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Deck Board Calculator

This calculator works out how many rows of decking fit across your deck, how many boards each row needs and the total number of boards to buy. It also estimates screws based on your joist spacing.

Enter your measurements

Measurements
ft
ft
in

A nominal 5/4×6 or 2×6 deck board is 5.5 in (140 mm) wide.

in

3/16 in (about 5 mm) is common. Follow the board manufacturer's spacing.

Material & assumptions
in

Used only to estimate screws (2 per board at each joist).

%

Extra for end cuts, staggered joints and defective boards.

Enter your measurements and press Calculate to see quantities, a material breakdown and an optional cost estimate.

How the calculation works

  • Rows = (Deck width + Gap) ÷ (Board width + Gap), rounded up
  • Linear length = Rows × Deck length
  • Boards = Linear length ÷ Board length × (1 + Waste %), rounded up

Each row needs a board width plus one gap; the final row needs no gap, which is why one gap is added to the deck width before dividing.

Assumptions used

  • Boards run parallel to the deck length and are joined end to end over joists.
  • Screw estimate assumes two screws per board at each joist. Hidden fastener systems differ.

Example calculation

A 16 × 12 ft deck with 5.5 in boards, 3/16 in gaps, 16 ft boards and no waste.

  1. 1Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35 → 26 rows
  2. 2Linear length: 26 × 16 ft = 416 ft
  3. 3Boards: 416 ÷ 16 = 26 boards

Result: 26 boards of 16 ft; with 10% waste, buy 29.

How to measure your project

  • Deck length is the direction the boards run; deck width is measured across the boards.
  • Use the actual board width. Most 5/4×6 and 2×6 boards are 5.5 in wide.
  • Choose board lengths that match the deck length or divide it evenly to reduce waste.

Frequently asked questions

How much gap should be between deck boards?

Around 3/16 in (5 mm) is a common gap for wood decking, but wet pressure-treated boards shrink and are often installed tight. Composite boards have specific manufacturer gaps.

Should deck boards run perpendicular to joists?

Yes, decking normally runs perpendicular to the joists. Diagonal decking needs closer joist spacing and produces more waste.

Related guides

  • Illustration of stacked lumber boards and a 1 by 12 by 12 inch board foot
    Lumber

    What Is a Board Foot in Lumber?

    A clear explanation of the board foot, how to calculate board feet for hardwood and lumber, nominal vs actual sizes, and how to estimate project cost.

    2 min read

Results are estimates based on the measurements and assumptions you enter. Always confirm product specifications, local building codes and final quantities with your supplier or a qualified professional. See our disclaimer.