Since this is a set of rectangle seat and back cushions, measure the seat and back separately, including dimensions:
Take the three basic measurements (a total of 6 measurements) for both the seat and back cushion area:
Length: Measure the length from the left to the right across the longest side
Width: Measure from the width from one end to the other across the widest side
Thickness: Measure from the top to the bottom, how thick you want your cushion to be
Always double-check your measurements before ordering. Make sure to take separate measurements for both the seat and the back. Always aim at having more thick seats than backs for optimal comfort.