Deck Calculator
Calculate how many decking boards, joists, posts, screws, and railing you need for your deck project. Enter your deck dimensions and board size to get a complete material list with cost estimate.
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 or 6x6 posts every 6 feet around the perimeter where beams intersect.
- Screws: Each deck board gets 2 screws at every joist crossing. Multiply total boards by the number of joist crossings per board, then by 2.
- Stairs: Divide total rise by 7.5 inches (standard riser height) to get the number of steps.
- Railing: Measure the perimeter minus the house side for total railing length. Posts every 6 feet, balusters every 4 inches.
Joist Spacing Guide
- 12" on center: Best for diagonal decking layouts and heavy-use commercial decks. Required by some composite brands for angled installations.
- 16" on center: Standard spacing for most residential decks. Required by many composite decking brands. Best balance of strength and cost.
- 24" on center: Acceptable for some pressure-treated lumber applications in low-traffic areas. Not recommended for composite decking.
Deck Board Types Compared
- 2x6 Pressure-Treated: $1.50-$3 per linear ft. Strong and affordable. Must be stained/sealed annually. Lasts 15-20 years with maintenance.
- 5/4x6 Pressure-Treated: $1-$2 per linear ft. Thinner profile, lighter weight. Standard for deck surfaces. Needs annual treatment.
- 5/4x6 Composite: $3-$8 per linear ft. Low maintenance, no staining needed. Lasts 25-30 years. Higher upfront cost but lower lifetime cost.
- 2x6 Cedar: $3-$6 per linear ft. Naturally rot-resistant. Beautiful grain. Needs periodic oiling. Lasts 15-25 years.
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. Always check local building codes for specific requirements.
Deck Screws and Fasteners
- Deck screws: Use #8 or #10 x 2.5-3 inch coated deck screws for face-fastening. Stainless steel for coastal areas.
- Hidden fasteners: Composite decking often uses proprietary hidden clip systems ($1-$2 per sq ft extra).
- Joist hangers: One per joist connection point. Use galvanized or stainless steel Simpson Strong-Tie hangers.
- Lag bolts: 1/2" x 4" lag bolts for ledger board attachment, spaced 16" apart in a zigzag pattern.
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 screws do I need for a deck?
Plan for approximately 350 deck screws per 100 square feet of decking when using 5.5-inch wide boards at 16-inch joist spacing. Each board gets 2 screws per joist crossing. A 1-lb box of #8 x 2.5-inch deck screws contains roughly 125 screws. For a 200 sq ft deck, you would need about 700 screws or 6 boxes.
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.
How much does it cost to build a deck per square foot?
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.
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 building permit for a deck?
Most municipalities require a building permit for decks that are more than 30 inches above grade, attached to the house, or over 200 square feet. Freestanding decks under 30 inches high may be exempt in some areas. Always check with your local building department before starting construction.