Deck Calculator
Calculate decking boards and joists needed for a deck.
Assumptions used in this calculation
- Board width excludes gap spacing: Uses the board's face width alone, it doesn't add the small gap (typically 1/8 to 1/4 inch) normally left between boards for drainage and expansion, which slightly undercounts real linear footage needed.
- Uniform joist spacing: Assumes joists run at constant on-center spacing across the full deck width, it doesn't add the extra doubled joists typically required at board end-joint seams.
About this calculator
Building a deck involves two separate material takeoffs that are easy to get wrong independently, the linear footage of decking boards needed to cover the surface, and the number of structural joists beneath it, and neither follows directly from the other. This calculator divides total deck area by board width to find base linear footage of decking, adds a waste percentage for trimming and staggered joints, and separately divides deck length by joist spacing (on-center) and rounds up, adding one for the starting joist, to find the joist count. Enter deck length, width, board width and joist spacing, and get both figures at once instead of estimating decking coverage and joist count as two disconnected calculations. It covers decking boards and joists only, footings, support posts, beams and railing are separate structural components that still need to be planned and priced on top of this output.
Worked example
16ft × 12ft deck, 5.5in boards, 16in joist spacing, 10% waste
Result: ≈ 461 linear ft of decking, 13 joists
How to use this
- 1Enter your deck's length and width.
- 2Enter the decking board width you plan to use and the joist spacing, on-center.
- 3Enter a waste allowance percentage for cuts and mistakes, then read the total linear feet of decking and joist count needed.
Troubleshooting
How much waste allowance should I use?
10% is a common starting point for a straightforward rectangular deck; increase it for a deck with an irregular shape, angled cuts, or a pattern layout that produces more offcuts.
Does this include the substructure (posts, beams, footings)?
No, this calculates decking boards and joists only, substructure sizing depends on deck height, span and local building code, and typically needs a separate structural calculation or a contractor's input.
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