Composite Decking Calculator
Calculate how many 5.5 in x 12 ft composite deck boards you need from your deck's area, with 10% waste and total cost.
How this calculator works
A standard composite decking board (5.5 in wide x 12 ft long) covers 5.5 square feet of deck surface. This calculator divides your deck area by that coverage figure and rounds up, since boards are sold as whole units.
Formula: Boards = Deck area (sq ft) ÷ 5.5 sq ft per board, rounded up.
Worked example
A 14 ft x 12 ft deck (168 sq ft):
- Area: 14 x 12 = 168 sq ft
- Boards: 168 ÷ 5.5 ≈ 30.5455 → rounds up to 31 boards
- At $35/board, that’s 31 × 35 = $1085.00 before waste.
Coverage reference
| Deck area | Boards needed |
|---|---|
| 42 sq ft | 8 boards |
| 168 sq ft | 31 boards |
| 672 sq ft | 123 boards |
How to measure your deck
Measure the full deck surface length and width, excluding any area that will be covered by built-in features like a hot tub pad. For L-shaped or wraparound decks, break the layout into rectangles, calculate boards for each section, and add the totals together before applying your waste allowance.
Frequently asked questions
How is composite decking different from wood decking for this calculation?
This calculator uses the same 5.5 in x 12 ft composite board (5.5 sq ft of coverage), which is a standard size across most composite decking brands. The board math is identical to wood decking — the real difference between the two shows up in maintenance and lifespan, not the square footage per board.
Do composite boards need a gap for expansion?
Yes — composite decking expands and contracts more than wood with temperature swings, so manufacturers specify a gap (often 1/8 to 3/16 in) between boards, and a larger gap at the ends and around fixed objects. Check your specific product's installation guide, since gap requirements vary by brand and board width.
Do I still need to seal or stain composite decking?
No — one of the main selling points of composite decking is that it never needs staining, sealing, or painting, unlike wood decking. It does still benefit from periodic washing to remove surface dirt and mildew, but there's no annual refinishing cycle to budget for.
How much waste should I add for composite decking?
10% covers standard end cuts and picture-frame borders on a simple rectangular deck. Diagonal board patterns or decks with lots of stair and post cutouts should use 15% or more, since composite offcuts can't be seamlessly patched the way wood sometimes can.
Does this include hidden fastener clips or screws?
No — this counts deck boards only. Hidden fastener systems (clips that run in the board's grooved edge) are priced and counted separately, typically at a rate of clips per linear foot of board edge, not per board.
Estimates only. Verify quantities with your supplier before purchasing.