Decking Boards Calculator
Calculate boards, joists, and fixings for timber decking
About This Calculator
This calculator helps you determine the materials needed for your timber decking project. It calculates the number of decking boards, joists required for the frame, and screws needed for fixing.
The calculator accounts for board width, gaps between boards for drainage/expansion, and recommended joist spacing based on board thickness.
How to Use
- Measure your deck dimensions (width × length in metres)
- Select your decking board width and thickness
- Choose gap size between boards (3-6mm typical)
- Set joist spacing (depends on board thickness)
- Decide if you want to include wastage for cuts
- Calculate quantities
Decking Board Sizes
Narrow Boards
90-100mm width
More traditional look, more boards needed
Standard Boards
120-150mm width
Most common, good balance
Thick Boards
25-32mm thickness
Durable, longer spans between joists
Thin Boards
19-22mm thickness
Economical, closer joist spacing needed
Joist Spacing Guide
| Board Thickness | Max Joist Spacing |
|---|---|
| 19mm | 300mm |
| 25mm | 400mm |
| 28mm | 450mm |
| 32mm | 600mm |
Building Tips
- Allow 3-6mm gaps between boards for drainage and expansion
- Joists should run perpendicular to the decking boards
- Use two screws per board per joist for secure fixing
- Consider adding 10% wastage for angled cuts and mistakes
- Pre-drill holes to prevent boards from splitting
- Use treated timber or hardwood for durability
Ready to Calculate?
Enter your deck dimensions and board specifications to get accurate quantities.
Launch CalculatorNAIBC / IRC codes
References & Sources
Standards Cited
- AWC DCA 6 — American Wood Council Prescriptive Residential Wood Deck Construction Guide
Provides guidance on joist spacing, board thickness, gap sizes, and structural requirements for timber decking - AWPA U1 — Use Category System: User Specification for Treated Wood
Guidance on timber treatment and preservation for external decking exposed to weather
Formulas Used
- Deck Area:
A = deckWidth x deckLength
Source: Standard geometry - Boards Across Width:
boards = ceil(deckWidth_mm / (boardWidth + gapSize))
Source: AWC DCA 6 decking guidance — Effective board width includes the gap between boards - Joists Required:
joists = ceil(deckLength_mm / joistSpacing) + 1
Source: AWC DCA 6 decking guidance — First joist at 0mm, then at each spacing interval - Screws Needed:
screws = boardsNeeded x joistsRequired x 2
Source: AWC DCA 6 decking guidance — 2 screws per board per joist intersection - Linear Metres of Board:
linearMetres = boardsNeeded x deckLength
Source: Standard calculation
Key Assumptions
- Recommended joist spacing: 300mm for boards <20mm thick, 400mm for <25mm, 450mm for <32mm, 600mm for >=32mm — AWC DCA 6 decking guidance
- Gap between decking boards typically 3-6mm for drainage and expansion — AWC DCA 6 decking guidance
- 10% wastage allowance for cuts at board ends — Industry standard practice
- 2 screws per board at each joist crossing point — AWC DCA 6 decking guidance