Free online deck calculator

How to use

1. Enter the deck length and width in feet.
2. Set the board face width — 5.5 in for standard 2×6 decking, 3.5 in for 2×4.
3. Choose the board length you plan to purchase (16 ft is the most common).
4. Select your joist spacing (16 in standard, 12 in for composite).
5. Set a waste factor — 10% is typical for a rectangular deck.
6. Read the board count, joist count, and fastener estimate.

Formulas

rows across width   = ceil(deckWidth × 12 / boardWidth)
decking linear ft   = rows × deckLength × (1 + waste% / 100)
decking boards      = ceil(decking linear ft / boardLength)

joists needed       = ceil(deckLength × 12 / joistSpacing) + 1
joist linear ft     = joists × deckWidth

rim joist linear ft = 2 × deckLength + 2 × deckWidth

screws (lbs)        = deckArea / 100 × 350 / 100

Worked example

16 ft × 12 ft deck, 5.5-in boards, 16-ft boards, 16-in joists, 10% waste:

rows across width   = ceil(12 × 12 / 5.5) = ceil(26.18) = 27
raw linear ft       = 27 × 16 = 432 ft
decking linear ft   = 432 × 1.10 = 475.2 ft
decking boards      = ceil(475.2 / 16) = 30 boards

joists needed       = ceil(16 × 12 / 16) + 1 = 12 + 1 = 13
joist linear ft     = 13 × 12 = 156 lin ft

rim joist linear ft = 2 × 16 + 2 × 12 = 56 lin ft

deck area           = 192 sq ft
screws              = 192 / 100 × 3.5 = 6.72 lbs ≈ 7 lbs

Notes

• The calculator does not size posts, beams, or footings — those depend on local code, load, and soil. Consult a structural engineer or your building department before framing.
• Add 5–10% to joist count for blocking between joists (required at mid-span for spans over 8 ft).
• Composite decking brands vary in width and gap requirements — check your manufacturer’s installation guide before ordering.