What it works out
Count deck boards, joists and fasteners. Enter the run along the boards and the width across them, then the board width and the gap between boards. Rows come from the width, boards in a row from the stock length, and joists from the spacing. Beams, posts and footings are structure and are not worked out here; follow your local code for them. Waste starts at the usual figure for the trade; change it in the tool.
The formula
- Rows
- width ÷ (board width + gap), rounded up
- Joists
- run ÷ spacing, rounded up, + 1
- Fasteners
- per crossing × rows × joists
In words: Rows = width divided by (board width plus gap), rounded up; joists = run divided by spacing, rounded up, plus 1; fasteners = per crossing times rows times joists
Worked example
A deck 16 ft long (boards run along it) and 12 ft wide, with 5.5 in boards, a 1/4 in gap and joists 16 in on centre
- Run 16 ft = 192 in
- Width 12 ft = 144 in
- Board width 5.5 in
- Gap 0.25 in
- Joist spacing 16 in
- Rows = 144 ÷ (5.5 + 0.25) = 25.04, rounded up to 26
- Joists = 192 ÷ 16 = 12, + 1 = 13
- Screw crossings = rows × joists = 26 × 13 = 338 (times the fasteners per crossing)
Boards per row come from the stock length you choose; waste is added on top.
Good to know
- Beams, posts and footings are structure and are not worked out here: follow your local code.
- Waste starts at the usual figure for the trade and can be changed.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
Does it size beams or footings?
No. It counts boards, joists and fasteners only.
Why does the gap matter?
Each board takes up its width plus one gap, so a wider gap means fewer rows.