Decking Calculator
Boards, joists and screws for a new deck.
Your Deck
Boards, joists and fixings update as you type.
Decking boards needed
44 boards (236.5 lineal metres)
5.4 m lengths, 43 rows of 90 mm boards, 10% waste included.
Plan view
Simplified and to scale: thin lines are board rows, dashed lines are joists, bold lines are bearers. House wall along the top. At most about 40 lines of each kind are drawn.
Materials list
| Item | Quantity | Lineal m | Cost |
|---|---|---|---|
| Decking boards5.4 m lengths, 215 m before waste | 44 | 236.5 | — |
| Joists450 mm spacing | 13 | 52 | — |
| Bearers1800 mm spacing | 4 | 20 | — |
| Screwstwo per board per joist; 3 × 500 | 1,118 | — | — |
Open “Pricing” and add supplier prices to see a cost estimate.
Materials estimate only. Footings, posts, flashing, stair, handrail and fastener extras are not included.
| Deck | Area | Boards | Lineal m |
|---|---|---|---|
| 3 × 3 m | 9 m² | 20 | 105.6 |
| 4 × 3 m | 12 m² | 27 | 140.8 |
| 5 × 4 m | 20 m² | 44 | 236.5 |
| 6 × 4 m | 24 m² | 53 | 283.8 |
Length × width at your current board size, gap, direction, stock length (5.4 m) and waste (10%).
How Much Decking Do I Need? Using the Decking Calculator
Enter the length and width of your deck, choose which way the boards will run and pick a board size, and the decking calculator shows how many boards to buy, the total lineal metres, and the joists, bearers and fixings underneath. Whether you call it a deck calculator, a deck estimator or a timber decking calculator, the aim is the same: a realistic shopping list before you ask for prices.
Add your supplier prices in the optional pricing section for a deck cost estimate, then use it to compare against any quotes you receive.
How to Measure Your Deck
Measure the length along the house wall and the width out from it, in metres, to the outer edge of the finished boards. For an L-shaped or stepped deck, split it into rectangles, calculate each one and add the results. Decide the board direction first: boards running along the length need fewer joins on a long deck, while boards running across may suit the look or the stock lengths you can buy. Joists always run the other way to the boards.
Decking Formulas
Lineal metres = rows × run length (m) × (1 + waste %)
Boards to buy = lineal metres ÷ stock board length, rounded up
The run is the dimension the boards lie along; the cover is the other dimension, which the rows fill. A 90 mm board with a 5 mm gap covers 95 mm per row.
Bearers = cover (mm) ÷ bearer spacing, rounded up, plus 1; each spans the run
Screws = rows × joists × 2 (or one hidden clip per board per joist for composite)
Screws are counted in boxes of 500 and clips in packs of 100. Boards are priced on the full lengths you would buy.
Worked Example: a 5 × 4 m Deck
A deck measures 5 m × 4 m (20 m²) with boards running along the 5 m length. The boards are hardwood 90 × 19 mm with a 5 mm gap, bought as 5.4 m lengths, with 10% waste, joists at 450 mm and bearers at 1,800 mm.
Each row takes 95 mm, so covering the 4 m width needs 43 rows. That is 43 × 5 = 215 lineal metres before waste and 236.5 lineal metres with 10% added, or 44 boards of 5.4 m (about 2.2 boards per m²). Underneath, the 5 m run needs 13 joists (52 lineal metres of 4 m joists) and the 4 m cover needs 4 bearers (20 lineal metres). Fixing with two screws per board per joist takes 1,118 screws, which is 3 boxes of 500.
Switching to the composite preset (approximate 137 mm boards with a 6 mm gap) cuts the rows to 28 and the boards to 29, with about 364 hidden clips. At the same settings a 3 × 3 m deck needs 20 boards, a 4 × 3 m deck 27 and a 6 × 4 m deck 53, as the quick reference table in the calculator shows.
Hardwood, Treated Pine or Composite?
Hardwood is dense and durable and is a popular choice for Australian decks, though it usually needs oiling to keep its colour. Treated pine is generally the lower-cost option and is typically available in thicker profiles for the money, but it may need more regular maintenance and care around moisture. Composite boards need little upkeep and are available in a range of finishes, but they usually cost more up front and have their own installation rules for gaps, expansion, joist spacing and fixings. Follow the manufacturer's guide for whichever you pick, because it can change the numbers on this page.
Joists, Bearers and Fixings
Boards are screwed or clipped to joists, joists rest on bearers, and bearers sit on posts and footings. How far each can span depends on the timber species, grade and size, which is why this calculator treats spacing as an input rather than a recommendation. Use span tables, or have an engineer or builder confirm the frame. Use fixings that suit the board material and the location, and remember that boards that are not square to the joists, or joins that do not land on a joist, can change the quantities.
Approvals and Building Rules
Whether a deck needs building approval, and what handrails or barriers are required, depends on your state, your council and your site. Height above the ground, the deck area, distance to boundaries and overlays such as bushfire areas can all matter. Check with your council or a private certifier before you start, because building without a required approval can be costly to fix.
Frequently Asked Questions
- How much decking do I need for my deck?
- Work out the deck area (length × width), then count how many rows of boards it takes to cover the width and multiply by the length each row runs. Add an allowance for waste, commonly around 10%, and divide by the length of the boards you will buy. A 5 m × 4 m deck in 90 mm boards with a 5 mm gap comes to 43 rows and 236.5 lineal metres including waste, or 44 boards at 5.4 m.
- How do I use a deck calculator to work out boards?
- Enter the deck length and width in metres, choose whether the boards run along or across the length, pick the board material and set the stock length you will buy. The calculator converts the board width plus gap into the number of rows, then returns lineal metres, the number of boards, joists, bearers and fixings. Change the waste percentage or spacing and the results update straight away.
- What is a decking calculator and what does it include?
- A decking calculator is a materials estimator for the deck surface and the frame beneath it. This one covers boards (in lineal metres and whole lengths), joists, bearers and either screws or hidden clips, and it adds a cost estimate if you enter your own prices. It does not size the structure, and it leaves out posts, footings, stairs, balustrades and finishing oil or stain.
- How is a deck cost calculator estimate worked out?
- Each material line is its quantity multiplied by the price you enter: boards by the dollars per lineal metre for the lengths you would buy, joists and bearers by their lineal metres, and fixings by the price per 100. Labour, if you add it, is a dollar rate per square metre. The total divided by the deck area gives a cost per m², which is handy for comparing options, but prices vary by region, species and supplier so use current figures from your own quotes.
- What should a decking quote include, and how do I compare quotes?
- A good decking quote should state the deck size, the board material and size, the framing (joist and bearer sizes and spacings), the footings, the fixings, any stairs, handrails or balustrades, finishes, removal of waste, and what is excluded. It should also say whether approvals and certifier fees are covered and what warranty applies. This calculator gives a materials estimate you can hold up against a quote: if a quote's board or framing quantities are far from yours, ask the builder to explain the difference. It does not set labour rates.
- How does a composite decking calculator differ from timber?
- Composite boards are usually wider than timber boards and are typically installed with a slightly larger gap, so fewer rows are needed. They are commonly fixed with hidden clips rather than two screws per joist, and they often need joists closer together, such as 400 mm or less, so check the manufacturer's guide. The composite size in this calculator is approximate, so overwrite the board width and gap with those of your actual product.
- What does a deck estimator allow for waste?
- A 10% allowance is a common starting point for straight runs of boards because of offcuts, end trimming and the odd damaged board. Diagonal patterns, picture-frame borders, curves and lots of joins can justify more, while a deck sized to suit the board lengths needs less. Set the waste percentage to match your layout and your supplier's available lengths.
- Is this decking calculator suitable for Australia?
- Yes. It works in metres and millimetres, and the presets reflect typical Australian decking sizes such as 90 × 19 mm and 140 × 19 mm hardwood and 90 × 22 mm treated pine, with stock lengths from 1.8 m to 6.0 m. Product ranges differ between suppliers, so confirm sizes and lengths with the supplier you plan to use.
- How far apart should the joists be for timber decking?
- Joists of 450 mm centres are a common starting point for timber decking, but the right spacing depends on the board species and thickness, the joist size and span, and the manufacturer's recommendations. Composite boards often need 400 mm to 450 mm or closer. Bearer spacing of 1,800 mm is a typical figure in this calculator but depends on the joist size. Use span tables or an engineer for the actual frame design.
- Do I need council approval to build a deck?
- It can depend on the deck's height above the ground, its size, how close it is to boundaries and whether the site has overlays such as bushfire or heritage controls. Rules differ between states and councils, and some small low decks are exempt while others need approval. Check with your local council or a certifier before you build, and ask about handrail and balustrade requirements too.