Decks & Fences
Decking Calculator — Boards, Joists, and Fixings
Enter metric deck dimensions and the approved board, joist, and fixing layout to get boards, joists, regular fixings, whole packs, and optional entered-price cost.
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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.
Regular fixings = board rows × joists × fixings per crossing, followed by a separate allowance and optional whole-pack rounding.
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.
- Regular fixings: 39 rows × 13 joists × 2 = 1,014; +5% = 1,064.7 → 1,065, or 11 packs of 100.
Buy 43 boards, 13 joists, and 1,065 regular fixings (11 packs of 100). Enter supplier prices; use 0% tax for GST-inclusive quotes or 10% for ex-GST pricing.
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.
- Confirm the board supplier's fixing count, corrosion resistance, pack size, butt-joint framing, starter/edge details, and bushfire-area material restrictions before ordering.
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 |
| Face fixings per regular crossing | 2 is a common plan value; supplier system governs |
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.
- Assuming the regular intersection count includes hidden-system starters, butt-joint clips, edge boards, stairs, fascia, and blocking.
- 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.
How many decking screws or clips do I need?
Use board rows × joists × the approved fixings per crossing for the regular field, then add your explicit allowance and round to whole packs. Count starters, perimeter details, butt joints, stairs, fascia, and proprietary clips separately.
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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:
- 2026-07-20
- Formula version:
- 1.1.0
- 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.
- Regular-layout fasteners = board rows × joists × fasteners per intersection. The editable fastener allowance is applied separately, then the quantity rounds up first to a whole fastener and, when entered, to whole supplier packs.
- The cost estimate multiplies board, joist, and optional fastener-pack counts by only 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. The regular-layout fastener count does not determine corrosion class, hidden-clip system, butt-joint blocking, starter clips, or edge details. Confirm the structural design, board fixing schedule, and quantities with qualified guidance, manufacturer instructions, and your supplier before ordering.