When planning cost and materials, calculate the number of deck boards required to complete the deck without running out or having a lot left over. The square footage of a deck also plays a role in planning your materials cost for the type of material used for the deck. The cost per square foot may be the determining factor in choosing materials such as less expensive pressuretreated lumber or higherpriced cedar or fabricated decking materials.
Things You'll Need
 Tape measure

Measure the area planned for the deck with a tape measure or use the measurements from deck plans. If there are outcroppings, use the measurements from these sections as individual calculations.

Multiply the width times the length of each section for the total square footage of each section and add the square footage of each section together. For example, a deck with a main portion of 16 by 24 feet with an additional section that is 10 by 10 feet, the square footage of the main section is 384 square feet and the square footage of the additional section is 100 square feet. In this example, the total square footage is 484 square feet.

Look up the specifications for the length and actual width of deck boards you will use for decking. Some interlocking aluminum and composite deck boards are actual size, while most wood decking boards are actually 5 1/2 inches wide although nominally stated as 6 inches wide. Eight, 10,12, 14 and 16 feet lengths are close to actual size.

Convert the actual width of the deck board to a decimal and add 0.13 for the 1/8inch spacing required between the boards. Divide the result by 12 for the decimal portion of one foot. For example, the decimal portion of a foot for a 5 ½inch deck board with 1/8inch spacing is 5.63 divided by 12 which is 0.47.

Calculate the coverage in square feet of each deck board by multiplying the length by the decimal portion of a foot of the width of the deck boards. For example, a 2by6 by 8 feet long pressuretreated deck board will cover 3.8 square feet.

Divide the square footage of the deck by the square feet each deck board will cover. In the example above, each deck board will cover 3.8 feet. If a deck is 484 square feet, round the result of 127.4 up to 128 deck boards.

Add 15 percent more boards for miscuts and miscalculations if the boards will be installed at 90 degrees and add an additional 15 percent more for deck boards that will be installed on a diagonal pattern. Round up to the nearest whole number and add that to the number of deck boards. In the example of 128 deck boards, adding 15 percent for a 90 degree installation will add 20 more boards and another 15 percent for a diagonal pattern will add a cumulative 42 boards.
References
 Photo Credit Deck de madeira image by CarlosNeto from Fotolia.com