How ManyBoards?

Deck material calculator

Enter the size of your deck and get one shopping list for the whole build: boards, frame, footings, fixings, stairs and railing. Works in feet or metres.

Units

Your deck from above

Boards are cut away on the right to show the frame.

Deck size
Width, along the house
ft
in
Depth, out from the house
ft
in
in
How is it supported?
Top view of the deck
Deck boards Joists Beams Posts and footings Railing
Deck boards
%
Boards run along the width, across the joists.
Frame
Railing and stairs
Sides with railing
in
Footings and post spacing
in
in
ft
Footings must reach below the frost line where the ground freezes. Your permit office can tell you the depth for your area.
192square feet of deck
29deck boards
13joists
—add prices for a cost

Shopping list

Enter what your store charges for one of each item. Prices stay in your browser.

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Estimated materials costAdd prices to see a total
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    Your current shopping list is sent with your message so it can be checked.

    How to use the calculator

    1. Measure the width of the deck along the house and the depth out from the wall.
    2. Measure from the ground to where the top of the deck boards will sit. This sets the post length and the number of stair steps.
    3. Pick the board width you plan to buy. The calculator chooses the board length that leaves the least waste, or you can set your own.
    4. Turn railing and stairs on or off. The list and the drawing update as you type.
    5. Print the list or copy it to your phone before you go to the store.

    How the numbers are worked out

    Every line in the shopping list has a "Show the working" link with the sum behind it. These are the main formulas.

    Deck boards

    Rows of boards = deck depth ÷ (board width + gap), rounded up
    Boards to buy = rows × deck width ÷ board length, plus waste

    For a 16 ft wide, 12 ft deep deck with 5½ in boards and a ⅛ in gap: 144 in ÷ 5⅝ in = 25.6, so 26 rows. Each row is one 16 ft board. Adding 10% for waste gives 29 boards.

    Joists

    Joists = deck width ÷ joist spacing, rounded up, plus 1

    A 16 ft (192 in) deck at 16 in spacing has 12 bays, so 13 joists. The joist size depends on how far each joist spans between the house and the beam. The calculator suggests a size from this table of typical limits, and adds a second beam when the deck is too deep for one span.

    Joist size12 in spacing16 in spacing24 in spacing
    2×69 ft 6 in9 ft7 ft 6 in
    2×813 ft11 ft 6 in9 ft 6 in
    2×1016 ft14 ft11 ft
    2×1218 ft16 ft 6 in13 ft 6 in

    These limits suit common softwood framing. Timber species and grade change them, so check the span table in your local building code.

    Posts and concrete

    Posts per beam = deck width ÷ post spacing, rounded up, plus 1
    Concrete per footing = 3.1416 × (hole width ÷ 2)² × hole depth

    A hole 12 in wide and 36 in deep holds 2.36 cubic feet of concrete, which is 4 bags of 80 lb mix.

    Screws

    Screws = rows of boards × joists × 2, plus 5% spare

    Stairs

    Risers = deck height ÷ 7¾ in (190 mm), rounded up
    Treads = risers − 1

    A deck 30 in high needs 4 risers of 7½ in each and 3 treads. Each tread is two deck boards.

    Railing

    Balusters = railing length ÷ (baluster width + gap), rounded up

    The gap between balusters is kept under 4 in (100 mm), which is the limit in most building codes.

    Common questions

    How many deck boards do I need for a 12 × 16 ft deck?

    With standard 5½ inch boards, a ⅛ inch gap and boards running the 16 ft length, a 12 × 16 ft deck needs 26 rows. That is 26 boards of 16 ft, or 29 once you add 10% for offcuts and defects.

    How many deck boards are in a square foot?

    A 5½ inch board with a ⅛ inch gap covers 5⅝ inches of deck, so one square foot takes about 2.13 linear feet of board. A 100 square foot deck needs about 213 linear feet before waste.

    How far apart should deck joists be?

    16 inches on centre (400 mm) is the usual spacing for timber deck boards. Many composite boards need 12 inches (300 mm), and boards laid diagonally need closer joists too. Check the board maker's instructions.

    How many screws do I need for a deck?

    Allow 2 screws wherever a board crosses a joist. At 16 inch joist spacing with 5½ inch boards that works out to about 350 screws for every 100 square feet of deck.

    How much extra decking should I buy for waste?

    10% covers a simple rectangular deck with boards laid straight. Allow 15% for boards laid diagonally or for a deck with angled corners.

    Do I need a railing on my deck?

    In most of the United States a deck more than 30 inches above the ground needs a guard railing. Canada and the United Kingdom use 600 mm, and Australia uses 1 metre. Local rules vary, so confirm with your permit office.

    What the list leaves out

    • Fascia or skirting boards around the edge of the deck.
    • Blocking between joists, which some codes and most composite boards require.
    • A landing pad at the bottom of the stairs.
    • Stain, sealer, joist tape and flashing.
    • Decks that are not rectangles. Split an L-shaped deck into two rectangles and run each one.

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