Deck Calculator
Calculate joists, decking boards and footings for a timber deck.
About Deck Calculator
Board count depends on the pitch rather than the width alone, because each board carries a drainage gap. Footing count comes from the span limits of the beams and joists rather than from the deck area. Omitting the board gap understates board count by around 4 per cent, and gaps are not optional because timber and composite both need drainage and movement room.
Formula
Deck
Boards = deck width ÷ (board width + gap); joists = length ÷ spacing + 1
- board pitch
- Board width plus the drainage gap, which is what repeats
- footing
- Concrete pad carrying a post, spaced by beam and joist span limits
Worked example
With the default values below, Deck Calculator returns Decking Boards Required of 25. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Deck lengthminimum 0 m
- 5 m
- Deck widthminimum 0 m
- 3.6 m
- Joist spacingminimum 100 mm
- 400 mm
- Decking board widthminimum 50 mm
- 140 mm
- Gap between boardsaccepts 0–30 mm
- 5 mm
Results
- Decking Boards Required
- 25
- Deck Area
- 18.000 m2
- Joists Required
- 13
- Joist Linear Metres
- 46.80 m
- Footings Required
- 12
Step by step
- 1Values used
- 2Deck
- 3Decking Boards Required= 25
- 4Deck Area= 18.000 m2
- 5Joists Required= 13
- 6Joist Linear Metres= 46.80 m
- 7Footings Required= 12
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Deck calculated?
Boards = deck width ÷ (board width + gap); joists = length ÷ spacing + 1 Board count depends on the pitch rather than the width alone, because each board carries a drainage gap. Footing count comes from the span limits of the beams and joists rather than from the deck area.
Why does Deck matter?
Omitting the board gap understates board count by around 4 per cent, and gaps are not optional because timber and composite both need drainage and movement room.
What values do I need to enter?
This calculator takes 5 inputs: Deck length, Deck width, Joist spacing, Decking board width, Gap between boards. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.