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Estimating Decking Materials

Estimating the amount of material you will need to build a deck is part art, part science. You may not hit the quantities exactly right, but you can get close with the formulas and methods on this page. It’s also wise to check the return policy of your supplier beforehand; make sure you can bring back any unused salable items for a full refund.

Estimating lumber

For a small deck (10 × 12 feet, for example), determine how much lumber you’ll need by using your detailed plans to count all the pieces of each size—12-foot 2 × 4s, 8-foot 2 × 6s, and so on—for all framing members, including stairs and railings. Add 10 percent to framing and 15 percent to decking to allow for waste.
Calculate the decking required based on the actual width of the boards you will use, including the gap between boards. The ends of stock decking lengths won’t always fall exactly on joists spaced on 16-inch centers. Figure the lengths necessary to fit the joist spacing with minimum waste. (Add an extra 15 percent for diagonal decking.)
For larger decks, you can calculate the total square footage of decking you need by multiplying the length of the deck surface times the width. Allow for overhangs. Then buy enough lineal feet of decking to make up the deck area plus the waste allowance. Make actual counts of posts, beams, joists, and other framing members.

Estimating concrete

For Round Footings:

1. Multiply half the hole diameter in inches times itself. (For a 12-inch hole, for example, this would be 6 inches × 
    6 inches = 36 square inches)
2. Multiply this by 3.14. (36 square inches × 3.14 = 113.04 square inches)
3. Multiply this by the depth of the hole in inches. (113.04 square inches × 48 inches = 5425.9 cubic inches)
4. Divide this by 1728 to find the amount of concrete required in cubic feet (1728 cubic inches = 1 cubic foot). 
    (5425.9 cubic inches ÷ 1728 = 3.14 cubic feet of concrete for one 12-inch-diameter hole that is 48 inches 
5. A 60-pound bag of premix concrete will make ½ cubic foot, so divide the total cubic feet for all footings by .5 to 
    find the number of bags (6.28 for the example; mix seven bags). Or tell your concrete supplier how many 
    cubic feet you need for delivery of ready-mixed concrete (see "Mixing Concrete"). To convert larger amounts 
    to cubic yards, divide cubic feet by 27.

For Rectangular Pads and Footings:

1. Multiply the length times the width of the area in inches. (Ex.: 36 × 36 = 1296)
2. Multiply this by the depth of the area in inches. (Ex.: 1296 × 4 = 5184)
3. Divide this by 1728 to find the cubic feet of concrete required. (Ex.: 5184 ÷ 1728 = 3 cubic feet of concrete for 
    a 36 × 36-inch pad that is 4 inches thick)
4. See Step 5, above.

Estimating hardware

Count the number of metal connectors and fasteners necessary for the framing (see Framing section). Estimate 5 pounds of screws (or nails) for every 100 square feet of decking.

Computing Stair and Railing Dimensions

Before you place your material order, determine how much lumber you’ll need for stairs and railings. Refer to your drawings to determine the number of posts, balusters, rails, stair treads and stringers. Because actual on-site dimensions can vary from drawings and estimates, be sure to take actual on-site measurements before constructing stairs or railings.

WORK SMARTER: Length of Lumber

Lengths of lumber come in even 2-foot intervals. Most stock is slightly longer than stated—a 12-foot board may measure 144¼ inches, for example. If you will cut several lengths from a board, allow for the saw's kerf (the empty space cut out by the blade) when estimating.