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When carrying out calculations in science, the answer must be given to the same level of accuracy as the values in the question. For example, if a balance weighs to two decimal places, the mass of substance weighed may be read as 5.02 g. The total number of significant figures in this number is three. This means we know the mass accurate to 3 significant figures.

The number can be rounded to a smaller number of significant figures as shown:

 Three significant figures = 5.02 g

 Two significant figures = 5.0 g

 One significant figure = 5 g

An answer should be given to the same number of significant figures as the number with fewest significant figures in the calculation. For example, if the mass of solid is 5.02 g and the mass of water the solid is added to is 50 g (the same as 50 mL or 50 cm³), the total mass of the solid and water would be 5.02 g + 50 g = 55 g. The answer can only be given to two significant figures as the mass of water is only known to this level of accuracy. In fact, the mass of water could actually be any mass between 49.50 g and 50.49 g, which are both equivalent to 50 g when given to two significant figures.

There are some rules for determining how many significant figures are in a number:

1 Any zeroes before a digit are not significant. For example, 0.005 is only accurate to 1 significant figure.

2 Any zeroes after a digit are significant. For example, 0.00500 is accurate to 3 significant figures.

3 Digits below the number 5 are always rounded downwards, and digits equal to or above the number 5 are rounded upwards, as in the following examples:The number 0.544 becomes 0.54 when written to two significant figures.The number 0.545 becomes 0.55 when written to two significant figures.The number 0.546 becomes 0.55 when written to two significant figures.

4 When performing a calculation, determine the required number of significant figures and round up or down at the end of the calculation, not at the steps in between.

5 Always give your answer to the same accuracy as that of the value known to the least number of significant figures in the calculation.