Deck calculator: boards, joists, posts and cost

How the board count is calculated

Decking is laid in rows across the width of the deck, with the boards running parallel to the house. A 5/4×6 or 2×6 board is actually 5.5 inches wide, and you leave a 1/8 inch gap for drainage, so each row takes 5.625 inches. Divide the deck width in inches by that and round up for the number of rows. Then divide the deck length by your board length, round up, and that is boards per row. Rows × boards per row × 1.1 for waste is the number in the green box.

The 10 percent waste covers bad boards, end cuts, and the last row that always needs ripping. Pick a board length that divides your deck length cleanly if you can: a 16 foot deck in 16 foot boards has no butt joints at all, which looks better and wastes nothing. A 14 foot deck in 16 foot boards wastes 2 feet on every single row, which is 12.5 percent before you have cut anything. If your length is awkward, staggered 12 and 8 foot boards often waste less than one long size.

Narrower 3.5 inch boards (2×4s) give a finer look and more rows, about 60 percent more boards, and take longer to screw down. They are also more likely to cup. Most decks are built with 5.5 inch boards for a reason.

Framing: joists, beams, posts and footings

Joists run the other way from the boards, from the house out to the beam. They are spaced 16 inches on center for wood decking laid straight, 12 inches for composite (most brands require it) and for any decking laid diagonally. Count is the deck length in inches divided by the spacing, plus one, with a rim joist across the outer ends and a ledger board bolted to the house at the inner ends. A freestanding deck swaps the ledger for a second rim and a second beam.

A 2×8 joist at 16 inches spans about 12 feet in most species tables, but the calculator uses 8 feet to keep the beam count conservative; if your width is more than 8 feet you get a second beam, and a mid-span beam is exactly what an inspector wants to see on a 12 foot wide deck with 2×8s. Beams are doubled 2×10s running the length of the deck. Posts sit under the beams every 8 feet for a low deck and every 6 feet for a deck 6 feet or more off the ground, on 6×6 posts. One concrete footing per post, sized and dug to your local frost depth.

Hardware is where beginners under-buy. Every joist gets a hanger at each end, so joists × 2. Deck screws run about 3.5 per square foot for 5.5 inch boards (two per board per joist). Composite with hidden fasteners uses about 90 clips per 100 square feet plus the screws that come in the clip box. Then there is the ledger flashing, the lag bolts or structural screws for the ledger, post bases, and the hurricane ties or hold-downs your code requires. Budget 10 to 15 percent of the framing lumber cost for hardware.

What decking costs in 2026

Per square foot of deck, materials only, including the framing underneath. Pressure-treated pine decking on treated framing is $15 to $25. Cedar decking on treated framing is $25 to $35. Composite is $35 to $60, with the spread driven by brand tier: entry-level capped composite at the bottom, PVC and the premium wood-look lines at the top. A high deck adds 15 percent for the longer posts, bracing and bigger footings; the calculator applies that on the 6 foot chip.

Installed, a deck builder charges roughly double the materials: $30 to $45 per square foot for pressure-treated, $40 to $60 for cedar, $55 to $90 for composite. Railings, stairs and a permit are on top of all of this. A 12 × 16 composite deck is a $7,000 to $11,000 materials job and a $12,000 to $18,000 installed one, before railing.

• Composite is 2 to 3 times the decking price of pine but the framing underneath is the same treated lumber, which is why the per-square-foot gap is smaller than the board price suggests.
• Railing is sold by the linear foot and is not in these numbers: $30 to $60 a foot for wood, $60 to $150 for aluminum or composite with cable or glass.
• A permit is required for almost any deck over 30 inches off the ground, and usually for any deck attached to the house. $100 to $500 depending on the town.

Decisions that change the count

Which way the boards run. The calculator assumes boards parallel to the house, joists perpendicular. Running boards perpendicular to the house swaps the roles of length and width in the framing, and it is usually the wrong call because the ledger then has to carry the ends of every board with a doubled joist behind it.

Diagonal decking. It looks great and uses 15 percent more boards, plus joists at 12 inches instead of 16 because the effective span between supports gets longer. Enter 12 inch spacing and add 5 percent on top of the count.

Picture framing. A border board around the edge hides the cut ends and is standard on composite decks. It takes one extra row's worth of boards around the perimeter plus doubled joists or blocking under the border seam. Add the perimeter in feet ÷ your board length to the count.

Stairs. Each step is a tread of two 5.5 inch boards at the stair width, plus stringers. A four-step stair 4 feet wide adds 8 boards of 4 feet, two or three 2×12 stringers, and its own footing.

Frequently asked questions