Deck Board Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The number of deck boards needed is calculated by dividing the total surface area by the area covered by one board, including the gap between boards. For 20 m² with 12 cm boards spaced 5 mm apart over 2.4 m in length, 67 boards are needed.
Explanation
Installing a wood or composite deck requires calculating the number of boards from the total surface area to cover, but without forgetting a detail often overlooked in a quick calculation: the gap left between each board during installation. This gap, generally 4 to 6 mm, both lets the wood expand slightly with humidity and temperature, and allows rainwater to drain between the boards rather than pooling on the surface. This gap is an integral part of the width actually occupied by each board once installed: a 12 cm board spaced 5 mm apart actually occupies 12.5 cm of width on the ground, a nuance that, added up across the full width of a deck, can amount to the equivalent of several extra boards on a large project if left out of the calculation. This calculator therefore multiplies the board's width, increased by this gap, by its length, to get the area actually covered by each installed board, before dividing the deck's total surface area by that value. The number of boards is always rounded up to the next whole number, since a board can't be bought as a fraction, which naturally includes a small margin for cuts along the deck's edge. For the surface area itself, from the plot's dimensions, see our surface area calculator; for a similar board-count calculation applied to an indoor flooring project, see our flooring quantity calculator.
Example: 20 m² deck, 12 cm boards, 5 mm gap, 2.4 m length
Inputs
Surface area: 20 m². Board width: 12 cm. Gap: 5 mm. Board length: 2.4 m.
Calculation
Area covered per board = ((12 + 0.5) ÷ 100) × 2.4 = 0.125 × 2.4 = 0.3 m². Number of boards = 20 ÷ 0.3 ≈ 66.67, rounded up to 67 boards.
Result
67 boards need to be ordered for this deck.
Frequently asked questions
Why does the gap between boards matter for this calculation?
Because it slightly increases the width actually occupied by each installed board, thereby reducing the area covered per board compared with its raw physical width alone. On a large deck with many rows of boards, ignoring this gap in the calculation can lead to underestimating the number of boards needed by several units.
What gap should I choose between boards?
A gap of 4 to 6 mm is commonly recommended for solid wood, which expands and contracts with ambient humidity. Composite wood, less sensitive to humidity changes, sometimes tolerates a slightly different gap: check the specific manufacturer's recommendations for the product you've chosen.
Does this calculation account for the cuts needed along the edge of the deck?
The systematic rounding up to the next whole board naturally absorbs a small margin for edge cuts. For a deck with a complex shape (many angles, irregular outlines around obstacles), an extra margin of a few boards beyond this basic calculation remains prudent, since it assumes a simple rectangular surface.