Project Guide

How to Estimate Deck Materials

Estimating Lumber
A wooden backyard deck
  • For a small deck, 10 × 12 feet, for example, determines 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.
  • Lengths of lumber come in even 2-foot intervals. Most stock is slightly longer than stated. A 12-foot board may measure 144 ¼ inches, for example. If you cut several lengths from a board, allow for the saw's kerf, the empty space cut out by the blade when estimating.
  • 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.
Estimating Composite Boards
A backyard deck made of composite boards
  • Composite boards look like natural wood, but they last longer, require less maintenance and are pre-finished upon purchase. 
  • These boards are measured in thickness and width, with 1-inch x 5-inches and 1-inch x 6-inches being the most common choices for a deck. Many options come in packs of 10 or 56. As with traditional lumber, determine how much you’ll need by using your blueprint.
  • Calculate the decking required based on the actual width of the boards you will use, and determine how many boards you’ll need according to square footage. Make sure to account for the gap between boards. 
  • The joist used for the framing should be 2-inches x 8-inches or larger. Figure the lengths necessary to fit the joist spacing with minimum waste, and add an extra 15 percent for diagonal decking. 
Estimate Railing and Post Quantities
A backyard deck with wooden railing
  • Consult your local building safety codes when estimating the materials needed for building a deck railing.
  • Measure the length of your deck to determine how much lumber you’ll need for the rail. Depending on how high the deck sits from the ground, you’ll likely need the rail to be at least 3-feet high.
  • To determine how many posts (also known as balusters or spindles), you’ll need to support your railing, confirm the minimum spacing required to meet safety ordinances. As a rule of thumb, use at least three posts for every 12 inches of railing.
  • Consider the spacing, length of the railing and post thickness to estimate your materials. 
Estimating Hardware
A closeup of an outdoor deck

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