Deck Material Calculator
Estimate decking, joists, screws and railing for a rectangular deck.
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What this tool does
Estimate decking, joists, screws and railing for a rectangular deck. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
How to use the Deck Material Calculator
- Enter or select length in ft.
- Enter or select width in ft.
- Enter or select actual board width in in.
- Enter or select board gap in in.
- Enter or select board length in ft.
- Enter or select waste allowance (%).
- Enter or select joist spacing in in.
- Enter or select railing length in ft.
- Read the calculated result; change any measurement to compare alternatives.
Formula
decking = board rows × boards/row with waste; joists = ceil(length × 12 / spacing) + 1; screws = boards × 2 × (ceil(boardLength × 12 / spacing) + 1)
- length
- Length (ft, before metric conversion)
- width
- Width (ft, before metric conversion)
- boardWidth
- Actual board width (in, before metric conversion)
- gap
- Board gap (in, before metric conversion)
- boardLength
- Board length (ft, before metric conversion)
- waste
- Waste allowance (%)
- joistSpacing
- Joist spacing (in, before metric conversion)
- railLength
- Railing length (ft, before metric conversion)
Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
Worked example
For deck material calculator, the following measurements illustrate the exact method: Length: 20; Width: 10; Actual board width: 5.5; Board gap: 0.125; Board length: 12; Waste allowance (%): 10; Joist spacing: 16; Railing length: 40.
Inputs
- Length20 ft
- Width10 ft
- Actual board width5.5 in
- Board gap0.125 in
- Board length12 ft
- Waste allowance (%)10
- Joist spacing16 in
- Railing length40 ft
Result
- Boards to buy49
- Deck area200 sq ft
- Board rows22
- Linear feet purchased588 ft
- Joists16
- Estimated decking screws980
- Railing length40 ft
Results explained
- Boards to buy
- Boards to buy from the formula above. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Deck area
- Deck area from the formula above, expressed in sqft before metric conversion. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Board rows
- Board rows from the formula above. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Linear feet purchased
- Linear feet purchased from the formula above, expressed in ft before metric conversion. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Joists
- Joists from the formula above. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Estimated decking screws
- Estimated decking screws from the formula above. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
- Railing length
- Railing length from the formula above, expressed in ft before metric conversion. Fasteners assume two screws per board at each support. Beams, foundations and structural connections need separate design.
Frequently asked questions
Deck board count uses coverage width; joist count uses center spacing and includes both ends.
Use actual board sizes, stock length, joist spacing and railing length.
Use 5–10% for simple layouts and 10–15% for diagonal cuts or complicated outlines; the allowance is editable.
Split the outline into rectangles or triangles and calculate each section separately. Do not count overlapping sections twice.
This is a purchasing estimate, not a structural design. Confirm measurements and local requirements before buying or building.