Decking calculator
We include the gap between boards, which most calculators forget.
Your inputs
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Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
Decking and the fence around the same yard share perimeter and posts.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Measure the deck length and width.
- Enter the width and length of the boards you will use.
- Set the gap between boards: it is part of the covered width.
- The result gives boards and linear metres.
How it is calculated
Each board covers its width plus the gap to the next one. Ignoring that gap is the classic mistake:
With a 5.5 in board and a 5 mm gap, each board covers 5.7 in. Across a 13 ft deck that is 27.4 boards per row instead of 28.4: almost a board less per row, in your favour.
The gap is not optional: timber and composite both expand, and without a joint the deck buckles in summer.
Worked example
A 16 x 13 ft deck with 5.5 in x 12 ft boards, 5 mm gap, 10 % waste:
- Area: 208 sq ft (19.3 m²)
- Effective width: 0.14 + 0.005 = 0.145 m
- Area per board: 0.145 x 3.66 = 0.53 m²
- Boards: 19.3 x 1.10 / 0.53 = 40.1 → 41 boards
- Linear feet: 492 ft
Good to know
- Joists and fixings excluded. Joists go every 16-20 in depending on board thickness; hidden clips run about 20 per square metre.
- Direction matters. Running boards the long way reduces cuts and joints.
- Composite and timber move differently. Follow the manufacturer's gap: it ranges from 3 to 8 mm.
- 10 % waste. Raise it for irregular shapes or if you are adding perimeter trim.
Frequently asked questions
How many deck boards do I need?
For a 208 sq ft deck with 5.5 in x 12 ft boards and a 5 mm gap, 41 boards including 10 % waste, which is about 492 linear feet.
What gap should I leave between deck boards?
3 to 8 mm depending on material and manufacturer. The gap allows expansion and drainage: without it the deck buckles.
Does the calculator include the gap?
Yes, and it matters. Each board covers its width plus the gap, so ignoring it makes you order more boards than you need.
How far apart should the joists be?
It depends on board thickness: 16 in is typical for 1 in boards. This calculator counts boards only, not the frame.