How the boards are counted
Boards run one way across the deck, so the number of rows is the deck width divided by the board width plus the gap. A 3 m wide deck with 145 mm boards and 5 mm gaps has 20 rows.
Total board length is the rows multiplied by the deck length, plus a waste allowance for cuts and joins.
Boards to buy is that length divided by the length the boards are sold in, rounded up. Longer boards mean fewer joins, which is both quicker and stronger.
Worked example
A deck 4 m by 3 m with 145 mm boards and 5 mm gaps needs 20 rows. At 4 m each that is 80 m of board, and with 10 per cent waste it is 88 m.
At 2.4 m board lengths that is 37 boards. Buying 3.6 m boards instead cuts the count and halves the number of joins.
Gaps, joists and what the count excludes
The gap is not decoration. It lets water off the deck and lets the boards move as they take on moisture, and 4 to 6 mm suits most timber.
Joists, bearers, fixings, fascia and a border are not in this figure. Joists are usually at 400 mm centres, which is a separate count along the deck.
Composite boards are wider and often longer than timber, and they move less. Enter their real dimensions rather than the timber figures.
Limits of this calculator
It assumes boards running in one direction at a constant width, with no border, picture frame or diagonal pattern.
It does not calculate the substructure, the fixings, the ventilation under the deck or the fall for drainage.
It does not check the span tables, which decide the joist size and spacing for the deck's height and loading.
For a paved surface instead, use the tile calculator.
Inputs for the decking calculator
Deck length
Length along the direction the boards run.
Deck width
Width across the boards, which sets the number of rows.
Board width
Face width of one board, 145 mm for a standard timber deck board.
Gap between boards
Drainage gap, usually 4 to 6 mm.
Board length
Length the boards are sold in, 2.4 m or 3.6 m for most timber.
Waste allowance
Extra for cuts and joins. Ten per cent suits a deck with a border.
CALCULATOR QUESTIONS
Decking calculator questions
How many boards do I need per square metre?
About 6.7 for a 145 mm board with a 5 mm gap, which is 20 rows over 3 m.
Should boards run along or across?
Along the longer dimension, and at right angles to the joists so every board is supported.
What gap should I leave?
4 to 6 mm. Too tight and water sits on the deck; too wide and heels catch.
Does this include joists?
No. Joists, bearers and fixings are separate, and joist spacing is usually 400 mm.