Deck Calculator

Calculate deck boards, linear footage, and approximate joists for a rectangular deck. Enter deck dimensions, board size, and gap to get a complete material estimate with waste.

ft

Length of the deck (direction boards will run).

ft

Width of the deck (across the boards).

in

Actual board width. 2×6 decking = 5.5". 2×4 = 3.5". 5/4×6 = 5.5".

ft

Length of boards you'll purchase. Common: 8, 10, 12, 16, 20 ft.

in

Spacing between boards for drainage. 1/8" (0.125) is standard for kiln-dried; 1/4" for pressure-treated.

%

Extra for cuts, defects, mistakes. Default 10%.

How this calculator works

This deck calculator estimates materials for a rectangular deck in four steps:

  1. Deck area: length × width.
  2. Rows of boards: deck width ÷ effective board width (board width + gap, converted to feet).
  3. Boards per row: if your board length ≥ deck length, 1 board per row. Otherwise, multiple boards per row (ceil of deck length ÷ board length).
  4. Total boards: rows × boards per row, plus waste allowance (default 10%).

The joist estimate assumes standard 16" on-center spacing perpendicular to the boards. This is a material estimate only, NOT structural engineering advice. Joist size, beam placement, footing design, and load calculations must be performed by a licensed structural engineer or per local building code.

Formula

deck_area              = deck_length × deck_width
effective_board_width  = (board_width + gap) / 12  (ft)
rows                   = ceil(deck_width / effective_board_width)
boards_per_row         = (board_length >= deck_length) ? 1
                         : ceil(deck_length / board_length)
boards_needed          = rows × boards_per_row
linear_footage         = boards_needed × board_length
boards_with_waste      = ceil(boards_needed × (1 + waste_pct / 100))

joist_count            = ceil(deck_length / (16/12)) + 1  (16" O.C.)
joist_linear_ft        = joist_count × deck_width

Example

For a 12 × 10 ft deck with 5.5" boards at 12 ft length, 1/8" gap, 10% waste:

  • deck_area = 120 sq ft
  • effective_board_width = (5.5 + 0.125) / 12 = 0.469 ft
  • rows = ceil(10 / 0.469) = 22 rows
  • boards_per_row = 1 (12 ft board ≥ 12 ft deck)
  • boards_needed = 22 × 1 = 22 boards
  • linear_footage = 22 × 12 = 264 linear ft
  • boards_with_waste = ceil(22 × 1.10) = 25 boards
  • joists (16" O.C.) = ceil(12 / 1.333) + 1 = 10 + 1 = 11 joists

Important assumptions

  • Default board width 5.5" matches 2×6 nominal decking (actual 1.5" × 5.5"). Adjust for 2×4 (3.5") or 5/4×6 (5.5").
  • Default gap 1/8" (0.125") for kiln-dried lumber. Use 1/4" (0.25) for pressure-treated (it shrinks).
  • Joist spacing: 16" on center (standard for residential decks). 12" O.C. for commercial or high-load decks.
  • Boards run along the deck LENGTH direction. Swap length/width if boards run across.
  • ⚠️ STRUCTURAL SAFETY: Joist count is an estimate. Joist SIZE (2×8, 2×10, 2×12), beam span, footing depth, and live load (40 psf residential, 100 psf commercial) MUST be verified with local building code or a licensed structural engineer.
  • For composite decking (Trex, TimberTech), follow manufacturer's gap and spacing recommendations — composites expand/contract differently than wood.

FAQ

How many deck boards do I need for a 12×12 deck?
A 12×12 deck (144 sq ft) using 5.5" boards at 12 ft length needs about 27 boards (26 + 1 for waste). Order 28-30 to be safe — breakage during cuts is common.
What's the difference between 2×6 and 5/4×6 decking?
2×6 decking is 1.5" thick (structural, can span 16-24" O.C.). 5/4×6 is 1.0" thick (typical for residential decks, spans 16" O.C. only). Both are 5.5" wide. Use 2×6 for high-traffic or commercial decks.
Should deck boards have a gap?
Yes — 1/8" gap for kiln-dried cedar/redwood, 1/4" for pressure-treated (it shrinks as it dries). Composite decking (Trex, TimberTech) follows manufacturer specs — typically 1/8" to 1/4". The gap allows drainage and prevents cupping.
How far apart should deck joists be?
Standard residential deck joist spacing is 16" on center for 5/4 decking and 24" O.C. for 2×6. Commercial or high-load decks use 12" O.C. Joist SIZE (2×8, 2×10, 2×12) depends on span — consult IRC Table R507.5 or a structural engineer.
Can I use this calculator for composite decking?
Yes — enter the composite board's actual width (typically 5.5" for Trex/TimberTech). Use the manufacturer's recommended gap (usually 1/8" to 1/4"). Composite boards come in 12, 16, and 20 ft lengths.
How much waste should I add?
10% is standard for rectangular decks with simple layouts. For diagonal patterns, add 15%. For multi-level decks with many cuts, add 20%. Always order extra — running short mid-project is costly.