Softonic

Deck Board Calculator

Step 1 / 3 Number of boards
Next

Enter your deck length and width in feet, the width of each board in inches, the board length in feet and the gap you plan to leave between boards. The calculator returns the deck area in square feet, the total number of boards and the total linear feet to order. Planning by board count and linear feet means you buy the right amount of decking the first time instead of guessing from square footage alone.

How to estimate your deck boards

  1. 1

    Measure the deck

    Enter the finished length and width of the deck surface in feet. The longer run is usually the direction the boards span; the width is measured across the boards.

  2. 2

    Set the board size and gap

    Enter the actual board width in inches (a nominal 6 in deck board is 5.5 in), the board length in feet, and the gap left between boards for drainage and expansion.

  3. 3

    Read the results

    The calculator shows the deck area, the total number of boards and the total linear feet. Add a waste allowance before you place the order.

How the board count is calculated

Deck boards are laid in rows across the width of the deck, with a small gap between each board. The effective coverage of one board is its face width plus the gap. Working in feet and inches the formulas are:

  • Deck area (ft²) = deck length × deck width
  • Effective board width (ft) = (board width in inches + gap in inches) ÷ 12
  • Rows of boards = deck width ÷ effective board width, rounded up
  • Boards per row = deck length ÷ board length, rounded up
  • Total boards = rows × boards per row
  • Total linear feet = total boards × board length

Both row counts are rounded up because you cannot buy a fraction of a board, and partial pieces at the end of a run are normal off-cuts.

Worked example

A 20 ft × 12 ft deck, 5.5 in boards, 12 ft board length, 0.125 in gap:

  • Deck area = 20 × 12 = 240 ft²
  • Effective width = (5.5 + 0.125) ÷ 12 ≈ 0.469 ft
  • Rows = 12 ÷ 0.469 ≈ 25.6 → 26 rows
  • Boards per row = 20 ÷ 12 ≈ 1.67 → 2 per row
  • Total boards = 26 × 2 = 52 boards
  • Linear feet = 52 × 12 = 624 ft

Common deck board widths

Nominal size Actual face width (in) Effective width with 1/8 in gap (in)
5/4 × 4 3.5 3.625
5/4 × 6 5.5 5.625
2 × 4 3.5 3.625
2 × 6 5.5 5.625

Pitfalls to avoid

  • Nominal vs actual width. A “6 inch” board is really 5.5 in. Use the actual face width or you will under-count rows.
  • Forgetting the gap. A 1/8 in (0.125 in) gap per board adds up across a wide deck and changes the row count.
  • No waste allowance. Diagonal layouts, picture-frame borders and off-cuts raise usage; add roughly 10–15% (more for 45° patterns).
  • Joist direction. Boards must run perpendicular to the joists. If you rotate the layout, swap the length and width in the calculator.

Frequently Asked Questions

Divide the deck width by the effective board width (actual width plus the gap) to get the number of rows, divide the deck length by the board length to get boards per row, and multiply the two, rounding each up. For a 20 ft × 12 ft deck with 5.5 in boards and a 1/8 in gap that is about 52 boards.

Use the actual face width. A nominal 6 in deck board measures 5.5 in, and a nominal 4 in board measures 3.5 in. Using the nominal size makes the deck appear to need fewer rows than it really does.

Add about 10% for a straight layout and 15% or more for diagonal or picture-frame patterns, which produce longer off-cuts. The calculator rounds each dimension up, but a waste allowance still protects you against defects and mistakes.

The numbers you enter are sent to our server so the calculation can run, and in the step-by-step mode they are also carried in the page link. They are not stored after the calculation and are not shared or sold.

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