Composite Decking Calculator

How many composite deck boards do I need?

This composite decking calculator multiplies deck length × width to get square feet, divides by 5.5 sq ft per board, and rounds up to whole boards. A 5.5 in wide board is 5.5 ÷ 12 = 0.458 ft wide, and 0.458 × 12 ft long = 5.5 sq ft. Tick the waste box to add 10% before rounding. The cost line multiplies boards by your price per board.

The count is for surface boards only. Fascia, stair risers, clips, and screws are separate line items.

The formula, step by step

1. Deck area = length × width in feet.
2. Boards = area ÷ 5.5.
3. With waste = boards × 1.10.
4. Round up to a whole board.
5. Cost = boards × price per board.

The 5.5 figure ignores the small gap between boards. That gap adds a little coverage, roughly 3% at a 3/16 in gap, which helps your margin.

Worked examples

1. The default: 14 ft × 12 ft deck, $35 per board

• Area: 14 × 12 = 168 sq ft
• Boards: 168 ÷ 5.5 = 30.5, rounded up to 31 boards, $1,085
• With 10% waste: 30.5 × 1.10 = 33.6, rounded up to 34 boards, $1,190

2. A small landing: 12 ft × 12 ft

• Area: 144 sq ft
• Boards: 144 ÷ 5.5 = 26.2, rounded up to 27 boards, $945 at $35
• With 10% waste: 28.8, rounded up to 29 boards, $1,015

On a 12 ft wide deck with boards running the 12 ft direction, each row uses exactly one board, so waste is low. On a 14 ft run, every row needs a 12 ft board plus a 2 ft piece, which is why the waste box matters.

Boards per deck size (12 ft boards)

Deck size	Area	Boards, no waste	Boards, +10%
12 × 8 ft	96 sq ft	18	20
12 × 12 ft	144 sq ft	27	29
14 × 12 ft	168 sq ft	31	34
16 × 12 ft	192 sq ft	35	39
24 × 12 ft	288 sq ft	53	58

Composite vs wood: what changes in the plan

Factor	Composite	Pressure-treated wood
Board coverage	Same 5.5 in face	Same 5.5 in face (5/4 × 6)
Joist spacing	Often 16 in o.c., 12 in for diagonals	Often 16 in o.c.
Gapping	Side and end gaps set by temperature	Wet boards shrink, so often butted tight
Fasteners	Hidden clips or colored screws	Deck screws or nails
Finish	None needed; wash periodically	Stain or seal every few years

Composite is heavier than most wood decking and sags more between joists, which is why span ratings matter. Check your existing framing with the joist spacing calculator before re-decking.

How to measure

• Measure the deck surface, outside edge to outside edge, along the direction the boards run and across it.
• Split odd shapes. Break an L-shaped or wraparound deck into rectangles, add their areas, and enter the total as length with a width of 1.
• Add the border separately. A picture-frame border is a run of boards around the edge. Add its length in feet, divide by 12, and round up.
• Count stair treads. Each tread typically uses two board widths. Multiply stair width × number of treads × 2, then divide by 12.

Common mistakes

• Laying composite on joists spaced for wood at an angle. Diagonal boards span farther between joists. Many products need 12 in on center for diagonals.
• Ignoring end gaps. Boards laid tight on a cold morning can buckle in summer heat.
• Mixing color lots. Buy all boards at once. Shades can vary between production runs.
• Forgetting fascia. The face boards that wrap the rim joist are sold separately in most lines.

For the frame underneath, use the deck joist calculator. For a full installed budget, see the deck cost calculator.