Once you decide on the size of the boards that you will be using for your deck, figuring how many of those boards you need to complete your deck is quite simple. Of course, once you've figured out how many boards you are going to need, it's always a good idea to purchase at least a couple of extra boards just in case you make an error in cutting a board or in case you find a board that can't be used for one reason or another.
Measure the width and length of your deck. Write down the dimensions.
Decide on the size of board you will be using for your decking. For purposes of this article we will assume that you will be using 2 X 6 planks.
Measure the actual dimension of the boards you are planning to buy. A board that is advertised as 6 inches wide is generally only 5 1/2 inches wide when it is ready for sale. Make certain of the exact width of the boards you will actually be using.
Decide on a spacing between boards. Many people like 1/8 inch between deck boards, and others prefer 1/4 of an inch spacing. Decide on what you like and add that amount to the actual width of the decking boards. In this example the actual width of decking boards is 5 1/2 inches with 1/4 of an inch of spacing between boards.
Get out your calculator. Each board is 5 1/2 inches wide with 1/4 of an inch of spacing between boards. That gives us a working width of 5 3/4 inches per board (5 1/2 + 1/4 = 5 3/4). Assuming your decking boards will run lengthwise on your deck, find the width of your deck in inches and simply divide the width of the deck area by 5 3/4 inches. As an example, let's assume that you deck is 24 feet wide. 24 feet is 288 inches (24 X 12 = 288). Divide 288 by 5 3/4 and you get 50.09. In other words you would need just a hair over 50 boards, each one 5 1/2 inches wide to cover 288 inches (24 feet) if there was 1/4 of an inch space between each board. To be safe you would want to buy at least 52 to 55 boards.
Increase the number of boards you need if your deck is longer than the longest decking board you can buy. For example, if your deck is 15 feet long and the longest deck board you can buy is 10 feet long, then you will need to purchase 50 additional boards, 5 feet long to make up the difference. In such a case you would need 50 boards 10 feet long plus 50 boards 5 feet long.
Always buy 5 per cent to 10 per cent more than you think you are going to need in order to cover all contingencies.