Decking Calculator — Deck Boards Estimator
Enter your deck dimensions and board size to find out exactly how many decking boards to buy, including a waste allowance.
Unit system
ft
ft
in
ft
in
%
Rounded up, with waste buffer included
- 1
Board pitch
5.5 + 0.125 = 5.625 - 2
Boards across deck width
144 ÷ 5.625 = 25.6 - 3
Rows along deck length
ceil(20 ÷ 16) = 2 - 4
Net boards (before waste)
25.6 × 2 = 51.2 - 5
Total boards with waste
ceil(51.2 × (1 + 10 ÷ 100)) = 57
How does this calculator work?
Divide deck width by (board face width + gap) to get boards across; multiply by number of rows (deck length ÷ board length, rounded up); add waste (10% typical) and round up. Nominal board sizes differ from actual widths — measure the real face. Does not include railings, stairs or fascia.
Formula
How this is calculated
The calculation starts with the number of board runs spanning the deck width: deck width (converted to inches or mm) divided by the effective board pitch, which is the board face width plus the drainage gap. Because a single board may not span the full deck length, the runs are divided by board length and rounded up to give the number of rows. Multiplying rows × runs per row gives the total board count before waste.
A waste allowance (typically 10% for a straight layout, 15% for diagonal) is applied to cover end-cuts, defective pieces and future repairs. The final count is rounded up to the next whole board — you cannot buy fractions.
This calculator estimates the flat deck surface only. Stairs, railings, picture-frame borders and angled layouts will need extra material. Always verify board dimensions against the actual product: nominal sizes (e.g. "2×6") differ from actual face widths (e.g. 5.5 in). Check local building codes for joist span ratings before finalising your material list.
Frequently asked questions
The most common gap is 1/8 inch (about 3 mm). This allows water drainage and seasonal wood expansion while keeping gaps small enough to be comfortable underfoot and to meet most building codes.
Budget 10% for a straight layout, 15% for diagonal or herringbone, and up to 20% for complex curved or pattern decks. This covers end cuts, defects and future repair boards.
Yes — the formula is the same regardless of material. Composite boards typically have a fixed face width matching common lumber sizes. Check the manufacturer spec sheet for the actual board width and recommended gap (some composites require a larger gap for thermal expansion).
Also known as
TG we-Calculate Editorial Team. (2026). Decking Calculator — Deck Boards Estimator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/decking-calculator
TG we-Calculate Editorial Team. "Decking Calculator — Deck Boards Estimator." TG we-Calculate. 2026. https://we-calculate.com/calculator/decking-calculator.
TG we-Calculate Editorial Team, "Decking Calculator — Deck Boards Estimator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/decking-calculator
@misc{wecalculate_decking_calculator, title = {Decking Calculator — Deck Boards Estimator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/decking-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?