Why use this calculator
Building a deck is a significant investment, and accurate material estimation ensures you stay on budget without running short or over-buying. This calculator accounts for decking boards, structural joists at standard 16-inch on-center spacing, and fasteners, giving you a comprehensive materials list. Proper planning also helps you coordinate delivery and staging for a smooth build process.
How to use it
Enter your planned deck length and width in feet, the width of your decking boards in inches (commonly 6 inches for standard lumber or composite), and the board length available. The calculator determines the total square footage, number of boards with a 10% waste factor, joists at 16-inch on-center spacing, and the approximate number of screws needed.
The formula
Total Area = Deck Length x Deck Width. Boards Across = Deck Length / (Board Width + Gap). Boards per Row = Deck Width / Board Length. Total Boards = Boards Across x Boards per Row x 1.1 (10% waste). Joists = (Deck Length / 1.33 ft) + 1 at 16" on-center. Screws = (Total Area / 100) x 350.
Worked examples
20 ft x 12 ft deck with 6-inch wide, 12-foot long boards
240 sq ft, approximately 44 boards, 16 joists, 840 screws
16 ft x 10 ft deck with 6-inch wide, 16-foot long boards
160 sq ft, approximately 22 boards, 13 joists, 560 screws
When people use it
- Planning a backyard entertainment deck build
- Estimating materials for a front porch or wraparound deck
- Budgeting for composite vs. pressure-treated lumber projects
- Creating a shopping list for deck renovation or board replacement
Tips
- Use 16-inch on-center joist spacing for standard decking; reduce to 12 inches for diagonal decking patterns or composite boards that require closer support
- Add 10-15% extra for waste due to cuts, defects, and end trimming. Complex shapes or angled cuts require more waste allowance
- Pre-drill screw holes near board ends to prevent splitting, especially with hardwoods and pressure-treated lumber
- Consider hidden fastener systems for composite decking for a cleaner appearance and easier installation
Questions people ask
- What does 16-inch on-center (OC) mean for joists?
- On-center means the distance is measured from the center of one joist to the center of the next. At 16 inches OC, joists are spaced 16 inches apart, which is the standard for most residential decks and provides adequate support for common decking materials.
- How many screws do I need per square foot?
- A general estimate is about 350 screws per 100 square feet of decking, or roughly 3.5 screws per square foot. This accounts for two screws per board at each joist crossing. Actual quantity may vary based on board width and joist spacing.
- Should I use screws or nails for decking?
- Screws are strongly preferred over nails for decking. They provide better holding power, resist pull-out from expansion and contraction, and are easier to remove if a board needs replacement. Use stainless steel or coated deck screws rated for your decking material.