Deck Calculator
Calculate how many decking boards, joists, posts, stairs, and railing you need for your deck project. Enter your deck dimensions to get a complete material list.
How to Calculate Deck Materials
Building a deck requires careful material estimation. Here is how each component is calculated:
- Deck boards: Calculate the number of board rows by dividing deck width (in inches) by the board width plus a gap (typically 1/4 inch). Each row needs boards to span the deck length.
- Joists: Divide deck length (in inches) by joist spacing and add 1 for the end joist. Joists run perpendicular to deck boards.
- Support posts: Place 4x4 posts every 6 feet around the perimeter where beams intersect.
- Stairs: Divide total rise by 7.5 inches (standard riser height) to get the number of steps.
- Railing: Measure the perimeter minus the entry side (house side) for total railing length.
Joist Spacing Guide
- 12" on center: Best for diagonal decking layouts and heavy-use commercial decks
- 16" on center: Standard spacing for most residential decks, required by many composite decking brands
- 24" on center: Acceptable for some pressure-treated lumber applications in low-traffic areas
Deck Board Types
- Composite decking: Made from wood fibers and plastic. Low maintenance, no staining or sealing needed. Lasts 25-30 years. Higher upfront cost but lower lifetime cost.
- Pressure-treated lumber: Chemically treated wood that resists rot and insects. Most affordable option. Requires annual staining/sealing. Lasts 15-20 years with maintenance.
Footing Requirements
Deck footings must extend below the frost line in your area (typically 36-48 inches deep). Each footing should be a minimum of 12 inches in diameter. Concrete tube forms (sonotubes) are the most common method for residential deck footings.
Frequently Asked Questions
What joist spacing should I use for composite decking?
For composite decking, use 16-inch on-center joist spacing for standard installations. Some manufacturers allow 12-inch spacing for diagonal layouts. Always check the specific manufacturer's requirements, as some composite boards may allow 24-inch spacing for residential applications.
How many footings do I need for a deck?
The number of deck footings depends on the deck size and local building codes. Generally, place footings every 6-8 feet around the perimeter and at beam intersections. A typical 12x16 deck needs 6-9 footings. Always check local codes as requirements vary by region.
What gap should I leave between deck boards?
Leave a 1/8-inch to 1/4-inch gap between deck boards for drainage and expansion. Pressure-treated lumber installed wet should be butted tightly as it will shrink. Composite decking typically requires 3/16-inch to 1/4-inch gaps between boards. Always follow manufacturer recommendations.
Do I need a ledger board for my deck?
If your deck is attached to the house, a ledger board is required. It is bolted to the house framing and supports one side of the deck. Freestanding decks do not need a ledger board but require additional footings and a beam on the house side. A proper ledger board must include flashing to prevent water damage.
How much does it cost to build a deck?
Deck costs vary widely by material. Pressure-treated wood decks cost $15-25 per square foot for materials. Composite decks cost $20-45 per square foot for materials. Labor adds $15-35 per square foot. A typical 12x16 (192 sq ft) deck costs $3,000-$8,000 for materials alone.