How do you work out how many decking boards you need?
Measure the deck, then follow four steps.
- Rows. Divide the deck width by the board width plus the gap, and round up. A 3 m width with 145 mm boards and a 5 mm gap is 3,005 divided by 150, so 21 rows.
- Linear metres. Multiply rows by the run, the length the boards run along. For a 4 m run that is 84 m.
- Boards. Divide by the board length and add waste. With 3.6 m boards and 10 percent waste, 84 times 1.1 divided by 3.6 gives 25.7, so 26 boards.
- Check the cuts. A 4 m run is longer than a 3.6 m board, so every row needs a joint. If you can’t reuse offcuts, the count can reach 42, and the calculator tells you.
When the board length matches the run, the sums get simple: 21 rows need 21 boards, with no cutting.
How much waste should I allow?
EnviroBuild says 5 to 10 percent for cuts and 15 percent or more for diagonal or herringbone layouts. Millboard also says a cut board needs support from at least three joists, which pushes you toward longer pieces and fewer offcuts.
Which direction should the boards run?
The direction changes the run and the number of joints, so try both. Turning a 4 by 3 m deck so boards run along the 3 m side gives 27 rows of 3 m, which is 81 linear metres instead of 84. The calculator shows both.
What about weight and length?
Hollow 150 mm boards weigh about 2.5 to 3.5 kg a metre and solid ones 4.5 to 6, according to EnviroBuild, so a 4 m solid 150 mm board weighs roughly 18 to 24 kg. Our board sizes guide shows how much each board length wastes against common runs.
Questions
What is a deck board estimator?
Another name for a decking board calculator: you enter the deck and board sizes and it returns how many boards to buy. The same tool is sometimes called a deck board calculator or a decking boards calculator.
How many decking boards do I need for 20 m²?
It depends on board width and length. With 145 mm boards and a 5 mm gap, each metre of board covers 0.15 m², so 20 m² needs about 133 linear metres. Add 10 percent waste and that is about 41 boards of 3.6 m. Plug in your own sizes to be exact.