Deck Calculator (2026)
Work out exactly how many deck boards, how many linear feet, and how much your deck will cost.
Calculate Your Deck Boards
Each board is cut to this full length, so measure the side your boards will run along.
Board rows are counted across this side, so it sets how many boards you need.
A standard 6-inch nominal board measures 5.5 inches wide; a 4-inch nominal board measures 3.5 inches.
The drainage and expansion gap between boards. One eighth of an inch (0.125) is the usual default.
Extra boards for offcuts, defects, and mistakes. Ten percent is standard; use 15 percent for diagonal or picture-frame layouts.
The price of one board at the deck length above, from your supplier. Enter the all-in price if tax is included in your quote.
Model: boards run along the deck length, so each board is one deck length long. Board count is the number of rows across the deck width, rounded up. Waste is applied to the board count, then rounded up again. Linear feet use the waste-included board count.
How the Math Works
The model is deliberately simple and transparent. Boards run along the deck length, so every board is one deck length long and the only question is how many rows fit across the width. Each row takes up one board width plus one gap, so the width in inches divided by board width plus gap gives the row count. Because you cannot buy part of a board, that count is rounded up. Waste is then added as a percentage and rounded up a second time, since a fraction of a board is still a whole board you must buy.
Effective board width (in) = board width (in) + gap (in)
Boards without waste = deck width (in) / effective board width (in), rounded UP
Boards with waste = boards without waste x (1 + waste percent / 100), rounded UP
Linear feet = boards with waste x deck length (ft)
Total cost = boards with waste x price per board
Worked Example: 16 x 12 Foot Deck
| Calculation | Result | |
|---|---|---|
| Deck area | 16 x 12 | 192 sq ft |
| Effective board width | 5.5 + 0.125 | 5.625 inches |
| Boards without waste | 144 / 5.625 = 25.6, rounded up | 26 boards |
| Boards with 10% waste | 26 x 1.10 = 28.6, rounded up | 29 boards |
| Linear feet of decking | 29 x 16 | 464 linear feet |
| Total cost | 29 x $12.50 | $362.50 |
Notice the double rounding: 25.6 rows becomes 26 boards, and adding 10 percent gives 28.6, which becomes 29 boards, not 28. Rounding down at either step would leave you short, and a short board run is the most common reason a deck project stalls mid-build.
Common Board Widths and Gaps
Most deck boards are sold by nominal size, which is larger than the actual width. A board labelled 6 inches wide actually measures 5.5 inches, and one labelled 4 inches measures 3.5 inches. Always enter the actual width, not the nominal size, or your count will come out low. The gap matters too: a wider gap means fewer boards per foot of width, but gaps that are too wide trap heels and debris, while gaps that are too narrow trap water and swell shut.
| Nominal board size | Actual width | Typical gap | Rows per 12 ft of width |
|---|---|---|---|
| 4 inch | 3.5 inches | 0.125 inches | 40 rows |
| 6 inch | 5.5 inches | 0.125 inches | 26 rows |
| 6 inch, wide gap | 5.5 inches | 0.25 inches | 26 rows |
| 8 inch | 7.25 inches | 0.125 inches | 20 rows |
The 6-inch board with a quarter-inch gap still gives 26 rows across 12 feet, because 144 divided by 5.75 is 25.04, which rounds up to 26. Gap size only changes the count when it pushes the division across a whole number.
How Much Waste Should You Add?
Ten percent waste covers straight runs on a rectangular deck: a few boards arrive warped or split, and end cuts eat a little length on every board. Raise the allowance to 15 percent if your deck is diagonal, picture-framed, or has angled edges, because angled cuts waste more of every board. For a simple rectangle with boards bought at exact deck length, 10 percent is the standard figure used by most suppliers and builders. If your supplier sells boards only in fixed lengths, you may need to buy longer boards and cut them down, in which case count the boards, not the linear feet, as your order quantity.
Board Length, Direction, and Layout Tips
- Run boards along the longer side when you can. Fewer butt joints means a cleaner look and less waste, and this calculator assumes boards run along the deck length you enter.
- Check fixed board lengths before ordering. If your supplier stocks 16-foot boards and your deck is 18 feet long, every row needs a butt joint and a second board piece, which this rectangular model does not cover.
- Buy all boards from one batch. Wood and composite shades vary between batches, so ordering the full waste-included count at once avoids a visible color change mid-deck.
- Let boards acclimate. Stack them on site for a few days before fitting, so the gap you planned for stays the gap you get after the boards settle.
- Price the whole job, not just boards. Joists, fasteners, railing, and stain or sealant are separate costs; this calculator prices deck boards only.
