Deck frame is down. Client wants 90x19mm merbau decking with 6mm gaps, and the deck is 7.2m x 4.5m. Before the lumber yard order is confirmed, you need the board count, fixings count and approximate lineal metres.
Decking Calculator
Decking
Common: 90mm, 140mm
Typical: 5–8mm for drainage
Boards across = Deck width ÷ (Board width + Gap)
Lineal metres = Boards across × Deck length × (1 + waste%)
Gap tips: Hardwood 5–6mm (expands in wet) · Treated pine 6–8mm · Allow for movement with seasoned timber.
Joist spacing: 450mm max for 90mm boards · 600mm for 140mm+.
Results are estimates for planning purposes. Verify with current standards and a qualified professional.
1 What this calculator does
Calculates the number of deck boards, lineal metres, approximate screws or hidden clip count, and total deck area from the deck dimensions, board size, gap width and waste percentage. Supports both surface screw and hidden clip fixing systems.
2 Formula & professional reasoning
Deck area = Deck length x Deck width
Effective board width = Board width (mm) + Gap (mm)
Boards across = Ceiling(Deck width / Effective width in m)
Lineal metres (with waste) = Boards across x Deck length x (1 + Waste%/100)
Boards to order = Ceiling(Lineal metres / Board length)
Screws: 2 per board per joist at joist spacing | Hidden clips: 1 per board per joist
Deck boards run the length of the deck. The number of rows is determined by dividing the deck width by the effective board width (including gap). Each row runs the full deck length. Fixings count is estimated from the board count and joist spacing -- surface screws require 2 per board per joist crossing to prevent twisting, hidden clips require 1 per board per joist. A 12% waste allowance for hardwood decking accounts for end cuts and defective boards.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
7.2m x 4.5m merbau deck, surface screws
Given: Deck: 7.2m x 4.5m | Board: 90mm x 19mm | Gap: 6mm | Board length: 4.8m | Waste: 12% | Joist spacing: 450mm | Fixings: surface screws
Working:
Area: 32.4 m² | Effective width: 96mm = 0.096m | Rows: ceiling(4.5/0.096) = ceiling(46.88) = 47 rows | LM: 47 x 7.2 x 1.12 = 379.0 lm | Boards: ceiling(379/4.8) = ceiling(79.0) = 79 boards | Joists per board length: ceiling(7.2/0.45) = 16 | Screws: 47 rows x 16 joists x 2 = 1,504Answer: 32.4 m² deck | 79 x 4.8m boards | 1,504 screws
💡 Order 80 boards and 1,600 screws (screws come in boxes). Check merbau requires pre-drilling and stainless steel or hot-dipped galvanised screws.
Standard
Same deck -- hidden clip system
Given: Same deck | Fixings: hidden clips (e.g. Camo or Durafix)
Working:
Same board count: 79 boards | Clips: 47 rows x 16 joists x 1 = 752 clips | Plus 1 row of face screws at the starting board: 16 screwsAnswer: 79 boards | 752 clips + 16 face screws
💡 Hidden clip systems add approximately 20-30% to fixing cost vs screws but give a clean screw-free surface. Confirm clip compatibility with the decking species and board width.
Advanced
Decking at 45 degrees -- increased waste
Given: Same deck 7.2m x 4.5m | Diagonal laying | Waste: 20%
Working:
Effective width still 96mm | Rows at 45 degrees: based on diagonal span | Approximate increase: diagonal increases required length -- add 20% waste factor instead of 12% | Boards: ceiling(47 x 7.2 x 1.20 / 4.8) = ceiling(84.6)Answer: 85 boards (vs 79 for straight) | 17.5% more boards needed for diagonal
💡 Diagonal decking creates larger off-cuts at each corner and requires longer runs. Always use 18-22% waste for diagonal applications. The board count and cost increase significantly.
4 Sanity check
Typical deck board widths and gaps
Merbau 90mm, 6mm gap | Spotted gum 86mm, 5mm gap | Treated pine 90mm, 5mm gap | Composite 140mm, 5mm gap
Waste allowance by application
Straight decking: 12% | Boards requiring matching: 15% | Diagonal: 18-22%
Fastener selection for hardwood decking
Stainless steel or hot-dipped galvanised screws | Grade 316 SS for coastal | Stainless steel hidden clips for composite
Standard zinc screws will stain and corrode in merbau and other hardwoods -- never use them.
Pre-drilling requirement
Hardwood decking (merbau, spotted gum, ironbark) must be pre-drilled to prevent splitting
Use a countersink bit sized for the screw head.
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Not using the effective board width (board + gap) when calculating rows | Dividing deck width by board width alone | Underestimating board count -- running short mid-job | Always add the gap to the board width before dividing. 90mm board + 6mm gap = 96mm effective width per row. |
| Using the wrong screw type for hardwood decking | Specifying zinc-plated screws instead of stainless or HDG | Screws corrode, staining the hardwood with rust streaks -- expensive to fix | Specify grade 316 stainless steel screws for coastal applications. Grade 304 stainless or hot-dipped galvanised for inland. Never use zinc-plated screws in hardwood decking. |
| Not allowing for the starting board face-fixed row with hidden clips | Specifying only hidden clip quantity | Starting board has no fixing -- bounces and moves | The first deck board must be face-fixed (screwed through the top) as hidden clips have nothing to clip to on the first board. Add one row of face screws equal to the number of joists crossed. |
| Not checking whether joist spacing is appropriate for the decking species and thickness | Using the standard 450mm joist spacing without verifying | Thin or lightweight boards flex between joists -- springy, uncomfortable deck | Check the decking manufacturer's maximum joist spacing for the board thickness. 90x19mm hardwood typically suits 450mm joist spacing. 90x19mm composite may require 300-350mm spacing -- confirm with the product data sheet. |
6 Reference & regulatory links
🇦🇺 Australia
7 Professional workflow
Common tools used alongside this one: