Deck board calculator
Enter the deck's length and width and pick your board, and this counts the decking boards to buy, with the deck area and a face-screw estimate. It is the same takeoff the CutOnce app runs, which also sizes the joists, beams, footings, and railing.
Updated
Boards run across the joists. Count is rows by board width times boards per row, rounded up after waste. Straight layout wastes less; diagonal wants 15% or more. Screws assume face-screwing at 2 per joist crossing.
- Deck area
- 192 sq ft
- Face screws
- 672
How the board count works
Decking runs across the joists, so the count is two numbers multiplied. First, rows: the deck's length divided by the board's actual width, rounded up, because a 5/4x6 is really 5-1/2 in wide, not 6. Second, boards per row: the deck's width divided by your stock board length, rounded up, since a run longer than one board needs a butt joint. Rows times boards per row is the raw count; then the waste factor covers cuts and culls, and the total rounds up to whole boards.
A worked example
A 16 ft long by 12 ft wide deck, boarded in 5/4x6 running the short way in 16 ft stock, with 10% waste. Here is the count and where it comes from.
| Deck area | 192 sq ft | 16 x 12 |
| Rows of decking | 35 | ceil(16 ft / 5.5 in board width) |
| Boards per row | 1 | ceil(12 ft / 16 ft board) |
| Raw board count | 35 | rows x boards per row |
| Boards with waste | 39 | raw x 1.10, rounded up |
| Face screws | 672 | area x 350 screws per 100 sq ft, at 16 in OC |
Nominal size versus actual width
The count uses the board's real milled width, not the name on the tag. A 2x6 and a 5/4x6 both cover 5-1/2 in; a 2x4 covers 3-1/2. Get this wrong and a big deck comes up a full row short.
| Nominal | Actual width |
|---|---|
| 5/4 x 6 | 5.5 in |
| 2 x 6 | 5.5 in |
| 2 x 4 | 3.5 in |
| 2 x 8 | 7.25 in |
Straight or diagonal
Boards run straight across the joists in most decks, which is what the default count assumes. Turn them 45 degrees for a diagonal deck and two things change: the joists have to be closer (a diagonal board spans farther between them) and every board meets the rim with an angled cut, so the offcuts add up. The calculator switches to an area-based count for a diagonal and leans on a higher waste factor, but a diagonal deck genuinely eats more board, so budget 15% waste or more.
A word on fasteners
The screw number is a face-screw estimate, two screws per board at every joist crossing, which is what most pressure-treated and solid composite decks use. Grooved boards with a hidden clip system are counted a different way, one clip per joist crossing plus starter screws, and the box states its own coverage. Treat the screw figure as a face-screw starting point, not a clip count.
What the app adds
This page counts the deck surface. The app frames the rest: joist and beam spans for your species and spacing, the footing layout and sizes, the railing with post and baluster spacing to the IRC 4 in sphere rule, and a diagonal picture-frame border. It prints the whole thing as a cut plan and a lumber list snapped to real stock lengths.
Common questions
- How many deck boards do I need for a 16x12 deck?
- In 5/4x6 (5-1/2 in actual) running the 12 ft direction in 16 ft boards, that is 35 rows of one board each, or 39 boards once you add 10% for waste. Change the board width or the direction the boards run and the count changes, which is why the calculator asks for both.
- Should the boards run the long way or the short way?
- Whichever crosses the joists, and that is set by how you framed. Boards must run perpendicular to the joists, so the joists run the other way. Running boards the short way often means no butt joints if your stock length covers the width, which is cleaner and wastes less; a long deck may force joints, which you stagger over joists.
- Does the calculator include the joists and framing?
- No, it counts the decking surface and the face screws. Joist and beam spans, footings, and posts depend on your lumber species, the spacing, and the load, which is framing math the app handles. This page is the board count you take to the yard.
- Why use actual width instead of the nominal size?
- Because that is what covers the deck. A 5/4x6 is sold as a six but mills out to 5-1/2 in, so ten of them cover 55 in, not 60. Counting on the nominal number leaves you short by a board or more on a wide deck. The calculator uses the milled width for every size.
- How much waste should I add for decking?
- Ten percent for a straight rectangular deck is the usual start. Go to 15% or higher for a diagonal layout, a deck with lots of angles or a picture-frame border, or short random-length stock, where the offcuts pile up. A simple deck in long uniform boards can run closer to 5%.
Count the boards, frame the whole deck
CutOnce counts the decking like this page, then sizes the joists and beams, lays out the footings, and spaces the railing balusters to code, all on the jobsite.
Free to download, with a 7-day trial of everything. The web calculator is the teaser; the app prints the cut plan and the lumber list.
See also