To calculate how much shiplap you need, start by measuring the height and width of each wall and multiply them together to determine the total square footage needed. Next, calculate the total square feet that each shiplap plank covers by multiplying the length of the board (in inches) by the width. Now, divide the SF of your wall by the SF covered by each plank to determine how many planks you need to buy. Lastly, add 10% more or round up to accommodate for waste.