Estimate deck boards, joists, footings and material cost for a wood or composite deck — metric or imperial.
Typical gap: 3-6mm (1/8-1/4in) for wood; check the manufacturer's spec for composite decking.
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Start on the Deck Boards tab: enter your deck's length and width, choose which direction the boards run, and enter your board's width, gap spacing and stock length. The calculator returns the number of rows, pieces per row and total boards to buy. On the Joists & Footings tab, the joist length and layout dimension are derived automatically from your deck size and board direction — just enter your maximum on-centre spacing for a joist count, and optionally your beam run and maximum post spacing for a footing count. Use the Cost tab with your local prices for a material cost estimate.
Deck board quantity depends on your deck's area, the direction the boards run, your board's width plus the gap you leave between boards, and the stock length the boards are sold in. The calculator works out how many rows are needed to span the deck's width (or length, depending on board direction), how many stock-length pieces make up each row, and multiplies rows by pieces per row for a total board count.
The "+ Gap" in the rows formula accounts for there being no trailing gap after the last board in a row — without it, a row that just fits can be undercounted by one.
The gap between deck boards allows for drainage and, for wood decking, seasonal expansion and contraction as the timber absorbs and releases moisture. A gap of 1/8 to 1/4 inch (roughly 3-6mm) is typical for wood decking — plastic deck board spacers are commonly used to keep this gap consistent during installation. Composite decking often has a manufacturer-specified gap requirement, since composite materials expand and contract differently to timber, so always check your specific product's installation guide rather than assuming the wood-decking figure applies.
Deck joists support the decking boards from underneath and run perpendicular to them, so their individual length and the dimension they're laid out across are derived automatically from your deck size and the board direction you chose on the Deck Boards tab — there's no separate "joist run" to re-enter. Enter the maximum on-centre spacing specified by your decking product, approved design or applicable requirements (12-24 inches / 300-600mm are shown only as convenience examples, not a recommendation), and the calculator fits the largest number of equal spaces that keeps the actual spacing at or under that maximum. Footings (posts) support the beams that carry the joists, and use the same maximum-spacing logic against your beam run and entered maximum post spacing. This calculator uses simple layout arithmetic; it doesn't verify these spacings against a span table or engineering requirements, so treat the results as a starting estimate to confirm against your local code.
The Cost tab multiplies your entered board, joist and footing prices by the quantities calculated above. This is a material-only estimate — fasteners, hardware, joist protection tape, footing concrete, labour and delivery aren't included unless you price them in separately. Decking material costs vary significantly between wood and composite products, and by brand and region, so use your local supplier's current prices for an accurate total.
A 4m × 3m deck uses 140mm-wide composite boards with a 5mm gap, running along the 4m length, with a 3.6m board stock length and 10% waste allowance. Rows across the 3m width = ⌈(3 + 0.005) ÷ 0.145⌉ = 21 rows. Each row is 4m, which with 10% waste is 4.4m, requiring ⌈4.4 ÷ 3.6⌉ = 2 pieces per row. Total boards = 21 × 2 = 42 boards. For the substructure, boards running along the 4m length automatically set the individual joist length to the 3m width and the joist layout dimension to the 4m length; at a 400mm maximum spacing that's ⌈4 ÷ 0.4⌉ = 10 spaces, so 11 joists at an actual spacing of exactly 400mm. A 4m beam run at a 1.8m maximum post spacing needs ⌈4 ÷ 1.8⌉ = 3 spaces, so 4 footings at an actual spacing of about 1.33m — not 3 footings, which would have spaced posts wider than the 1.8m maximum entered.
A common mistake is dividing span by (board width + gap) without adding a gap back in first — since there's no trailing gap after the last board, that undercounts the final row on an exact-fit deck. Another is forgetting that a waste allowance should apply to each row's length before dividing by the stock length, not just to the final total, since offcuts happen per board, not just once overall. For post/footing spacing, rounding down to the nearest whole number of spaces (instead of rounding up) can silently produce an actual spacing wider than the maximum entered — this calculator always rounds up the number of spaces first, then divides back down for the actual spacing. Finally, this calculator's simple spacing arithmetic is not a substitute for your decking product's official span table, which accounts for board thickness, species or composite formulation, and load requirements.
This calculator estimates material quantities from the dimensions and spacing you enter. It assumes straight, full-width board rows and does not account for picture-framing, diagonal or herringbone layouts, or cut-outs for posts and features, all of which typically increase waste. Joist and footing counts use simple on-centre spacing arithmetic and do not check span ratings, load capacity, or local building code requirements — always confirm against your decking product's span table, local code, or a structural professional before building.
It depends on your deck's size, your board's width, the gap you leave between boards, and the stock length the boards are sold in. Enter these on the Deck Boards tab above for an exact count.
Deck board spacing (the gap between boards) is typically 1/8 to 1/4 inch (3-6mm) for wood decking to allow for drainage and seasonal movement, and per the manufacturer's spec for composite decking. Enter your chosen gap on the Deck Boards tab to see how it affects board count.
Joist count depends on your deck's run length and on-centre spacing (commonly 16in/400mm for composite decking, or 12-16in/300-400mm for wood). Use the Joists & Footings tab above to calculate this from your dimensions.
Common on-centre joist spacings are 12in, 16in or 24in (300mm, 400mm or 600mm), with composite decking often requiring tighter spacing (commonly 12-16in) than wood decking - always check your specific decking product's span/spacing requirements.
This depends on the beam run length and the post spacing your design or local code requires, commonly 6-8ft (1.8-2.4m) apart. Use the Joists & Footings tab above with your beam run and post spacing for an estimate.
Material cost depends on decking type (wood vs composite), size and local prices. Use the Cost tab above with your own board, joist and footing prices for a material estimate specific to your project; labour is priced separately.
1/8 to 1/4 inch (about 3-6mm) is typical for wood decking to allow drainage and seasonal expansion/contraction. Composite decking often has a manufacturer-specified gap, sometimes using plastic spacers - check your product's installation guide.
This is usually a design choice, though boards typically run perpendicular to the joists beneath them for adequate support. This calculator estimates quantity either way based on the deck dimensions and board length you enter.
The same way as wood decking - based on your deck area, the composite board's width, the manufacturer's recommended gap spacing, and the board's stock length. Enter these on the Deck Boards tab above.
Railing and baluster opening requirements vary by jurisdiction and deck configuration - confirm your local requirements. This calculator does not calculate railing or baluster spacing.
No - it estimates material quantities from the dimensions and spacing you enter, not structural load capacity or local building code requirements. Confirm joist spans, footing sizing and railing requirements against your local code or a structural professional.