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.
Ejemplo resuelto
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.
In metric: a 4.8 m × 3.6 m deck in 140 mm boards with a 3 mm gap. Each row covers 143 mm, so 3,600 ÷ 143 = 25.2 → 26 rows; 26 × 4.8 m is 124.8 linear metres. In 4.8 m boards, with 10% waste, 124.8 × 1.1 ÷ 4.8 = 28.6, so buy 29 boards.
En la obra
Boards shorter than the deck. A 20 ft × 12 ft deck in 5.5 in boards with a ⅛ gap is 144 ÷ 5.625 = 25.6 → 26 rows of 20 ft. But the yard's boards are 12 ft and 16 ft, not 20. Each row is a 12 and an 8 — the 8 being half a 16 — so 26 rows take 26 twelves and 13 sixteens, and the butt joints stagger themselves if you start every other row with the 8. The tool's answer with 16 ft stock (520 lin ft × 1.1 ÷ 16 = 36 boards) is the right total; the yard order is the row-by-row version of it.
A picture-frame border. The border eats one board width off each side, so the field is narrower and shorter by 11 in total each way, and the border itself is the perimeter in linear feet: 2 × (20 + 12) = 64 lin ft, four 16s. Run the field at 19 ft 1 in × 11 ft 1 in, add the four border boards, and push the waste to 15% — every field board now has a cut on both ends.
Composite is sold by the piece and priced by the foot. The count is the same arithmetic; the difference is a 20 ft composite board is a real length you can buy, so put 20 in the stock field and the rows fall out one board each with no joint. Worth the money on a deck this size if the joints would otherwise show.
What went wrong?
Quick referencefor decking
- Rows
- deck width in inches ÷ (board width + gap), rounded up
- 5.5 in boards
- ⅛ gap: 2.13 rows per foot · ¼ gap: 2.09 · tight: 2.18
- Pies lineales
- rows × deck length (linear metres in metric)
- Boards per 100 ft² of deck, 5.5 in, ⅛ gap, 16 ft stock, 10% waste
- about 15–16
- Actual widths
- 5/4 × 6 and 2 × 6 are 5½ in · 2 × 4 is 3½ in · most composite 5½ in
- Métrico
- 140 mm board + 5 mm gap = 145 mm a row · 6.9 rows per metre
Waste & overagewhat to add, and why
- Deck builders order 10% over for a straight lay and 15%+ for a diagonal or a picture-frame border, because every board cut on the angle loses its offcut. The waste field starts at 10%.
- Buy board lengths that divide the deck without a joint (a 16 ft board on a 16 ft deck) — joints have to land on a joist and each one costs a cut and a wasted end.
- Cull a few percent for crook and wane when the stock is treated lumber; the pile is never all straight.
Mistakes to catchbefore you order
- Typing the nominal board width (6) instead of the face width (5½) — the row count comes out 10% low.
- Measuring deck width to the outside of the rim without the fascia; the boards run past the rim by the overhang, so add it.
- Setting the gap to 0 for wet treated lumber that will shrink — the tool lays the deck with the gap you type; the wood will open more later.
What an inspector looks forthe building inspector, at framing and at final
On this part of the job
- Joist spacing against what the decking is approved to span. Composite and PVC boards have their own maximum spacing, in their own printed instructions, and it is often tighter than for timber — tighter again on a diagonal layout.
- Fasteners suited to the material and to treated timber, and installed the way the board maker specifies (hidden clips have their own instructions).
- Gapping between boards and at the ends, and clearance where the deck meets the house wall.
- Guards: height, infill, and above all how the posts are anchored — through-bolted into blocking, not lag-screwed to a rim.
- The ledger. Fastener type, pattern, flashing over it, and that it is fixed to structure rather than to sheathing. This is the single most-failed part of a deck.
Have this ready
- The decking manufacturer's span and fastening instructions.
- The deck plan with joist spacing and the guard detail.
- The ledger and its flashing left visible until it has been looked at.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La fórmula
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.
Preguntas
- 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 (about 3 mm) 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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