Enter Your Deck Details
How the Deck Calculator Works
This calculator estimates materials based on your deck's square footage and configuration. It calculates the number of decking boards by dividing the deck area by the coverage of each board (approximately 5.5 inches wide), then adds 10% for waste from cuts and end trimming.
Joist and Beam Sizing
Joists are spaced at either 16 or 12 inches on center, running the width of the deck. The calculator determines the number of joists based on the deck length divided by your chosen spacing. Beams run perpendicular to the joists and are typically doubled 2x10s, spaced about 8 feet apart under the joists. Support posts sit under the beams on concrete footings.
If you need to validate framing depth before ordering lumber, use our deck joist span calculator to compare joist size, spacing, and species before you lock in the beam layout.
Once the beam layout is set, use our deck footing calculator to estimate footing count, diameter, and concrete volume for the support posts.
When you need to check how far the support beam can run between posts, use our deck beam span calculator to compare beam assemblies against the joist span you plan to frame.
Railing and Stair Estimates
Railing posts are placed every 6 feet with balusters spaced no more than 4 inches apart (code requirement). Stair stringers are calculated at 3 per set of stairs for widths up to 4 feet. Each step needs two tread boards.
Tips for Your Deck Project
- Always check local building codes — most decks over 30 inches high require a permit
- Pressure-treated lumber is the most affordable but needs annual sealing
- Composite decking costs more upfront but requires almost zero maintenance
- Cedar is naturally rot-resistant and splits less than pressure-treated wood
- For decks attached to a house, use a properly flashed ledger board
Use this estimate as a starting point and adjust for your specific design. Complex layouts with angles, multi-level sections, or built-in benches will need additional materials.
Frequently Asked Questions
How much does it cost to build a deck?
A pressure-treated wood deck costs $15-$25 per square foot for materials. Cedar runs $20-$30 per sq ft, and composite decking costs $25-$45 per sq ft. A typical 12x16 deck (192 sq ft) costs $2,900-$8,600 in materials. Labor adds $15-$35 per sq ft if hiring a contractor.
What size deck can I build without a permit?
Rules vary by jurisdiction, but many areas allow decks under 200 sq ft and 30 inches above grade without a permit. Attached decks (connected to the house) almost always require a permit regardless of size. Check with your local building department before starting.
How far apart should deck joists be?
16 inches on center is standard for most residential decks using 5/4 decking boards. Use 12-inch spacing for composite decking, diagonal board patterns, or when expecting heavy loads like hot tubs. Closer joist spacing reduces bounce and increases the deck's load capacity.
How many decking boards do I need?
Divide your deck's square footage by the width of each board (5.5 inches for standard 5/4x6 decking) to get the number of board-feet of decking. Then add 10-15% for waste from cuts and end trimming. For a 192 sq ft deck, expect to need about 40-45 boards at 16-foot lengths.
Should I use pressure-treated wood or composite for my deck?
Pressure-treated wood is cheaper upfront ($2-$4 per board foot) but needs annual staining and sealing. Composite decking costs 2-3x more but requires almost zero maintenance and lasts 25-50 years. If you want the lowest total cost over 20 years, composite often wins despite the higher initial price.