How to Use This Calculator
Enter your deck's length, width, board width, joist spacing, and local material cost. The calculator estimates board count, joists, posts, and total material cost — useful for planning lumber purchases and budgeting your project.
Formula & Method
Board count = ⌈(deck width ÷ board width in ft) × 1.1⌉ to include 10% waste. Joists = ⌈deck length ÷ spacing in ft⌉ + 1. Posts ≈ ⌈length ÷ 6⌉ × 2 + 2. Material cost = deck area × price per sq ft.
Calculation Example
12 × 16 ft deck, 5.5" boards, 16" joist spacing, $8/sq ft: ~34 boards, 13 joists, 8 posts. Estimated material cost $1,536. Add 10–15% contingency for cuts and defects.
Expert Tips
Buy at least 10% extra boards for end cuts and defects. Composite decking costs 2–3× more than pressure-treated lumber but eliminates staining and annual maintenance. Most jurisdictions require permits for decks over 30 inches above grade — check with your local building department.
Frequently Asked Questions
What size joists should I use?
For residential decks, 2×8 joists span up to 10 ft at 16" spacing. 2×10 joists span up to 14 ft. Always check local building codes.
Do I need a permit?
Most areas require a permit for decks over 30 inches above grade or larger than 200 sq ft. Check with your local building department.