Deck board calculator
These numbers come straight from the sizes you type. Deck framing, fasteners and the substructure are a separate takeoff, and this tool does not judge whether a board or a joist carries the load. Lay out a row on the actual deck to check the gap before you commit the whole order.
Enter the deck size, the board face width and the gap you are leaving, and the tool counts the rows of decking, totals the linear length, and rounds up to whole boards of the length you buy. Every figure — board width, gap, waste — is a field you can change.
Worked example
A 16 ft × 12 ft deck with 5.5-inch boards and a 1/8-inch gap: each row covers 5.625 inches of width, so 12 ft (144 in) ÷ 5.625 ≈ 26 rows. Twenty-six rows × 16 ft is 416 linear feet of decking. In 16-foot boards that is 26 boards; add 10% waste and you would buy 29 boards.
The formula
rows = ⌈width ÷ (board + gap)⌉ · boards = ⌈(rows × length × (1 + waste)) ÷ stock length⌉
A board plus its gap is the width one row eats up, so the deck width divided by that is the row count. Each row is one deck length of decking; multiply out, add waste, and divide by the board length you are buying to get whole boards.
Questions
- How many deck boards do I need?
- Work out how many rows of decking cross the deck — the deck width divided by one board plus its gap — then multiply by the deck length for the total linear length of decking, and divide that by the length of the boards you are buying. This tool does all three steps and rounds up to whole boards.
- What gap should I leave between deck boards?
- A small gap lets water drain and gives the boards room to move. An eighth of an inch is common for boards installed dry; pressure-treated lumber that is still wet is often laid tight because it shrinks as it dries. The gap is an editable field here — set it to what you are actually leaving.
- Do deck boards run the length or the width?
- Enter the deck so that a single board runs across the width and rows stack along the length. If your boards run the other way, just swap the two dimensions before you type them in — the math is the same, it only cares which direction a board spans.
- How much waste should I add for a deck?
- Ten percent is a fair default and what this tool uses. A simple rectangle with boards that match the deck length wastes little; a deck with angles, a picture-frame border, or lengths that force awkward cuts wastes more, so bump the factor up for those.
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