How It Is Calculated
This calculator estimates the number of deck boards, total linear feet, number of joists, and material cost for a rectangular deck. The calculations are based on the deck dimensions, board width, gap between boards, joist spacing, waste factor, and price per linear foot.
First, convert deck length and width from feet to inches: L = deck length × 12, W = deck width × 12.
Total deck area: A = deck length × deck width (ft²).
Raw number of board rows: R = W / (B + G), where B is board width (in) and G is board gap (in).
Adjusted rows with waste: R_adj = R × (1 + waste factor / 100). Round up to the nearest whole number to get number of boards N.
Total linear feet: T = N × deck length.
Number of joists: J = floor(L / S) + 1, where S is joist spacing (in).
Deck board cost: C = T × price per linear foot.
Assumptions
- Deck boards are laid parallel to the deck length (perpendicular to joists).
- Board length is sufficient to cover the deck length without joints; boards are cut to deck length.
- Joist spacing is measured on center.
- Waste factor accounts for cuts, mistakes, and offcuts.
- Board width and gap are in inches.
- Price per linear foot is for the deck board material only; does not include joists, fasteners, or labor.
- Standard board widths are nominal; actual width may vary slightly.
Worked Example
For a deck with length = 20 ft, width = 10 ft, board width = 5.5 in, board gap = 0.125 in, joist spacing = 16 in, waste factor = 10%, and price per linear foot = $2.50/ft:
- Total deck area = 20 × 10 = 200 ft².
- Raw rows = (10 × 12) / (5.5 + 0.125) = 120 / 5.625 ≈ 21.33 rows.
- Adjusted rows = 21.33 × 1.10 ≈ 23.47, rounded up to 24 boards.
- Total linear feet = 24 × 20 = 480 ft.
- Number of joists = floor((20 × 12) / 16) + 1 = floor(240/16) + 1 = 15 + 1 = 16 joists.
- Deck board cost = 480 × 2.50 = $1,200.
Practical Tips
- Always add a waste factor of at least 10% to account for cuts and mistakes.
- Consider the direction of deck boards: laying them parallel to the house length often reduces waste.
- If you need to estimate materials for the substructure, use the Roofing calculator for joist spacing and fasteners.
- For ground cover or garden beds near the deck, the Mulch calculator can help estimate mulch needed.
- Check local building codes for required joist spacing and board gaps.