1.3.3—Assess the uncertainty in a derived quantity by simple addition of absolute

For addition or subtraction, add absolute uncertainties. For multiplication, division or powers, add fractional or percentage uncertainties multiplied by the relevant power.

Keep units consistent, avoid overstating precision and round the uncertainty to an appropriate significant figure before matching the value’s decimal place.

For L=2.0±0.1 m and W=3.0±0.1 m, perimeter 2L+2W has absolute uncertainty 2(0.1)+2(0.1)=0.4 m.

Adding percentage uncertainties to an addition is wrong; the operation determines which uncertainty form is appropriate.