Deck Calculator
Decking length in feet is the deck area with waste, divided by the board width. Joists span the width and are spaced along the length, with a joist at each end: the count is the length divided by the spacing, rounded up, plus one. Posts sit on two beams, each with a post at each end and at the spacing you type. That is the model. It is not a code span table.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Area = length × width. Linear metres = area × (1 + waste %) ÷ board width. Joists = ceil(length ÷ joist spacing) + 1. Posts = 2 × (ceil(length ÷ post spacing) + 1). Joists run across the width. Posts are two lines along the length.
Worked example
A 16 × 12 ft deck is 192 sq ft. Boards 5.5 in wide, at the form’s waste, are 460.79 ft of decking. Joists: 13. Posts on two beams: 8.
Boards, joists, and posts for a platform
A ground-level deck is boards, joists, and posts. Enter the platform, the board cover width, and the two spacings. The decking length is the area with waste divided by the board width. Joists run across the width and are counted along the length, including both ends. Posts are two lines. A picture-frame border and a breaker board are extra linear length this rectangle does not draw.
Measuring a deck platform
The deck model is stated so you can disagree with it on purpose. Decking is the square-metres-to-linear-metres division: area times waste, divided by board width. Twelve square metres of 140 mm boards is 85.71 linear metres before waste. Ten percent waste is 94.29 linear metres. Picture-frame borders and breaker boards are extra linear metres. Add them, or raise waste. Joists run the short way in this model, across the width, and they are placed along the length. The count is ceil(length divided by spacing) plus one, so there is a joist at 0 and a joist at the far end. Four metres at 0.4 m is ceil(10) + 1 = 11. If your joists run the other way, swap length and width in your head before you type, or swap the fields so the spacing is measured along the right side.
How the platform becomes boards and joists
Posts are not a grid under every joist. The model is two beams, one at each long edge, with posts along the length. ceil(4 ÷ 1.2) + 1 is 5 posts on one beam and 10 posts for two beams. A third beam in the middle is another row you add yourself: half of 10, which is 5, if the spacing matches. A corner post is included. Concrete for those holes is the post-hole calculator, one hole per post, full cylinder, post not subtracted. A footing strip under a ledger is the footing calculator. This page does not pour either.
Ordering decking, joists, and posts
Nothing here checks a span. A 3 m joist span may be too far for the timber you are buying. The spacing field starts at the form’s inch default because that is a common centre to edit, not because a code table was opened. Bearers, ledgers, bolts, and hangers are not counted. Railings are the fence calculator if you want posts and pickets, and a deck rail is often a different baluster. The stair calculator gives rise, run, and a stringer length for the steps off the deck.
Checking a deck order
There is no price per metre of decking and no price per post. Order the linear metres, the joist count, and the post count as pieces of the lengths you will cut. The linear-metre page can turn a board length into a piece count if you type the stock length. Do not add waste a second time there if you already applied it here.
Deck fields and starting values
Deck length starts at 16 ft. Deck length in feet. Switch to m / cm for metres. Deck width starts at 12 ft. Deck width in feet. Switch to m / cm for metres. Board width starts at 5.5 in. Board cover width in inches. Waste allowance starts at 10 %. Cuts at the ends and a picture-frame border. The percentage is yours. Joist spacing starts at 16 in. Joist spacing, centre to centre in inches. Post spacing starts at 6 ft. Post spacing along each beam in feet. Switch to m / cm for metres.
Picture-frame borders and breaks
Swap the fields if the joists run the other way. The spacing is always measured along the length field, and the joists span the width field. A 3 m width at 0.4 m spacing with the fields swapped is ceil of 3 divided by 0.4, plus 1, which is 9 joists, and the posts follow the new length. Decking linear metres do not change when you swap, because area is the same. Breaker boards and a picture frame are extra length. Add the perimeter of the frame if every edge has a border board. A 4 by 3 deck has a 14 m perimeter. That 14 m is not in the 85.71 m figure. Concrete for the posts is the post-hole calculator once you know the hole size.
Deck joists and posts for common platforms
Each row is the deck calculator at 5.5 in boards, 16 in joist spacing, 6 ft post spacing, and 10% waste. Linear length is the decking, not the joists.
| Platform | Joists | Posts | Decking |
|---|---|---|---|
| 10×10 ft | 9 | 6 | 73.2 m |
| 12×12 ft | 10 | 6 | 105.3 m |
| 16×12 ft | 13 | 8 | 140.4 m |
| 20×16 ft | 16 | 10 | 234.1 m |
Joists include one at each end. Posts are two beams. Span rules are not in this count.
Common mistakes
- Reading joist spacing as a permitted span.
- Forgetting the model is two beams, not a full post grid.
- Entering board width in the wrong unit. 5.5 in is a common board, and 140 mm is 0.14 m.
- Using the stair calculator's stringer as a joist.
- Expecting concrete bags. The post-hole page does that after you know the holes.
- Adding a price.
Frequently asked questions
How many deck boards do I need?
Divide the area with waste by the board width. The form’s deck at 5.5 in boards is 460.79 ft of decking.
How are joists counted?
They span the width and step along the length, including both ends. Four metres at 0.4 m spacing is 11 joists.
How are posts counted?
Two beams, posts at the ends and at the spacing along the length. Four metres at 1.2 m is 5 posts on a beam and 10 posts altogether.
Does this include a footing?
No. Take the post count to the post-hole or footing calculator.
Is 400 mm joist spacing required?
No. It is the number in the field. Span tables and the building department decide what you may build.
What about stairs?
Use the stair calculator for rise and run. This deck is the platform.