Bob explains in his LBM Journal article how to calculate the number of deck boards you need from square feet. Below is an excerpt from the article.

In the past, I’ve discussed efficient ways that can help contractors estimate the cost of building a deck, which I refer to as deck math. As I’ve stated in previous columns, deck builders most often use 12- and 16-ft. boards during a deck project, not always being aware that the deck is going to cost more than if they were to use 10- and 14-ft. boards. This is due to the fact that if the length of the boards aren’t evenly divisible by 12, the boards are more likely to cost less.

This issue, I’m going to focus on some additional techniques such as joist quantity, determining the length of the longest piece of lumber you’ll need in a diagonal project and estimating the total square footage in lineal feet. By mastering these methods, deck builders will be able to estimate a deck more efficiently and accurately.

One factor to consider when estimating the framing of a deck is: How many joists is it going to take to cover the area of the deck?

The calculation is relatively simple. For example, with a 14-ft. x 100- ft. deck, you need to know how many joists it’s going to take to cover the deck with each joist 16-in. on center (OC). This can be done using a simple math formula. Take the width of the deck (100 feet) and multiply that by .75, plus one joist to either start or finish. If you do the math, you can see that you’re going to need 76 joists to cover up that area. [For 16-in. OC, use (width x .75) + 1; for 12-in. OC, use (width x 1.0) + 1.]

Now that you know how many joists you will need for the project, the next thing you’re going to want to know is how much decking material it’s going to take to cover the deck and how much it will cost to cover the deck with wood or composite decking—and that’s something you can easily and quickly calculate.

While we sell these materials by the piece, not lineal footage, it’s quite easy to convert. You take the price of the board, divide it by the length of the board, and that tells you your cost per foot (Cost ÷ by Length of Board) = Price per lineal foot. Now, good quality cedar board might be $1.30 per lineal foot, while a quality cap-stock composite is probably closer to $4.00.

If we use the same example of a 14-ft. x 100-ft. deck, we would end up with 1,400-sq. ft. of deck surface. Then, you want to take the square feet of 1,400 and multiply it by 2.3. The reason we use 2.3 is to convert lineal footage to square footage is because there is a gap between the boards and the boards really are not 6 inches wide. It takes 2.3 lineal feet of product to cover 1-sq. ft. of deck area. If you do the math, it will tell you that you need 3,220 lineal feet of 5-1/2-in. material to cover this deck.

To determine the cost based on the material, you just multiply the price of the material per lineal foot (ex: $1.30 for cedar, $4.00 for composite) by the total lineal feet of the deck. Another heads up rule I have is, a composite railing generally costs as much as the decking. To calculate decking cost: (deck area in S.F. x 2.3) x lineal foot cost of decking).

Finally, another rule to consider if you’re going to build your deck diagonally is, what’s going to be the longest available piece you will need? There is an equation that works for any size deck. Again, we’ll use a 14-ft. deck for this example. Divide the 14-ft. by the magic number .707. The answer will be the longest length piece required to reach 14 feet (remember to add the width of the deck board to the answer). (Length in feet ÷ by .707) +5.5-in. = total length of a deck board applied at a 45° angle. For reference, .707 is a shortcut to the Pythagorean Theorem and the sine of a 45° angle. Since you’re cutting at an angle, you have to add 5-1/2-in. to that or make a border around the deck, but you’ll know the needed measurements by some simple math.