How to use this calculator
- Enter the deck length along the house (the direction the boards run) and the width out from the house (the direction the joists run).
- Choose your deck board. 5/4 × 6 and composite boards are the US standard; 120 or 144 mm boards are common in the UK and Europe, and 90 or 140 mm boards in Australia and New Zealand.
- Set the gap between boards and the board length you plan to buy, such as 16 ft or 4.8 m.
- Pick the joist spacing: 16″ (400 mm) on center suits most decks. Keep 10% waste, or use 15% for diagonal boards.
- Enter local prices for a deck board, for joist lumber per foot or meter and for a pack of 100 screws to see the material cost.
How the deck calculation works
Each row of decking covers one board width plus one gap. The number of rows is the deck width divided by that, rounded up. A 16 × 12 ft deck with 5½″ boards and a 3/16″ gap needs 144 ÷ 5.6875 = 25.3, so 26 rows. Each row is 16 ft long, which makes 416 linear feet of board; with 10% waste that is 457.6 ft, or 29 boards of 16 ft.
Joists run across the width at the chosen spacing, with one at each end: 192″ ÷ 16″ = 12 spaces, so 13 joists of 12 ft. Adding a ledger board against the house and a rim joist at the front gives 13 × 12 + 2 × 16 = 188 linear feet of framing lumber.
Every board is fixed with two screws at each joist: 26 rows × 13 joists × 2 = 676 screws, or 710 with 5% spare. At $20 a board, $1.50 per foot of joist lumber and $12 per 100 screws, the materials come to $580 + $282 + $85.20 = $947.20, about $4.93 per square foot. In metric, a 4.8 × 3.6 m deck with 120 mm boards and 5 mm gaps takes 29 rows, 32 boards of 4.8 m, 13 joists at 400 mm centers and 792 screws.
Pro tips
- Lay out the deck so its length is a multiple of your board lengths, or plan staggered butt joints that always land on a joist. Doubling the joist at each joint gives both board ends something solid to screw into.
- Wet pressure-treated boards shrink as they dry, so they are often laid with only a small gap. Composite boards need the gap the manufacturer specifies, plus end gaps that change with temperature.
- Use coated or stainless deck screws rated for treated lumber, and stainless steel within a few miles of the coast.
- Run boards with the bark side up (growth rings curving down) where you can see the end grain; it helps them shed water.
- This calculator covers the decking and joists. Beams, posts, footings, joist hangers and railings are extra, and their sizes should come from local span tables or your building inspector.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5/4 × 6 boards (5½″ wide) and a 3/16″ gap, a 12 × 12 ft deck needs 26 rows. Using 12 ft boards and 10% waste, that is 29 boards.
How many screws do I need for a deck?
Two screws per board at every joist. A 16 × 12 ft deck with 5½″ boards and joists at 16″ on center has 26 rows crossing 13 joists, which is 676 screws, or about 710 with 5% spare. That works out at roughly 3.5 screws per square foot.
How many joists do I need for a deck?
Divide the deck length by the joist spacing, round up and add one. A 20 ft deck with joists at 16″ on center needs 240 ÷ 16 = 15 spaces, so 16 joists. In metric, a 4.8 m deck at 400 mm centers needs 13 joists.
What joist spacing do I need for decking?
For boards laid square to the joists, 16″ (400 mm) on center is the usual maximum for 5/4 (1″) and 28 mm boards, and 24″ (600 mm) is allowed for 2× boards. Most composite decking is rated for 16″. For diagonal boards, drop to 12″ (300 mm). Always follow the board maker's instructions and your local code.
How much gap should there be between deck boards?
About 3/16″ (5 mm) is typical for composite and kiln-dried wood. Pressure-treated boards that are still wet can be laid with a smaller gap, around 1/8″ (3 mm), because they shrink as they dry.
How many deck boards per square meter?
With 120 mm boards and a 5 mm gap, each meter of deck width takes 8 rows, so you need about 8 linear meters of board per square meter, or 8.8 m with 10% waste. That is just under two 4.8 m boards per square meter.
How much do the materials for a 16x12 deck cost?
With 5/4 treated boards at $20 each, joist lumber at $1.50 per foot and screws at $12 per 100, the decking, joists and screws for a 16 × 12 ft deck come to about $947, or $4.93 per square foot. Composite boards typically cost two to three times as much as treated wood. Beams, posts, footings and railings are extra.
These results are estimates for planning and budgeting. Actual quantities depend on site conditions, products and workmanship. Check structural and code requirements with a licensed professional before you build.