Deck Calculator
Estimate deck boards, joists, beams, posts, screws or hidden clips and concrete footings for a rectangular deck, in metric or imperial units.
Boards run along the length and rest on joists that span the width, from the house (ledger) or a beam to the next beam. Swap the two values to lay the boards the other way.
Use the real width: a 5/4×6 board is 5½ in (14 cm) wide; common metric deck boards are 120 to 145 mm. A gap of 3 to 6 mm (⅛ to ¼ in) lets water drain.
Take the maximum spans from your local code or the span tables for your lumber species, size and grade. Composite boards usually require 12 or 16 in (30 or 40 cm) joist spacing.
- Deck area
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- Deck boards (with waste)
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- Board length to buy
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- Joists
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- Ledger and rim joists
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- Beams
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- Posts and footings
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- Deck screws (face fastening)
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- Hidden fasteners (clips)
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- Concrete for the footings
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- Concrete bags
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Estimate only. This calculator does not replace your local building code, the span tables for your lumber or a permit review: joist and beam sizes, spans, footing size and frost depth, ledger attachment and guard rails are set by the code. Call before you dig to locate buried utilities.
How to use the deck calculator
- Choose Metric or Imperial units.
- Enter the deck Length (along the boards) and Width (along the joists). The boards run along the length; the joists span the width, from the house or a beam to the next beam.
- Under Deck boards, enter the actual Board width, the Gap between boards, the Board length you will buy and a Waste and offcuts (%) allowance.
- Under Framing, pick the Joist spacing (centre to centre), then enter the Maximum joist span and Maximum post spacing along a beam taken from your span tables. Tick Attached to the house with a ledger board if one side of the deck is fixed to the house.
- Under Footings, enter the hole diameter and depth, and pick a concrete bag size.
The results update as you type. The steps below the results show every calculation, so you can check each quantity.
What the calculator counts
- Deck boards: the number of rows is (width + gap) ÷ (board width + gap), rounded up; the last row may need to be ripped. When a board is at least as long as the deck, several rows can be cut from one board. When the deck is longer, each row needs several boards joined end to end over a joist. The waste percentage is then added and the result is rounded up to whole boards.
- Joists: length ÷ joist spacing, rounded up, plus one for the end. Each joist is as long as the deck width. The ledger and the outer rim joist are listed separately.
- Beams and posts: an attached deck needs width ÷ maximum joist span beam rows (rounded up), because the ledger supports the house side; a freestanding deck needs one more. Each beam gets posts at no more than the maximum post spacing, plus one at the end.
- Fasteners: face screwing uses two screws per board at each joist, plus two more at each butt joint where two boards meet. Hidden clips use about one clip per board edge at each joist, which is (rows + 1) × joists.
- Concrete: each footing is a cylinder, π × (diameter ÷ 2)² × depth, multiplied by the number of posts, then divided by the yield of one bag.
Tips and limits
Joist spacing depends on the decking: 16 in (40 cm) is standard for wood boards laid square to the joists, and composite or diagonal decking often needs 12 in (30 cm). A gap of 3 to 6 mm lets water drain and leaves room for wood movement. Order about 10 % extra for straight lay-outs and 15 % or more for diagonal or picture-frame patterns.
This is an estimate for planning and budgeting. It does not replace your local building code, the span tables for your lumber species and grade, or a permit review. Joist and beam sizes, cantilevers, footing size, frost depth, ledger flashing and fastening, stairs and guards are all set by the code and must be checked. Blocking, hangers, post bases and hardware are not counted.
Frequently asked questions
How many deck boards do I need for a 12 × 16 ft deck?
With 5.5 in boards, a 3/16 in gap and 16 ft boards laid along the 16 ft side, you need 26 rows, so 26 boards, or 29 with 10 % waste. Enter your own sizes to get your count.
Which way should the boards run?
Usually parallel to the house, square to the joists. If you want them the other way, swap the length and width: the calculator always runs the boards along the length.
How many screws per deck board?
Two screws at every joist the board crosses, plus two at each end joint. For a deck with 25 rows and 13 joists, that is 650 screws; buy a little more for losses.
How deep should the footings be?
Below the frost line required locally, often 1.2 m (4 ft) in Quebec and much of Canada, and never less than the depth set by your code. Check with your municipality.
Does the calculator size the joists and beams?
No. It counts pieces from the spans and spacings you enter. Use the span tables of your building code or lumber supplier to choose sizes and maximum spans.
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- Flooring and Square Footage Calculator Calculate the square footage or square metres of several rooms and irregular shapes, then the flooring boxes to buy with a waste allowance.