Deck Board Calculator

What this calculation does not cover

• Board width and the gap between boards are fixed together as a single 5.625 in pitch — a 5.5 in board plus a 1/8 in drainage gap — and neither is offered as an input, so the row count only describes nominal 5/4x6 stock. Narrower composite or hardwood boards change it quickly: at 5.25 in wide the same deck needs about 5 per cent more rows, and the wider gap that grooved hidden-fastener clips set gives you fewer.
• Boards per row is worked out once and then multiplied by the number of rows, so an offcut is never carried across to start the next row. On the default 6 m deck that is two full 12 ft boards bought for every one of the 28 rows and a 4.3 ft tail discarded 28 times, when in practice many of those tails would be reused — whether they can be depends on the seams landing on a joist and on how you stagger them, and neither is modelled here.
• No waste, breakage or cutting allowance is added anywhere in the figure. Boards culled at the yard for crown, cup or splits, the ripping the outermost row usually needs to bring the deck to its finished width, and cuts around posts, notches and openings all come out of the same count, so ordering exactly this number leaves nothing to reject.
• The footprint is treated as one plain rectangle with every board running the same way: area is length times width, and rows come straight off the width figure. An L-shaped or wrapped deck has to be entered as separate rectangles and the counts added, and any surface where the boards turn to run the other way has to be worked out on its own, because the width input is what defines the direction rows are counted in.
• The board-length choices are held in feet and the metric labels are rounded: choosing 2.4 m (~8 ft) still puts a full 8 ft into the arithmetic, which is 2.4384 m and about 1.6 per cent longer than the board you would actually be handed. On a deck length that only just divides into the nominal figure, that difference is enough to leave each row one board short.
• The square-foot figure in the breakdown is the gross rectangle, not the timber it takes to cover it — roughly one part in 45 of that surface is drainage gap once the boards are down. Pricing decking from the area and pricing it from the board count will not agree, and the board count is the one to order from.