If you're doing an experiment to calculate the volume of something you would work out half of the range of the values you took for the width, length and height.
Then for each of the errors which is the half range you would calculate a percentage error for the width, length and height by doing the (half range/ the width )*100. Finally add all the percentage errors together since your multiplying the width, length and height to work out the volume.
Hope that made some sense