How to Estimate Deck Materials: Lumber, Hardware, and Concrete

Start with the Deck Footprint

Every material calculation flows from two numbers: length and width. Measure these in feet. If you're building against the house, length runs parallel to the house wall and width extends outward from the house to the far edge of the deck.

Multiply length by width for your square footage. A 16x12 deck is 192 square feet. This number drives everything that follows.

Decking Boards

Standard deck boards are 5/4x6 (actually 1" thick by 5.5" wide). They're sold in lengths of 8, 10, 12, and 16 feet. Choose a length that minimizes cuts and waste. For a 16-foot-long deck, 16-foot boards run the full span with no seams. For a 14-foot deck, you'll cut 16-foot boards and waste 2 feet each, or use two shorter boards with a mid-span joint over a joist.

To calculate the number of boards: divide the deck width (in inches) by the board width plus gap (5.5" + 0.125" gap = 5.625"). A 12-foot-wide deck needs about 26 rows. Then multiply by how many boards per row based on your board length versus deck length. Add 10% for waste from cuts, splits, and defective boards.

Joists: The Structural Backbone

Joists run perpendicular to the deck boards, spaced 16 inches on center for most applications. For composite decking, many manufacturers require 12 inches on center. For lightweight use with pressure treated boards, 24 inches works but feels bouncy underfoot.

To count joists: divide the deck length (in inches) by the joist spacing, then add 1 for the starter joist, plus 2 rim joists (the boards that cap the ends). A 16-foot deck at 16-inch spacing needs 13 joists plus 2 rim joists = 15 total.

Joist size depends on span (the unsupported distance). Up to 10 feet of span, 2x8 joists work. Beyond 10 feet, use 2x10. Always use pressure treated lumber for joists regardless of what your deck boards are made of.

Posts, Beams, and Footings

Support posts carry the deck's weight to the ground through concrete footings. For decks under 4 feet off the ground, 4x4 posts work. Above 4 feet, use 6x6 posts for stability.

Beams sit on top of posts and support the joists. They're typically doubled 2x10 or 2x12 lumber bolted together. Post spacing along the beam should be no more than 8 feet apart.

Each post needs a concrete footing that extends below the frost line. In Arkansas, that's about 18 inches deep. In Minnesota, it's 42 inches. Each footing needs approximately 2 bags of 50-pound concrete for a 10-inch diameter hole.

Choosing Your Material

Pressure treated pine is the most affordable option at roughly $8-12 per 12-foot board. It requires staining or sealing every 2-3 years and lasts 15-25 years with maintenance. Let it dry 2-3 weeks before applying finish.

Cedar costs roughly $18-26 per 12-foot board. It's naturally rot-resistant and can be left to weather to a silver-gray or sealed to maintain its golden color. Lasts 15-20 years with minimal maintenance.

Composite decking runs $32-48 per 12-foot board. No staining ever. Won't rot, split, or splinter. 25-year warranty is standard. However, it gets hotter in direct sun than wood, and the framing underneath still needs to be pressure treated lumber.

Redwood is premium at $28-40 per 12-foot board. Beautiful natural color, excellent durability, and natural insect resistance. Increasingly difficult to source in many regions.

Hardware: Don't Forget the Small Stuff

Joist hangers connect each joist to the ledger board or rim joist. You need one per joist connection. Post brackets attach posts to concrete footings. Lag bolts secure beams to posts. Deck screws attach every board to every joist: figure roughly 350 screws per 100 square feet of decking (2 screws per board at each joist intersection).

For composite decking, use the manufacturer's recommended hidden fastener system. Standard screws leave visible holes and void some warranties.

Railing

Building code requires railing on any deck 30 inches or more off the ground. Railing must be 36 inches high (42 inches in some jurisdictions). Balusters must be spaced so a 4-inch sphere cannot pass through (approximately 4.25 inches on center).

A simple wood railing uses 4x4 posts every 6 feet, 2x4 top and bottom rails between posts, and 2x2 balusters. Pre-made aluminum or composite railing systems cost more but install faster and require zero maintenance.