Deck calculator
How much material do I need for a deck?
Estimate joists and decking boards for your deck based on dimensions, joist spacing, and board width.
Deck calculator
Deck calculator result
Deck building considerations
- Footings: Concrete piers below frost line, typically 4x8 feet apart
- Beams: Support the joists, sized based on span and load
- Joists: 2x6 at 16-inch spacing for most residential decks
- Decking: 5/4x6 or 2x6 boards, leave 1/8 inch gap for drainage
- Railings: Required for decks over 30 inches high
- Hardware: Use galvanized or stainless steel fasteners for outdoor use
The calculator estimates primary framing materials. Add 10-15% for waste, cuts, and additional blocking.
Worked example
For example, for a 12×10 ft deck, joists at 16-in spacing with 5.5-in wide deck boards:
- Deck area = 12 × 10 = 120 sq ft
- Joists: (10 × 12 ÷ 16) + 1 = 8.5 → 9 joists at 12 ft = 108 ft
- Deck boards: (12 × 12 ÷ 5.5) = 26.2 → 27 boards at 10 ft = 270 ft
- Add 10% for waste: 300 ft of decking, 120 ft of joists
- Also plan for hardware: joist hangers, screws, and flashing
Common mistakes
| Mistake | Why it matters |
|---|---|
| Not using proper flashing | Tape the top of each joist before decking to prevent rot and extend deck life |
| Spacing boards incorrectly | Use a 16d nail as a spacer between deck boards for proper drainage |
| Setting posts in concrete without drainage | Concrete above grade traps water against posts — slope the concrete away |