The first step in figuring out is to compute the square footage of your deck by multiplying the length by the breadth (L x W). Secondly, determine how each length will fit into your deck structure and whether or not each board will require a cut – the fewer the cuts, the better. Thirdly, after you've decided on the board length, the following step in figuring out "how to determine how much decking I need" is to compute the board's square footage. To do so, multiply the board's length by its width. Moreover, subtract the square footage of the deck from the square footage of the boards. To figure out how many deck boards you will need for your project, multiply the size of your deck by the board area. Furthermore, the distance among all deck boards spanning the deck surface will account for this amount of overall square footage.