Wood & Trim
Deck Board Calculator
Estimate the number of deck boards from deck size, board width, gap and stock length.
Enter your measurements
Results update as you type.
About this calculator
Enter the deck dimensions and which way the boards run, the board's actual (dressed) width, the gap between boards, and the stock length you will buy. The result counts rows and the boards to purchase, including an extra allowance.
How to use this calculator
- Measure the deck and decide which direction the boards run.
- Use the board's actual dressed width and your planned gap.
- Enter the stock length you will buy.
- Calculate; leftover length is reported, not hidden.
Formula
Row pitch = board width + gap; rows = ⌈(across dimension + gap) ÷ row pitch⌉, with gaps only between boards
Stock pieces per row = ⌈along dimension ÷ stock length⌉; offcuts are not reused between rows
Boards = ⌈rows × pieces per row × (1 + extra ÷ 100)⌉
Assumptions
- Each row is estimated using whole stock pieces, with no offcut reuse across rows. If a row needs multiple pieces, a separate supported joint layout is required.
- Uses actual board width; a nominal name is not a measurement.
- Extra allowance applies once, after row counting.
What to count separately
| Project part | Planning step |
|---|---|
| Straight deck boards | This calculator |
| Stair treads and borders | Separate measured quantities |
| Joist spans and footings | Separate structural design |
A board quantity is not a structural design.
Common mistakes
- Counting a trailing gap beyond the last board.
- Using nominal board width instead of actual installed width.
- Forgetting that board direction changes the required run length.
Worked example
Try it: press “Load example” above the form.
12 ft across at a 5.75 in row pitch needs 26 rows; 12 ft boards cover 12 ft rows, so 26 boards.
Limitations
Quantity estimate only: no joist design, fastener schedule, supported-joint layout or structural guidance. Rectangular decks only.
Frequently asked questions
Are gaps counted after the final board?
No. Gaps occur between adjacent rows only. The row count must cover the width with no imaginary gap beyond the last board.
Does it reuse offcuts?
The model uses its stated stock-length and row assumptions, not a full cut optimizer. Stairs, borders, picture framing and structural supports require separate quantities.
What does the worked example show?
12 ft across at a 5.75 in row pitch needs 26 rows; 12 ft boards cover 12 ft rows, so 26 boards.
Which assumptions should I check before using the result?
No joist design, fasteners or structural guidance. Rectangular decks only.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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