Deck calculator: deck boards, joists and screws

How many deck boards do I need?

This decking calculator counts boards the way a deck builder does: row by row, reusing offcuts that are long enough. It also gives joists, supports, screws or clips, waste and a rough cost, with a top-view drawing. Imperial units are the default.

Measure the deck and pick a board direction

1. Under 1. The deck, enter the Length and Width. For an L-shaped deck, tick L-shaped deck: add a second rectangle and split the L into two rectangles.
2. Choose the Board direction: Parallel to the length or Parallel to the width. Running the long way means fewer butt joints.
3. Under 2. Deck boards, enter the board width, length and thickness, and the Gap between boards. You can type fractions such as 3/16 or 5 1/2.

Counting boards: a worked example

A 16 × 12 ft deck, 5.5 in × 12 ft boards, a 3/16 in gap, boards running along the 16 ft side.

• Rows: 144 in ÷ (5.5 + 0.1875 in) = 25.32, rounded up to 26 rows. The last row is ripped to about 1 13/16 in.
• Each 16 ft row takes one full 12 ft board plus a 4 ft piece. Every other row starts with the piece, so the joints are staggered.
• The first board you cut gives a 4 ft piece and an 8 ft offcut, which gives two more pieces. Each new board therefore yields three 4 ft pieces: 26 pieces need 9 boards.
• Total: 26 + 9 = 35 boards, 420 ft bought for 416 ft laid. One 4 ft offcut is left, so waste is 4 ÷ 420, about 1%.

Joists and joist spacing

Joists run across the boards. Leave Joist spacing, on center (blank: rule of thumb) empty to use a common figure: 16 in on center for 1 in thick wood boards, 12 in for thinner boards, often 12 to 16 in for composite. Follow the span table of your boards. In the example, 192 in ÷ 16 in = 12 spaces, so 13 joists of 12 ft. Each board end needs its own bearing, so the tool adds a doubled joist at every butt-joint position: here at 4 ft and 12 ft, for 15 joists and 180 linear feet.

Supports

For a ground-level deck on blocks or pedestals, enter the Support spacing along each joist. At 48 in: 144 ÷ 48 = 3 spaces, 4 supports per joist, 52 for the 13 joists. A raised deck on beams and posts needs a proper design.

How many deck screws?

The usual rule is 2 screws per board at every joist it crosses. Each row crosses 13 joists plus the doubled joist at its joint: 14 crossings × 26 rows = 364, so 728 screws. That is 8 Boxes of 100 or 4 Boxes of 200. For composite, Hidden clips (1 per board per joist) replaces screws: 364 clips, plus the starter and finishing fasteners the maker calls for.

Wood or composite

Pressure-treated pine, cedar and tropical hardwoods are screwed down and allow wider joist spacing for the same thickness. Composite expands more with heat, so the gap, the spacing and the clips come from each maker's instructions.

Drainage, airflow and permits

Give the deck a slight fall away from the house, often around 1/8 in per foot, and keep air moving underneath. In much of the United States a permit is needed for decks attached to the house or more than about 30 in above grade, but rules vary by city and county: check with your local building department before you start.

Frequently asked questions

Should I buy extra boards?

Cuts and offcuts are already counted. Many builders add one or two spare boards.

Why not just divide the area by the board area?

Counting each board with its gap, 192 sq ft ÷ (5.6875 in × 12 ft) = 33.8, so 34 boards: one fewer than the row count, because a whole row is needed for the ripped last row and a 4 ft offcut is left over. Turn the boards to run along the 12 ft side and each of the 34 rows takes exactly one board, with no offcut.

Does the gap matter?

Yes. With 5.5 in boards and a 3/16 in gap, each row takes 5.6875 in and the deck needs 26 rows. Without a gap, 144 ÷ 5.5 = 26.2 rounds up to 27 rows: the gap saves a whole row here.

Can I work in meters?

Yes. Metric (m, cm, mm) converts every value: deck in meters, board width in millimeters, spacing in centimeters.