Deck Board Calculator
How many deck boards does your deck need, and how many screws hold them down? Enter the deck size, board width and gap. Results update as you type. The calculator assumes boards run along the length of the deck.
Your decking
Assumptions, stated plainly: boards run along the deck length, screws are 2 per joist crossing, and joists sit at the spacing you chose (400 mm centres by default). Add about 10 percent more boards for cuts, end splits and the odd warped board.
Worked example
A 4.8 m by 3.6 m deck, 140 mm boards, 5 mm gap
Each board covers its width plus one gap: 140 + 5 = 145 mm. Rows across the 3.6 m width: 3.6 ÷ 0.145 = 24.8, so 25 boards.
Linear metres: 25 boards × 4.8 m = 120 m of decking.
Joists at 400 mm centres along the 4.8 m length: (4.8 ÷ 0.4) + 1 = 13 joists. Screws: 25 × 13 × 2 = 650 screws. These are the default results shown above.
How this calculator works
The number of boards is the deck width divided by the pitch of the boards, where the pitch is the board width plus one gap: boards = width ÷ (board width + gap), rounded up because partial rows still need a board (you can rip the last board to fit).
Linear metres are simply the board count times the deck length. This is the figure a lumber yard needs if your boards are cut to order; if you buy standard lengths (3.66 m and 4.88 m are common), round each board up to the next standard length when you order.
Screws: every board is fastened at every joist it crosses with 2 screws. The joist count is (deck length ÷ joist spacing) + 1, counting the joist at each end. Hidden fastener systems use clips instead of face screws; most clip systems need roughly one clip per board per joist, so the crossing count above is your clip estimate too.
Common questions
Why leave a 5 mm gap between boards?
The gap drains rain and snowmelt so water does not sit between boards, and it gives wood room to expand in humid weather. Pressure treated lumber is often installed nearly tight because it shrinks as it dries; kiln-dried and composite boards are installed with the full gap. Follow the board manufacturer spacing guidance for your product.
How many extra boards should I order?
About 10 percent. Some boards will have knots, checks or warp you do not want in a visible spot, and end cuts consume length. For the worked example above, order 28 boards instead of 25, or about 132 linear metres. Keep the receipt; most yards take back uncut boards in good condition.
Does joist spacing change for composite decking?
Often, yes. Many composite and PVC boards require 300 mm (12 inch) centres for a firm feel, and 400 mm can feel springy; diagonal layouts need closer spacing still. The calculator lets you switch to 300 mm so the screw and clip counts stay right, and your joist plan should follow the board maker specifications.
What if my boards run the other direction?
Swap the length and width in the form. Boards running across the width means the width becomes the run direction. The board count and screw math follow whichever way you enter it.
Estimates only, verify before you buy materials. This calculator counts boards and fasteners; it does not design the deck structure. Deck framing, footings and guards must meet your local building code, and most decks need a permit.