Softonic

Deck Calculator

Step 1 / 3 Boards needed
Next

Enter your deck length and width, the size of the decking boards you plan to use, the gap between boards and a waste allowance. The calculator returns the number of boards to buy, the deck area in square feet and the total linear feet of decking to order. It accounts for the gap between boards, which quietly adds up across dozens of rows, so you order the right amount the first time.

How to estimate decking boards

  1. 1

    Enter the deck size

    Deck length and width in feet. For an L-shape or wrap-around, split it into rectangles and add the results.

  2. 2

    Enter the board details

    Board face width in inches (5.5 in is typical), board length in feet, the gap between boards and a waste percentage for cuts and defects.

  3. 3

    Read the material list

    Boards needed, deck area in square feet and total linear feet. Round up and order from one batch so the colour matches.

How the deck board count works

The calculator lays boards across the width of the deck and runs them along the length. Each board occupies its own face width plus the gap on one side:

  • Effective board width (ft) = (board width in inches + gap in inches) ÷ 12
  • Rows = deck width ÷ effective board width, rounded up
  • Boards per row = deck length ÷ board length, rounded up
  • Boards needed = rows × boards per row × (1 + waste ÷ 100), rounded up
  • Deck area (ft²) = deck length × deck width
  • Linear feet = area ÷ (board width in feet) × (1 + waste ÷ 100), rounded up

Worked example: a 16 × 12 ft deck

Using 5.5 in boards, a 1/8 in (0.125 in) gap, 16 ft board lengths and 10% waste:

  • Effective board width = (5.5 + 0.125) ÷ 12 = 0.469 ft
  • Rows = 12 ÷ 0.469 = 25.6 → 26 rows
  • Boards per row = 16 ÷ 16 = 1
  • Boards needed = 26 × 1 × 1.10 = 28.6 → 29 boards
  • Deck area = 16 × 12 = 192 ft²

Typical board sizes

Board face width Common lengths Best for
3.5 in (5/4×4) 8, 12, 16 ft Narrow boards, tight gaps
5.5 in (5/4×6) 8, 12, 16, 20 ft Standard residential decks
7.25 in (5/4×8) 12, 16 ft Wide-plank, fewer seams

Pitfalls to avoid

  • Skipping the gap. A 1/8 in gap over 26 rows adds more than 3 in of deck width, ignore it and you come up a board short.
  • Forgetting the waste factor. Cuts, mitres around posts and the odd cupped board mean 10% is a sensible minimum; bump it to 15% for diagonal or picture-frame layouts.
  • Mixing batches. Order all decking at once. Different production runs of composite or stained wood can show a visible shade line.
  • Ignoring joist spacing. This tool sizes the deck surface; check your joist span and fastener count separately.

Frequently Asked Questions

With standard 5.5 in boards, a 1/8 in gap, 16 ft lengths and 10% waste, you need about 29 boards covering 192 ft². Wider boards reduce the count; a larger gap or more waste increases it.

For pressure-treated lumber installed wet, 1/8 in (0.125 in) is common because the boards shrink as they dry. For dry or composite boards, follow the manufacturer’s spec, often 1/8 to 3/16 in for drainage and expansion.

Use 10% for a straight, rectangular deck. Add 15% or more for diagonal boards, a picture-frame border or lots of cut-outs around posts and stairs, since each angled cut produces an offcut you cannot reuse.

No. The numbers you enter are sent to our server only to run the calculation. They are not stored or shared with anyone. In the step-by-step view, the values also travel in the page link between steps, so avoid sharing links that contain numbers you want to keep private.

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