How this works
Calculating deck boards is essentially a coverage problem: figure out the effective width of one installed board (the actual board width plus the expansion gap between boards), divide the deck width by that to get how many boards run along the length, and multiply by the deck length to get total linear feet. The math is straightforward; the trap is forgetting the gap. Wood expands across the grain when wet — a 6-inch dry board can be 6.05" after a rainy week — and tight installations buckle, popping fasteners and creating tripping hazards. The standard 1/8-inch (3 mm) gap on installation lets seasonal moisture move without stressing the structure.
Board widths matter for both look and material count. The two dominant North American sizes are 5/4 × 6 (actual 1" × 5.5") and 2 × 6 (actual 1.5" × 5.5"); 5/4 is lighter, cheaper and the most common, while 2 × 6 is used for longer joist spans or where you want extra heft underfoot. In Europe and Australia, decking boards are typically metric 25-32 mm thick × 120-150 mm wide. Some specialty hardwoods (ipê, cumaru) come in 5.25" or 5.75" widths — small differences that compound across a 16-foot wide deck. Always measure the actual board you're buying before doing the count.
Three practical points. (1) For long decks (more than your stocked board length), you'll need to butt-joint boards mid-run, which means each row uses two pieces with a joint that should land on a joist. The calculator above gives you total linear footage; divide by your stocked length and round up to get pieces. Stagger the joints between rows for both strength and aesthetics. (2) Standard waste allowance is 10% for straightforward rectangular decks, 15-20% for decks with cutouts, angles or picture-frame borders. Hardwood deck boards in particular often have 5-10% defective sections you'll cull on install, so don't be shy with the buffer. (3) Composite decking (Trex, Fiberon) installs by the same maths but with much smaller gaps (often 1/16" / 1.5 mm) since composite expands less than wood. Check the manufacturer spec — using the wood-spec gap on composite leaves visible lines.
The formula
Board width and gap are in inches; deck length and width are in feet. The result is the linear footage you need to buy, plus a piece count if you tell the calculator the stocked board length you're using. The "deck width" is the dimension perpendicular to the direction the boards run — boards typically run along the long dimension for fewer cuts and a cleaner look, but you can install either way; just enter whichever dimension is perpendicular to the boards as the "width".
Example calculation
- 12 ft × 16 ft deck, 5/4 × 6 boards (5.5" actual width), 1/8" gap, 16 ft stock length.
- Effective board width = 5.5 + 0.125 = 5.625". Boards along 12 ft (144"): ceil(144 / 5.625) = 26.
- Total linear feet = 26 × 16 = 416 ft. With 10% waste = 458 ft. Pieces (16 ft stock) = ceil(458 / 16) = 29.
Frequently asked questions
Why include a gap if the boards are already cut to size?
Wood is dimensionally stable along the grain (the long direction) but not across it. As outdoor boards absorb moisture from rain or humidity, they swell perpendicular to the grain — typically 0.5-1% per 10% change in moisture content for stable softwoods, considerably more for some hardwoods. A 5.5" wide board going from 8% to 18% MC can swell 0.05-0.10 inches across its width. If you butt them tight, that expansion has nowhere to go; the boards push against each other, the deck cups upward in the middle of each board, and fasteners pop or shear. The 1/8" gap absorbs ordinary seasonal moisture cycles. Some installers go to 3/16" or even 1/4" with high-shrinkage hardwoods like ipê — read the species' specific moisture-movement guide.
How do I handle non-rectangular decks?
Break the deck into rectangles, calculate each separately, and add the linear footage. For an L-shape, treat it as two rectangles — the calculator gives a clean answer for each, then you sum them. For decks with cut corners or curves, draw the largest enclosing rectangle and add 15-20% waste rather than 10% — the cuts at the perimeter eat material faster than a straight rectangle does. For decks with large fixed obstructions (a hot tub footprint, a tree well), subtract that area from the largest enclosing rectangle. The calculator above handles single rectangles; for multi-rectangle decks, run it once per rectangle and add the totals.
What about composite decking?
Same calculation, different gap. Composite decking (Trex, Fiberon, TimberTech and similar wood-plastic composite or capped polymer products) expands and contracts with temperature rather than moisture, and the movement is much smaller than wood — typically 1/16" (1.5 mm) or even less between boards is the manufacturer's recommended gap. Each manufacturer publishes specific spec sheets; check yours before installing because it varies with the product line and the installation temperature. The same is true of butt joints between board ends — composite typically wants smaller end gaps than wood. Capped composite expands less than uncapped; PVC decking expands the most and needs the largest gaps. Read the spec, set the gap accordingly in the calculator, and the rest of the math is identical.