Percentage uncertainties are added. For example the maximum possible uncertainty in the value of resistance R of a conductor determined from the measurements of potential difference V and resulting current flow I by using R = V/I is found as follows: V = 5.2 ± 0.1 V I = 0.84 ± 0.05A
The %age uncertainty for V is =(0.1 V / 5.2 V) X (100 / 100) = about 2 %
The %age uncertainty for I is = (0.05 A / 0.84 A) X (100 / 100) = about 6 %
Hence total uncertainty in the value of resistance R when V is divided by I is 8%. The result is thus quoted as R = = 6.19 VAˉ¹ = 6.19ohms with a %age uncertainty of 8%
that is R = 6.2 ± 0.5 ohms
The result is rounded off to two significant digits because both V and R have two significant figures and uncertainty, being an estimate only, is recorded by one significant figure.