How this instrument works
Decking is laid as a series of parallel boards running the length of a deck, with a small gap left between each board for drainage, airflow and seasonal wood movement. The distance from the start of one board to the start of the next — its 'pitch' — is simply the board's face width plus that gap, and dividing the total deck width by this pitch tells you how many boards are needed to span it.
This instrument works perpendicular to the boards: Deck width, perpendicular to boards (ft) is the distance the boards need to cover side to side, not the length each individual board runs. It converts that width to inches, divides by the board width plus gap, and rounds up to the next whole board, because a partial board at the end of a run still has to be cut from a full-length piece — there is no such thing as buying 25.6 boards.
The gap value matters more than it looks. A 1/8 in (0.125 in) gap is a common composite-decking default, but pressure-treated wood is often spaced wider to allow for shrinkage as it dries, and some manufacturers specify their own minimum gap for warranty compliance — checking the board manufacturer's installation guide before entering a value here avoids a board count that is technically correct for the wrong spacing.
- Enter Deck width, perpendicular to boards (ft) — the total side-to-side distance the boards need to cover.
- Enter Board face width (in), the visible width of a single board as installed, 5.5 in by default for standard 6 in nominal decking.
- Enter Gap between boards (in) — the spacing left between adjacent boards, 0.125 in (1/8 in) by default.
- Read Deck boards needed — the count is always rounded up, since a partial board still requires cutting a full one.
- Add extra boards beyond this count for waste from end cuts, picture-frame borders, or any boards damaged during installation.
Worked example — a 12 ft wide deck, 5.5 in composite boards
Enter 12 into Deck width, perpendicular to boards (ft), 5.5 into Board face width (in), and 0.125 into Gap between boards (in) — a 12 ft wide deck built with standard composite boards and a 1/8 in gap. Twelve feet converts to 144 in, and the board pitch is 5.5 + 0.125 = 5.625 in.
Dividing 144 by 5.625 gives 25.6, and Deck boards needed rounds that up to 26 — twenty-five full-pitch boards plus one more to close out the remaining 0.6 of a board's worth of width, since decking is never sold or cut to a fractional board.
Questions
Why does the result always round up instead of to the nearest board?
Because a deck can't be built with a fraction of a board — if the math works out to 25.6 boards, the leftover 0.6 still has to come from a 26th physical board, ripped down to fit. Rounding down would leave a literal gap in the deck surface, so the instrument always rounds up (ceiling) rather than to the nearest whole number.
Should I measure the width parallel to the boards or across them?
Across them — Deck width, perpendicular to boards (ft) is the side-to-side distance the boards need to span, not the length each board runs along the deck. If your boards run the long way across a 12 ft by 20 ft deck, enter 12 ft here, since that's the distance being divided into board-width increments; the 20 ft length only determines how long each individual board needs to be.
What gap should I use between boards?
Check the board manufacturer's installation guide first — many composite brands specify a 3/16 in to 1/4 in gap for expansion, while a common default like this instrument's 0.125 in (1/8 in) suits kiln-dried composite or some wood species. Using a gap narrower than the manufacturer requires can cause boards to buckle as they expand in heat or humidity, and using one that's too wide can void a product warranty.
Does this account for waste from cutting boards to length?
No — this instrument only counts how many boards are needed to fill the width, based purely on board width and gap. It does not add extra for cutting boards to the deck's length, mitred corners, picture-frame borders, or boards damaged during installation; most contractors add 10% or more on top of this count to cover those separately.
Does the board count change if I switch from wood to composite decking?
Only through the numbers you enter — the formula itself doesn't care what material a board is made of, only its face width and the gap you plan to leave. Composite boards commonly run 5.5 in wide same as many wood boards, but always check the actual product's specified face width, since nominal lumber sizes (like a '1x6') are usually narrower than their name implies.