects are usually weighed in units of grams as the gram is a far more convenient amount to work with. Even though we use a balance to weigh a substance, we report its mass in grams, not its weight.

One kilogram is equivalent to one thousand grams, 1 000 g. 1 kg = 1 000 g. Therefore 1 g = or 0.001 kg. This can also be written as 1 × 10⁻³ kg.

Another common measure of mass is the milligram, mg. One milligram is one‐thousandth of a gram:

You should always choose a balance appropriate to the level of accuracy of the mass required. If you need ‘about two grams’, for example, a two‐figure balance is perfectly appropriate. If you are told to weigh ‘accurately approximately two grams’, this implies you need to use a four‐figure balance to weigh around two grams of substance but record its weight accurately to four decimal places, e.g. 0.0001 g.