Online Tool Store
Deck Board Calculator
Work out how many deck boards, joists, and screws a deck needs from its dimensions, board width, and the gap you plan to leave.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Unit & Converters
Deck Board Calculator
Frontend preview — no upload or external service.
Materials list
4.2 x 3.0 m deck: 21 boards at 4.2 m, 8 joists at 400 mm centres, roughly 340 screws at two per board per joist, plus 8% wastage on boards.
How the Deck Board Calculator works
- Enter the deck dimensions and the actual board width, not the nominal size.
- Set the gap between boards — timber that arrives wet needs a smaller gap because it shrinks.
- Choose joist spacing according to the board thickness and the manufacturer's span table, then add wastage for cuts.
The method
Board count comes from the deck width divided by the effective width of each board, which is the board plus its gap.
boards = deck width / (board width + gap)
3.0 m of deck width with 145 mm boards and 5 mm gaps: 3000 / 150 = 20 boards, plus one to allow for the trimmed edge.
FAQ
How big should the gap between boards be?
Around 5 mm for kiln-dried timber, smaller for wet timber that will shrink, and check the manufacturer's figure for composite boards, which move with temperature rather than moisture.
What joist spacing do I need?
It depends on board thickness and the span table for the product — 400 mm centres suits most 25 mm timber boards, and many composites require 300 to 400 mm. Wider spacing makes a deck feel bouncy.
Why include wastage?
Cuts, defects, and diagonal or picture-frame edges all consume material. Eight to ten percent is normal, more for a complex shape.
How we compare
| Feature | Online Tool Store | A search-engine answer box | A reference table |
|---|---|---|---|
| Boards, joists and fixings together | ✓ | Manual | Yes |
| Accounts for the board gap | ✓ | Manual | ✓ |
| No account required | ✓ | ✓ | ✗ |
| Product-specific span tables | ✗ | Datasheet | Sometimes |
Deck Board Calculator counts materials from the effective board width, and defers to the manufacturer's span table for joist spacing, which is a safety matter.