Decks & Fences
Decking Calculator
Enter your deck length and width in metres to get the number of decking boards and joists to buy — works for 90 mm and 140 mm timber boards and for composite systems.
Formula tested · Local units · No sign-up
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Show the calculation methodFormula, conversions, rounding, and assumptions+
Rows of boards = deck width ÷ (board width + gap), rounded up. Australian hardwood decking such as merbau or spotted gum is commonly sold in 90 mm and 140 mm widths; with a 4 mm gap a 90 mm board takes 94 mm per row.
Boards per row = deck length ÷ board length, rounded up; boards to buy adds your wastage allowance and rounds up.
Joists = deck length ÷ joist spacing rounded up, plus one closing joist — 450 mm centres is a common spacing for standard decking boards.
Real-world example
Worked example: 5.4 m × 3.6 m deck, 90 mm boards, 4 mm gap, 5.4 m boards, 450 mm centres
- Row width: 90 + 4 = 94 mm. Rows: 3.6 ÷ 0.094 = 38.30 → round up to 39 rows.
- Boards per row: 5.4 ÷ 5.4 = 1 board.
- Boards to buy: 39 × 1 × 1.10 (10% wastage) = 42.9 → round up to 43 boards.
- Joists: 5.4 ÷ 0.45 = 12 exactly, + 1 closing joist = 13 joists.
Buy 43 boards and 13 joists. Enter your supplier's prices for a cost estimate — advertised consumer prices in Australia normally include 10% GST, so enter 0% tax for GST-inclusive quotes.
Before you start
How to measure
- Measure the deck length (the direction the boards run) and width in metres.
- Enter the actual board width from your supplier — 90 mm and 140 mm are the common Australian widths; 140 mm boards cover faster but move more with moisture, so many installers prefer 90 mm for hardwoods.
- Enter the stock length you can buy; hardwood decking is often sold in random or set lengths — planning rows so butt joints land on joists reduces waste.
Local guidance
Notes for Australia
- Raised decks commonly need council approval in Australia — height thresholds and rules differ by state and council, so check before building; decks attached to the house or over a certain height usually need a building approval and compliant balustrades.
- In bushfire-prone areas, the site's BAL (bushfire attack level) rating can restrict decking materials — confirm requirements with your council or certifier before choosing boards.
- Common choices are hardwoods (merbau, spotted gum, blackbutt), treated pine, and composite boards — all fit this calculator; for composites use the manufacturer's stated width, gap and joist spacing.
- GST is 10% and consumer prices are usually advertised GST-inclusive; enter 0% tax for GST-inclusive quotes, or 10% for ex-GST trade pricing.
Quick reference
Common Australian decking planning values
| Item | Common value |
|---|---|
| Board widths | 90 mm or 140 mm |
| Drainage gap | 4–5 mm |
| Joist centres | 450 mm (closer for some composites/diagonal) |
| Board thickness | 19 mm hardwood is common |
Planning values only — supplier guidance, span tables and council rules govern.
Good to know
Common mistakes to avoid
- Using 140 mm boards at spacings meant for 90 mm without checking the supplier's span and fixing guidance.
- Forgetting the gap — hardwoods like merbau move with moisture and need drainage room.
- Building a raised deck without council approval — thresholds are lower than many people expect and balustrade rules kick in with height.
- Comparing GST-inclusive retail prices against ex-GST trade quotes.
Need help?
Frequently asked questions
How many 140 mm boards for a 4.2 m × 3 m deck?
Row width is 143 mm with a 3 mm gap, so 3 ÷ 0.143 = 20.98 → 21 rows. With 4.8 m boards that's one per row, and 21 × 1.10 = 23.1 → 24 boards with 10% wastage. Joists at 450 mm: 4.2 ÷ 0.45 = 9.33 → 10, + 1 = 11 joists.
Should I choose 90 mm or 140 mm boards?
90 mm boards are the traditional hardwood choice — they move less across their width and suit tighter joist details; 140 mm boards cover the deck with fewer rows and fewer fixings. Run the calculator with both widths and compare the counts before pricing.
Do I need approval to build a deck?
Often, yes — especially for raised decks, decks attached to the house, and anything needing balustrades. Rules differ by state and council, and bushfire-prone sites add material restrictions. Check with your council or a private certifier before ordering materials.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- Australia
- Spotted an error?
- Report a correction
Methodology
- Boards are assumed to run parallel to the deck length; joists run parallel to the width, spaced along the length. All dimensions are converted to metres internally before any arithmetic.
- Rows of boards = deck width ÷ (board width + gap), rounded UP. Boards per row = deck length ÷ board length, rounded UP — part boards count as whole boards because that's what you buy.
- Boards to buy = rows × boards per row × (1 + wastage%), rounded UP. Joists = deck length ÷ joist spacing, rounded UP, plus one closing joist. Exact multiples are not bumped up.
- The cost estimate multiplies the board and joist counts by the prices you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Board dimensions: 90 mm and 140 mm are common Australian decking widths; confirm your supplier's actual profile.
- Joist centres: 450 mm centres is common practice for standard boards; span tables and supplier guidance govern.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size joists, bearers, posts, footings or ledger connections — deck structure is an engineering and permit matter. Confirm the structural design with a qualified person and quantities with your supplier before ordering.