Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate deck boards and joists needed, using each board's real (actual) width, not its nominal size.
This calculator sizes a deck-board order from your deck's area, plus a joist count from the framing spacing beneath it. The important detail it gets right: deck boards are sold by a nominal size (like "5/4x6") that doesn't match their real, actual coverage width -- a 5/4x6 board's true width is 5.5 inches, not 6.
Enter your deck's total area, the board's real actual width (check your specific product -- 5.5in is common for a "6-inch" composite or lumber deck board), and the board's length. Optionally add a joist run and on-center spacing to also get a joist count.
Board Coverage (sqft) = (Actual Width in ÷ 12) × Board Length (ft)
Boards = ROUNDUP((Deck Area ÷ Board Coverage) × (1 + Waste))
Using the nominal width instead of the actual width would understate how many boards you need -- a small error per board that adds up significantly across a whole deck.
Use this when planning materials for a new deck build or a resurfacing project. Once you have a board and joist count, the Deck Cost Calculator turns those quantities into a cost estimate using your own supplier's prices.
Always check your specific decking product's real actual width rather than assuming a standard figure -- composite decking, in particular, varies by manufacturer more than dimensional lumber does.
The calculator converts area into a board count using each board's coverage, which assumes boards run the full deck length or are cut with little scrap. In practice, boards are cut to fit, and the length you buy affects how much ends up as offcuts. If your deck is 14 feet across the run direction, 16-foot boards leave a 2-foot offcut per row, while 14-foot boards (if your supplier stocks them) leave almost none. Compare a couple of board lengths in the calculator and choose the one that wastes the least.
Butt joints, where two boards meet end to end, must land on a joist and are usually staggered from row to row. Longer boards mean fewer joints, and fewer joints mean fewer chances for visible seams.
Take a hypothetical deck measuring 10 feet by 20 feet, so 200 square feet. Boards are 5.5 inches wide and 12 feet long, and you set waste to 10 percent. Coverage per board is 5.5 square feet, and ROUNDUP((200 / 5.5) x 1.1) = 40 boards. Note that a 10-foot joist run at 16 inches on center needs 10 x 12 / 16 = 7.5, so around 8 or 9 joist positions across the span, depending on how you count the ends. Use the joist fields to check this against your framing plan.
Boards per row = Deck Width ÷ Actual Board Width (plus a gap allowance)
Joist count ≈ (Run in ft × 12) ÷ Spacing + 1
The coverage formula uses the board's width only. Most decking is installed with a small gap between boards for drainage and expansion, commonly around an eighth to a quarter of an inch, though the correct gap depends on the product and installation instructions. Over a 10-foot span that gap can consume a full extra row, so if you want an exact count of rows, add the gap to the board width when working out how many boards fit across. The waste allowance usually absorbs the difference, but on a tight fit it is worth checking.
If you lay boards at 45 degrees, expect more offcuts because every board end is angled. Increasing the waste percentage to 15 percent is a common allowance for diagonal patterns, though you should confirm with your installer or supplier for your specific layout.
Lumber nominal sizes describe the rough-cut size before milling and drying, not the final planed dimension -- a "5/4x6" board's actual finished width is typically 5.5 inches.
16 inches on-center is standard for most residential decking, though some composite products require closer 12-inch spacing -- always check your specific decking manufacturer's span requirements.
Composite boards are often bought in fixed lengths and cannot be reused as easily when cut, so many builders keep a modest waste factor. Follow the manufacturer's guidance for your product.
No. Enter only the flat deck surface, then calculate stair treads and risers separately and add them to your order.
No, it is optional. It is useful for a new build or for checking that your existing joist spacing matches what your decking product requires.
Some homeowners buy an extra board or two from the same production batch, since color can vary between lots. That is a personal choice rather than a calculation.