Deck Calculator
Enter your deck size, board width, and joist spacing to estimate how many decking boards, joists, and screws you need. Assumes 16 ft boards with a 10% waste allowance.
How to calculate deck
Multiply deck length × width for the area. Divide the area by the board width (in feet) to get the linear feet of decking, divide by the board length (16 ft here), and add 10% for waste. Joists run every 16 inches: floor(length ÷ spacing) + 1. Budget about 3.5 deck screws per square foot.
Boards = (Deck area ÷ board width ÷ 16 ft) + 10% · Screws ≈ 3.5 × ft²
Worked example
A 20 ft × 12 ft deck with 5½-inch boards and 16-inch joist spacing.
- Area = 20 × 12 = 240 ft²
- Linear feet = 240 ÷ 0.458 = 524 ft
- Boards = 524 × 1.1 ÷ 16 = 36
- Screws = 240 × 3.5 = 840
Result: ≈36 boards, plus joists and ~840 screws
Frequently asked questions
How many deck boards do I need?
Divide the deck area by the board width (in feet) to get linear feet of decking, divide by the board length, and add 10% for waste. A 240 ft² deck with 5½-inch × 16 ft boards needs about 36 boards.
How far apart should deck joists be?
16 inches on center is standard for most residential decks. Use 12 inches for diagonal decking or heavier composite boards.
How many screws do I need for a deck?
Plan on about 3.5 deck screws per square foot (two per board at each joist), so roughly 350 screws per 100 ft² of decking.
Should I add extra for waste?
Yes, add about 10% for straight decking and more for diagonal or picture-frame patterns, to cover cuts and the occasional warped board.