Siding Calculator
Calculate how many siding boards you need for an exterior wall, based on wall measurements, board width and standard board length.
How to use the siding calculator
- Enter the total wall length and wall height (the height the boards need to cover, e.g. from foundation to soffit).
- Enter the exposed board width — the width each board actually covers on the wall after overlap, not the board's full width.
- Enter the standard board length of the product you plan to use.
How the number of boards is calculated
The calculator first finds how many horizontal rows are needed to cover the wall height, then works out the total number of boards based on the standard board length:
Number of boards = (Number of rows × Wall length) ÷ Standard board length
If you're using horizontal siding (the most common), the boards run horizontally and "rows" means the number of boards stacked in height. If you're using vertical siding, swap length and height in the calculation.
The ventilation gap behind the cladding
External cladding is not meant to be watertight. It is a screening outer layer, and the precondition for it lasting is that water getting in behind it can also get out again. Cladding is therefore mounted on battens outside the wind barrier, so that a continuous, ventilated cavity runs behind the boards. The cavity provides both drying and pressure equalisation, and must be open at both bottom and top for air to actually circulate.
This is often the difference between cladding that lasts for decades and cladding that rots from the bottom up. The calculator works out the number of boards — counter-battens, battens, wind barrier and vermin mesh come in addition, and the batten direction depends on whether the cladding runs horizontally or vertically.
Frequently asked questions
What's the difference between horizontal and vertical siding?
Horizontal siding (lap siding) is installed horizontally with overlap, while vertical siding (board-and-batten) is installed vertically. The installation method affects how you should enter length and height in the calculator.
Should I add extra for window and door openings?
This calculator doesn't automatically subtract area for openings. If they're large, you can reduce the wall area manually. For smaller openings, it's often easier to just account for a bit of extra waste.