Deck calculator
Calculate the number of deck boards, screws and joists for your deck.
Deck (boards)
Calculate the number of deck boards, linear metres and optional screws from area and board dimensions.
Direction boards run
Across the boards
Typically 4–6 mm for pressure-treated timber
Allowance for cuts and defects (typically 10 %)
Distance between supporting joists, typically 600 mm
Typically 100, 200 or 500 pcs/pack
Your deck project
53 boards (190.8 lm) for 12 m²
Area
12 m²
Linear metres
190.8 lm
Boards
53 pcs
Screws
423 pcs
Joists
8 pcs
Materials
190.8 lm for 12 m²
Buy 3 packs of 200 pcs = 600 pcs (surplus 177 pcs)
ℹThe result is an estimate based on your dimensions, board sizes, spacing and waste.
ℹScrews are a practical estimate and should be double-checked against the manufacturer's recommendations.
Example: 4 × 3 metre deck
A 4 × 3 metre deck with 120 mm boards, 5 mm gaps and 10 % waste needs about 53 boards and 3 packs of screws.
The calculation looks like this:
Numbers may vary slightly depending on board length and waste percentage. The calculator always rounds up.
How to calculate deck boards
Board width and spacing
Deck boards typically come in 95, 120 and 145 mm widths. A 5–8 mm gap between each board lets rainwater drain and allows the wood to expand. The "effective board width" is board width + gap — e.g. 120 + 5 = 125 mm.
From dimensions to board count
Number of rows = deck width ÷ effective board width (rounded up). If the deck is longer than a single board, multiple boards per row are needed. The calculator accounts for both.
Always add waste
In practice boards must be cut at corners, edges and posts. There is also a risk of defective boards. 10–15 % extra is typically recommended. The calculator automatically adds your chosen waste percentage.
Joist spacing affects screws and structure
Joists are typically placed 600 mm apart (centre to centre). Thin boards (19 mm) need closer joists (400 mm). The number of screws depends directly on the number of joists × the number of board rows × screws per crossing.
How to use this calculator
Four quick steps – you don't need to be a tradesperson to follow them.
- Measure the width and length of the area you want to deck.
- Pick a deck board type – it sets the board width and how many you'll need.
- Enter the joist spacing (usually 40–60 cm) and add a bit for waste.
- Check the result and save it to a material list if you want to come back to it.
Common mistakes
Small things that often cost an extra trip to the hardware store.
- Forgetting waste – add 5–10 % extra boards for cuts and offcuts.
- Skipping joists and screws – plan for around 25–30 screws per m².
- Spacing joists too far apart, which makes the deck feel bouncy underfoot.
- Starting on uneven or poorly drained ground.
Example: a 4 × 5 m deck
A 4 × 5 m area is 20 m². With 14 cm wide boards and joists every 60 cm you'll need around 130 linear metres – usually 30–35 boards at 4 m each. Add about 10 % for offcuts so you don't run short halfway through.
Frequently asked questions
Short answers to the questions we hear most often about this calculator.
How much waste should I add?
5–10 % is plenty for a simple rectangular deck. If you have lots of cuts or angled corners, push it to about 15 %.
How far apart should the joists be?
Usually 40–60 cm centre-to-centre for standard 21–28 mm boards. Always double-check what the board manufacturer recommends – it varies a bit between brands.
Does the calculator count screws?
Yes – it gives a rough screw count based on the number of boards and joists.
Can I use this for a raised deck?
It fits the deck surface. The substructure (posts and beams) needs to be sized separately based on height and load.
Use the results in a real project
With MyPlanDIY you can save materials, create budgets, track tasks and bring everything together in one project.
No credit card required. Free demo project included.
See also
Keep going with one of these – they pair well with what you just read.
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