Deck Board Calculator

Calculate exactly how many deck boards you need. Enter deck dimensions, board size, gap, and orientation to get board count, linear footage, and waste — without the joist framing estimate.

ft

Length of the deck.

ft

Width of the deck.

in

Actual board width. 2×6 = 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

1/8" (0.125) for kiln-dried, 1/4" (0.25) for pressure-treated.

Direction boards run. Typically along the longer dimension for fewer joints.

%

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

How this calculator works

This deck board calculator focuses on board-level estimation:

  1. Effective board width: (board width + gap) / 12, converting inches to feet.
  2. Rows: ceil(cross dimension / effective board width). The cross dimension is the direction boards span across.
  3. Boards per row: if your board length ≥ the run dimension, 1 board per row. Otherwise, ceil(run dimension / board length).
  4. Total boards: rows × boards per row, plus waste allowance (default 10%).

You can select board orientation: along deck length (default) or along deck width. This calculator does NOT include the joist estimate — use the Deck Calculator for that.

Formula

effective_board_width = (board_width + gap) / 12  (ft)
rows = ceil(cross_dimension / effective_board_width)
boards_per_row = (board_length >= run_dimension) ? 1 : ceil(run_dimension / board_length)
boards_needed = rows × boards_per_row
linear_ft = boards_needed × board_length
boards_with_waste = ceil(boards_needed × (1 + waste_pct / 100))

Example

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

  • 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_ft = 22 × 12 = 264 linear ft
  • boards_with_waste = ceil(22 × 1.10) = 25 boards

Important assumptions

  • Default board width 5.5" matches 2×6 nominal decking (actual 1.5" × 5.5").
  • Default gap 1/8" (0.125") for kiln-dried lumber. Use 1/4" (0.25) for pressure-treated.
  • Boards typically run along the LONGER deck dimension to minimize joints.
  • This calculator does NOT include joist estimates — use the Deck Calculator for framing.
  • For composite decking, follow manufacturer gap and spacing specs.

FAQ

How is this different from the Deck Calculator?
The Deck Board Calculator focuses on board count and linear footage only. The Deck Calculator includes an approximate joist estimate and broader material overview. Use this for board-specific planning; use the Deck Calculator for full project estimation.
How many deck boards do I need for a 12×12 deck?
A 12×12 deck using 5.5" boards at 12 ft length needs 22 boards (exact) or 25 with 10% waste. Order 25-26 to be safe.
Can I change which direction the boards run?
Yes — use the Board Orientation selector. Boards can run along deck length (default) or along deck width. Typically, boards run along the longer dimension to minimize joints.
What gap should I leave between deck boards?
1/8" (0.125") for kiln-dried cedar/redwood. 1/4" (0.25") for pressure-treated (it shrinks). For composite decking, follow manufacturer specs — typically 1/8" to 1/4".