To calculate the area of each of your rectangles, multiply the length by the width. For this project, that sum is 3.3m x 1.1m = 3.63m2 for the first rectangle, and 2.6 x 5.2 = 13.52m2 for the second. Because we are measuring the whole deck, we add these numbers together to get the final area measurement. So that sum is 3.63 + 13.52 = 17.15m2. Make sure to add the 10 per cent contingency to this final sum. You can do this by multiplying the total by 1.1. So 17.15 x 1.1 = 18.865m2.