A deck calculator turns the size of a deck into a material list. Enter the length along the house, the width out from it, your board size and joist spacing, and Contruva’s free deck calculator gives you the number of decking boards, joists, joist hangers, rim and ledger boards, beam lumber, posts, footings and deck screws or hidden clips, plus a material cost at your prices.
Open the free deck calculator in your browser. No login, no email, nothing to pay.
Open the free toolIt works in feet and inches or metres and millimetres, and needs no login. Below is exactly how it counts each line, a worked example you can check by hand, and the limits of a takeoff: it tells you how much to buy, not how big the joists and beams have to be. That part comes from your local code.
How to use the deck calculator
Length along the house and width out from it. The joists span the width, so the width is the number that matters for joist length.
Actual board width (5.5 in or 140 mm for 5/4×6 and 2×6), the gap between boards, the board length you’ll buy (8, 10, 12 or 16 ft), the direction and a waste percentage.
Joist spacing (12 or 16 in on centre), the joist size for your list, a ledger on the house or a freestanding deck, the maximum post spacing along the beam, and a 2-ply or 3-ply beam.
Face screws or hidden clips, then your supplier’s price per deck board, joist, post, hanger and per 100 fasteners.
Boards, joists, rim, ledger, beam boards, posts, footings, hangers and fasteners, with a cost breakdown by line.
Measure the finished deck, not the house wall. If the deck wraps a corner or has a step-down, run each rectangle through the calculator separately and add the lists.
How many deck boards do I need?
With boards running parallel to the house, each row of decking covers one board width plus one gap. The calculator divides the deck width by that, rounds up to whole rows, then works out how many boards of your chosen length each row takes.
| Step | Formula |
|---|---|
| Rows of decking | (width in inches + gap) ÷ (board width + gap), rounded up |
| Row shorter than a board | One board covers as many rows as it can be cut into (a 16 ft board does two 8 ft rows) |
| Row longer than a board | Row length ÷ board length, with offcuts used in the next staggered row |
| Boards to buy | Rows × boards per row × (1 + waste %), rounded up |
On a 12 ft wide deck with 5.5 in boards and a 1/8 in gap, that’s 144.125 ÷ 5.625 = 25.6, so 26 rows. If the deck is 16 ft long and you buy 16 ft boards, it’s one board per row: 26 boards, or 29 with 10% waste.
Board length matters more than most people think. Stretch the same deck to 20 ft and buy 12 ft boards, and every row needs a butt joint: 26 × 20 ÷ 12 = 43.3 boards, or 48 with waste. Where you can, buy boards that match the deck length or divide into it cleanly, and plan butt joints to land on a doubled joist.
Diagonal decking
Run the boards at 45° and the calculator switches to an area method: deck area × 12 ÷ (board width + gap) gives the linear feet of board, which it divides by the board length. Diagonal layouts throw off more offcuts, so the tool bumps waste to 15% and joist spacing to 12 in when you pick it. Trex’s own install guide calls for 12 in joist spacing when boards run diagonally (checked November 2026), and most board makers say the same. Check yours.
How many joists do I need?
Joists run out from the house and are spaced along the length of the deck, with one at each end.
| Line | How the calculator counts it |
|---|---|
| Joists | Length in inches ÷ spacing, rounded up, + 1 |
| Joist length | Deck width rounded up to the next even foot (lumber comes in 2 ft steps), 8 ft minimum |
| Rim (band) joist | One run the length of the deck, across the joist ends |
| Ledger | One run the length of the deck on the house wall (attached decks only) |
| Joist hangers | One per joist at the ledger. None on a freestanding deck where joists sit on beams |
A 16 ft deck at 16 in on centre is 192 ÷ 16 = 12 spaces, so 13 joists. At 12 in on centre it’s 17. The joist size field doesn’t change the count; it only labels your list. Size comes from a span table, covered below.
Beams, posts and footings
An attached deck has a ledger on the house and one beam near the outside edge. A freestanding deck has two beams, one at each side. Each beam is made of two or three boards nailed together, the same size as your joists in this tool, running the length of the deck.
- Beam lumber = number of beams × plies × deck length. One 2-ply beam on a 16 ft deck is 32 ft of board.
- Posts per beam = deck length ÷ your maximum post spacing, rounded up, + 1. At 8 ft spacing on 16 ft, that’s 3.
- Footings = one per post.
- Hangers = one per joist on an attached deck. Joists bearing on a drop beam usually get hurricane ties or other connectors instead, which aren’t counted.
Rim, ledger and beam boards are listed in feet so you can buy them in whatever lengths your yard stocks. For the cost, the calculator prices them per foot at the same rate as your joists.
How many deck screws do I need?
Face-screwed boards get two screws at every joist they cross. Hidden clips are one per board at every joist. The calculator counts every board-to-joist crossing, multiplies by two for screws or one for clips, then adds 5% for drops and stripped heads. Small decks come out a little higher per square foot because the end joists are a bigger share of the frame.
Per 100 sq ft, on the 16 × 12 ft example deck below:
| Setup | Fasteners per 100 sq ft (before spare) |
|---|---|
| 5.5 in boards, 16 in o.c., screws | About 352 |
| 5.5 in boards, 16 in o.c., hidden clips | About 176 |
| 5.5 in boards, 12 in o.c., screws | About 460 |
| 3.5 in boards, 16 in o.c., screws | About 542 |
That first line lines up with the manufacturers. Simpson Strong-Tie rates a 350-count box of its DCU composite deck screws for about 100 sq ft at 16 in on centre, and TimberTech sells its CONCEALoc hidden clips in a 175-clip pack rated for about 100 sq ft (both checked November 2026). Buy by the box size closest to the count, and check the screw length and type your board maker requires; many composite warranties depend on it.
Worked example: a 16 × 12 ft deck
An attached deck 16 ft along the house and 12 ft out. 5/4×6 boards (5.5 in actual) with a 1/8 in gap, 16 ft boards, straight, 10% waste. Joists 16 in on centre, a 2-ply beam with posts no more than 8 ft apart, face screws. Sample prices: $30 a board, $22 a 12 ft joist, $28 a post, $3 a hanger and $12 per 100 screws. These are placeholders to show the math, not market prices.
16 × 12.
26 rows × 1 board, + 10% = 28.6, so 29 boards of 16 ft (416 linear ft before waste).
192 ÷ 16 = 12 spaces, + 1. Plus 13 hangers at the ledger.
16 ft rim + 16 ft ledger + 32 ft for a 2-ply 16 ft beam.
16 ÷ 8 = 2 spaces, + 1.
26 rows × 13 joists × 2 = 676, + 5%.
| Line | Quantity | Cost |
|---|---|---|
| Deck boards | 29 | $870.00 |
| Joists | 13 | $286.00 |
| Rim, ledger and beam | 64 ft at $22 ÷ 12 ft | $117.33 |
| Posts | 3 | $84.00 |
| Joist hangers | 13 | $39.00 |
| Deck screws | 710 | $85.20 |
| Material total | $1,481.53 |
Change two inputs and the list changes with them. Lay the same deck diagonally at 12 in on centre with 15% waste and you need 17 joists, 30 boards and 647 screws. Make it freestanding and you get two beams (64 ft of beam board), 6 posts and no hangers. Switch to hidden clips and the 710 screws become 355 clips.
Spans, footings and permits
The calculator is a takeoff, not a structural design. It doesn’t check whether a 2×8 can span 12 ft, whether a 2-ply beam can carry the load between posts 8 ft apart, or how big the footings need to be. Those answers depend on species, grade, snow load and your local code.
| Where you build | Where the spans come from |
|---|---|
| Most of Canada | Part 9 of the National Building Code of Canada, as adopted by your province, plus your municipality’s deck guide |
| Ontario | The Ontario Building Code; many municipalities publish a deck guide with joist, beam and footing tables |
| United States | IRC Section R507 (deck joist spans in Table R507.6) and the American Wood Council’s DCA 6 guide |
Footings are where Canadian decks differ most. Several Ontario municipal deck guides call for footings at least 1.2 m (48 in) deep for a deck attached to the house, with exceptions for some low, freestanding decks (checked November 2026). Frost depth varies across the country, so get the number from your building department. Many municipalities require a permit for attached decks or decks above a set height; check before you dig, and get underground utilities located first.
Deck takeoff mistakes
| Mistake | What happens | The fix |
|---|---|---|
| Using nominal board width | 6 in instead of 5.5 in leaves you a row short | Enter the actual width |
| Forgetting the gap | Wrong row count and boards that cup or trap water | Use the board maker’s gap spec |
| Buying boards shorter than the deck | More butt joints, blocking and waste | Match board length to deck length |
| Diagonal boards on 16 in joists | Bouncy deck, voided warranty on composite | Frame at 12 in or check the install guide |
| Leaving out stairs and railings | A second trip and an unhappy margin | Take them off separately and add them |
| Pricing joists without the rim, ledger and beam | Framing comes in short on the quote | Use the full framing list |
Stairs, railings, post bases, ledger fasteners, flashing and fascia aren’t in this tool. Count them separately. For a concrete pad under the stairs, the construction material calculator works out yards and bags, and the board foot calculator handles hardwood or cedar for trim and benches.
From material list to a deck quote
The material list is about a third of a deck quote. Add delivery, disposal and permit fees, your crew’s hours at the right rate, then your markup on the lot. The markup calculator turns that cost into a price, and the fence calculator covers the privacy screen or yard fence the customer asks about at the same visit.
If decks are your trade, Contruva is built for fence and deck contractors. Your lumber, hardware and markup live in a price catalog, so a deck quote takes minutes, and the customer signs and pays the deposit online through card payments. When the deck is built, job costing shows what you quoted against what you spent. It’s all in Contruva Pro at $249.99 CAD a month. Try it free for 7 days, no credit card needed.