Deck Calculator
Calculate decking boards, joists and fixings for a deck.
Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.
What the Deck Calculator does
Deck board count comes from the deck width divided by the board width plus its expansion gap, giving the number of rows, then the deck length divided by board length for pieces per row. Joists follow the same spacing-plus-one rule as studs.
Formula
Rows = ⌈Deck width ÷ (Board width + Gap)⌉Boards per row = ⌈Deck length ÷ Board length⌉Total boards = Rows × Boards per row × (1 + Waste %)Joists = ⌊Deck width ÷ Joist spacing⌋ + 1
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Deck length | number | Yes | Accepts more than 0. |
| Deck length unit | one of 6 options | Yes | — |
| Deck width | number | Yes | Accepts more than 0. |
| Deck width unit | one of 6 options | Yes | — |
| Board width | mm | Yes | Accepts 10 or more. |
| Gap between boards | mm | Yes | Accepts 0 or more. |
| Board length | number | Yes | Accepts more than 0. |
| Board length unit | one of 6 options | Yes | — |
| Joist spacing | mm | Yes | Accepts 50 or more. |
| Waste allowance | % | Yes | Extra material for cuts, breakage and offcuts. 5–10% is typical, more for complex layouts. |
| Price per board | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Deck length unit and Deck width unit.
- Enter Deck length, Deck width, Board width and Gap between boards and 3 more.
- Optionally add Price per board.
- Select Calculate.
Worked example
A 5 m × 4 m deck with 144 mm boards, 5 mm gaps, 3.6 m board lengths, 400 mm joists, 10% waste.
- Length
- 5 m
- Width
- 4 m
- Board
- 144 mm
- Gap
- 5 mm
- Joists
- 400 mm
4 ÷ 0.149 = 27 rows, 5 ÷ 3.6 = 2 per row = 54 boards, +10% = 60 boards. 11 joists at 5 m.
Frequently asked questions
Why leave gaps between deck boards?
Timber swells when wet. Without a gap the boards press together and cup or buckle. The gap also lets water and debris drain through.