How It Is Calculated
The deck board calculator estimates the number of boards and material cost for a rectangular deck. The calculation proceeds step by step:
- Total deck area: Multiply deck length by deck width.
- Effective board width: Add the gap between boards to the board width. For diagonal orientation, multiply the board width by cos(45°) ≈ 0.7071 before adding the gap.
- Theoretical linear feet: Divide total deck area (in square feet) by the effective width (in inches) and multiply by 12 to convert to linear feet.
- Adjusted linear feet: Increase theoretical linear feet by the waste factor percentage.
- Number of boards: Divide adjusted linear feet by board length and round up to the nearest whole board.
- Total linear feet ordered: Multiply number of boards by board length.
- Material cost: Multiply number of boards by price per board.
A = L × W
E = B + G (straight) or E = B × 0.7071 + G (diagonal)
T = A × 12 / E
F = T × (1 + P / 100)
N = ceil(F / C)
D = N × C
M = N × R
Where: L = deck length (ft), W = deck width (ft), A = total deck area (ft²), B = board width (in), G = gap (in), E = effective board width (in), T = theoretical linear feet (ft), P = waste factor (%), F = adjusted linear feet (ft), C = board length (ft), N = number of boards, D = linear feet ordered (ft), R = price per board ($), M = material cost ($).
Assumptions
- The deck area is perfectly rectangular.
- Boards are laid either perpendicular to joists (straight) or at a 45° angle (diagonal).
- The gap between boards is uniform and measured perpendicular to board edges.
- Board length is the standard length you purchase; partial boards are allowed in the calculation, but the waste factor covers small offcuts and end cuts.
- For diagonal orientation, a 45° angle is assumed.
- Price per board is for boards of the specified length; cost does not include installation, fasteners, or other materials.
- A waste factor of 10% is typical for straight orientation; 15% is recommended for diagonal.
Worked Example
Suppose you have a deck that is 12 ft long and 10 ft wide. You plan to use boards that are 5.5 in wide and 16 ft long, with a 0.125 in gap between boards, laid straight (orientation = 1). You allow 10% waste, and each board costs $7.50.
- Total deck area: 12 ft × 10 ft = 120 ft².
- Effective board width: 5.5 in + 0.125 in = 5.625 in.
- Theoretical linear feet: 120 ft² × 12 / 5.625 in = 256 ft.
- Adjusted linear feet: 256 ft × (1 + 10/100) = 281.6 ft.
- Number of boards: ceil(281.6 ft / 16 ft) = ceil(17.6) = 18 boards.
- Total linear feet ordered: 18 boards × 16 ft = 288 ft.
- Material cost: 18 boards × $7.50 = $135.00.
The results match: Total deck area = 120 ft², Linear feet of decking (with waste) = 288 ft, Number of boards needed = 18 boards, Deck board material cost = $135.
Practical Tips
- Always measure your deck area carefully and account for any irregular shapes by breaking them into rectangles.
- For diagonal layouts, increase your waste factor to at least 15% because of more angled cuts.
- Order a few extra boards beyond the calculated number to have spares for future repairs or mistakes.
- If you are also planning a concrete base or posts, consider using the Concrete calculator or Fence post calculator for those parts of the project.
- Check the actual width of your chosen deck boards—some may be slightly different from the nominal width.