Every measured value carries an uncertainty, and when measured quantities are combined in a calculation, their uncertainties combine as well. A reading with three significant figures, such as the length 16.2 cm measured with a metre scale, is written as 16.2 ± 0.1 cm, which equals 16.2 cm ± 0.6 %. Similarly a breadth of 10.1 cm is written as 10.1 ± 0.1 cm, or ± 1 %. For a product of two or more experimental values, the combination of errors rule states that the percentage errors add: the area 16.2 × 10.1 = 163.62 cm² has an error of about ± 1.6 %, that is ± 2.6 cm², so the result is quoted as 164 ± 3 cm². Here 3 cm² is the uncertainty in the estimate of area. If data are specified to n significant figures, a result obtained by combining them is also valid to n significant figures; however, when data are subtracted the number of significant figures can reduce, as 12.9 g − 7.06 g can only be given as 5.8 g. The relative error also depends on the size of the number and not only on the number of significant figures: 1.02 g and 9.89 g are both accurate to ± 0.01 g, but their relative errors are ± 1 % and ± 0.1 %. Finally, intermediate results should carry one extra significant figure to avoid accumulated rounding errors.