Decking Calculator

How a decking calculator counts boards

Board count is a coverage problem, not an area problem. Each board covers its own face width plus the gap you leave beside it, so the deck width divided by that combined figure gives the number of rows. The deck length divided by the board length gives how many boards make up one row. Multiply the two, add waste, and you have an order quantity. Square footage alone will always under-count, because it ignores the gaps and the offcut at the end of every run.

Worked example: a 16 ft x 12 ft deck

Take a rectangular deck 16 ft long with boards running the long way, 12 ft across, built from 5.5 in wide pressure-treated 2x6 boards at 12 ft, with a 1/4 in gap and 10% waste.

Deck width in inches: 12 x 12 = 144 in. Each board occupies 5.5 + 0.25 = 5.75 in. Rows = 144 / 5.75 = 25.04, rounded up to 26 rows. Boards per row = 16 ft of run / 12 ft boards = 1.33, rounded up to 2. Base count = 26 x 2 = 52 boards. Add 10% waste: 52 x 1.1 = 57.2, so buy 58 boards.

Now the rest of the order. Deck area is 16 x 12 = 192 sq ft, so face screws at 350 per 100 sq ft come to 672, rounded up to 680 — buy a 5 lb box, which typically holds around 350 screws, so two boxes. Joists at 16 in on centre run perpendicular to the boards, spaced along the 16 ft length: 192 in / 16 in = 12 bays, plus one for the closing joist, which is 13 joists at 12 ft each.

Why 58 boards and not 52

Look at that second row calculation again: a 16 ft run built from 12 ft boards means each row is one full board plus a 4 ft piece, and the 8 ft leftover from the second board is only useful if another row needs a piece 8 ft or shorter. In practice some of those offcuts land in the skip. Switching to 16 ft boards makes each row a single board — 26 boards plus waste instead of 52 — and removes every butt joint from the deck surface. Run the calculator both ways before ordering; the longer board is usually cheaper per square foot once waste is counted.

Layout planning: start with a full board

The rounding up on rows means the last row is almost never a full board. A 144 in width covered by 5.75 in rows leaves 0.04 of a row, which is a sliver about a quarter of an inch wide — impossible to fasten and ugly at the edge. Three ways out: widen the gap slightly across all rows to absorb the remainder, rip the first and last board to equal widths so the sliver is shared between both edges, or plan a picture-frame border that swallows the difference. Deciding this before you order matters, because a ripped border row can change how many boards you need.

Fastener schedule

Two screws per board at every joist crossing is the standard, which is where the 350 per 100 sq ft figure comes from. Screws must be rated for the treatment chemicals in the lumber — plain zinc corrodes fast in modern copper-based treatments, so stainless or coated deck screws are the norm. Hidden fasteners give a clean face with no visible heads but cost more and are sold by coverage area, typically 100 or 175 sq ft per box, so buy by the coverage figure on the label. Composite boards almost always want the manufacturer's own clip system.

Gaps and seasonal movement

Wet pressure-treated lumber leaves the yard saturated and shrinks as it dries, so a 1/8 in gap can open to 1/4 in within a season. That is why 1/8 in is the usual spacing for wet stock — the board does the work for you. Kiln-dried lumber and composite have already stabilised, so those are laid with the gap you want to keep, commonly 1/4 in for drainage and debris clearance. Composite also expands lengthwise in heat, which is why manufacturers specify a butt-end gap that changes with the temperature on installation day.

Limitations

This calculator assumes a rectangle with boards running in one direction. Octagons, curves, wrap-around stairs and multi-level decks need to be broken into rectangles and added up. It also assumes a single board size across the whole deck, so a picture-frame border in a contrasting colour should be priced separately. Joist counts here are a spacing estimate only, not a span table: joist size, spacing and beam positions depend on span, load and your local building code, and that is a plans-review question rather than an arithmetic one.