How many deck boards do I need?
Boards usually run parallel to the house, so count rows across the deck's depth. Divide the depth by the board width plus the gap, and round up. Then multiply by the boards each row needs and add waste.
Rows = deck depth (in) ÷ (board width + gap), rounded up. Boards = rows × boards per row + waste
A 16 × 12 ft deck with 5/4×6 boards (5.5 in actual) and a 1/8 in gap: 144 ÷ 5.625 = 25.6, so 26 rows. Each row is one 16 ft board. With 10% waste, buy 29 boards. Joists at 16 in on center take 16 × 12 ÷ 16 + 1 = 13 joists, 12 ft long. Face screwing at 2 screws per joist takes 26 × 13 × 2 = 676 screws, or about 350 per 100 sq ft.
Pick a board length that matches the deck. A 16 ft deck uses 16 ft boards with no joints. A 20 ft deck with 16 ft boards needs a butt joint in every row, and each joint has to land on a joist.
Deck framing: joists, beams and posts
The calculator counts joists at your spacing, a ledger and rim joist, 2-ply beams, 6×6 posts along each beam, joist hangers and concrete for the footings. It doesn't size any of them. Joist size, beam size, post spacing and footing size depend on spans, loads, wood species and your local code. Take them from your plans, the span tables your building department uses, or a design guide such as the American Wood Council's DCA 6.
| Part | How it's counted |
|---|---|
| Deck boards | Rows × boards per row + waste |
| Joists | Length ÷ spacing + 1, cut to the depth |
| Ledger and rim | 2 × deck length |
| Beams | 2 plies × deck length per beam |
| Posts | Deck length ÷ post spacing + 1, per beam |
| Screws | Rows × joists × screws per crossing + 10% |
Composite deck calculator
Composite boards are usually 5.5 in wide and gapped at about 3/16 in. Many are installed with hidden clips, one per board at each joist. Most composite brands call for joists at 16 in on center for boards laid straight and 12 in for diagonal. Check the install guide for your board, because it decides the gap, the spacing and the warranty.
Deck calculator cost
The shopping list prices every line from the defaults in the price fields. Decking is priced per linear foot, so a 16 ft board at $1.40 per foot costs $22.40. The defaults are $1.40 per foot for pressure-treated and $3.50 for composite, so the decking line is where the two options differ most. Replace the defaults with your supplier's prices for a real number.
Free deck calculator for stairs and railings
Stairs and railings aren't included here. Use the stair calculator for risers, treads and stringers, and add both to the same job in contractor mode.
How much deck stain do I need?
Deck stain typically covers about 150 to 300 sq ft per gallon. Rough, dry or new pressure-treated wood soaks up more, and smooth or previously stained wood less. A 192 sq ft deck takes roughly 1 to 1.5 gallons per coat, plus extra for railings and stairs. Follow the coverage on the can.
How much weight can my deck hold?
Under the International Residential Code, decks are typically designed for a 40 psf live load plus a 10 psf dead load. That's the load the framing is designed to carry, not a tested rating for your deck. Hot tubs and other heavy loads need their own design. If you're not sure, ask a structural engineer or your building department.
Pricing a deck job
Deck builders often quote by the square foot for the deck and by the linear foot for railings. Switch to contractor mode to add your labor and markup, add the stairs from the stair calculator, and send it as an estimate.
Questions
How many screws do I need per 100 square feet of deck?
About 350 when face screwing 5.5 inch boards with 2 screws at each joist 16 inches on center. Add about 10% for drops and ends.
How many deck boards do I need for a 12x16 deck?
With 5.5 inch boards and a 1/8 inch gap, 26 rows of 16 ft boards. With 10% waste, buy 29.
How much deck stain do I need?
Deck stain typically covers 150 to 300 square feet per gallon, depending on the wood and the product. Rough or new wood takes more. Check the can.
How much weight can my deck hold?
Decks built to the IRC are typically designed for a 40 psf live load plus 10 psf dead load. That's a design load, not a guarantee. Ask an engineer about hot tubs or other heavy loads.
What joist spacing do I need for composite decking?
Most composite boards call for joists 16 inches on center laid straight and 12 inches on a diagonal. Follow the manufacturer's install guide.