How to use the deck calculator
- Enter the deck size. Length is the direction the decking boards run. Width is the direction you count rows of boards.
- Pick your board. A "5/4 × 6" or "2 × 6" board is actually about 5.5 inches wide. Many composite boards are about 5.36 inches.
- Set the gap and board length. Longer boards mean fewer butt joints. Follow the manufacturer's gap for composite boards.
- Choose joist spacing. 16 inches on center is standard. Diagonal decking and some composites need 12 inches.
Deck formulas
Linear feet = rows × deck length × (1 + waste %)
Boards = linear feet ÷ board length, rounded up
Joists = deck length (in) ÷ spacing + 1
Screws ≈ 2 per board at every joist
Worked example
A 16 × 12 ft deck (192 sq ft) with 5.5 inch boards and a 3/16 inch gap needs 26 rows. That is 416 linear feet, or about 458 feet with 10% waste, so buy 39 boards of 12 ft. With joists 16 inches on center, the frame needs 13 joists 12 ft long, and the decking takes about 700 screws.
Wood or composite?
| Pressure-treated wood | Composite | |
|---|---|---|
| Upfront cost | Lowest | About 2 to 3 times wood |
| Maintenance | Stain or seal every 2 to 3 years | Wash occasionally |
| Lifespan | 10 to 20 years | 25 years or more |
| Joist spacing | Usually 16 in | Often 12 or 16 in, check the brand |
Tips before you build
- Get a permit. Most towns require one for attached decks or decks over about 30 inches high.
- Plan board lengths so joints land on joists and are staggered between rows.
- Use hidden fasteners for grooved composite boards instead of face screws.
- Footings matter. Pour concrete footings below the frost line; use our concrete calculator for post holes.
Need the square footage of an odd-shaped deck first? Try the square footage calculator.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
With 5.5 inch boards and a 3/16 inch gap, a 12 × 16 ft deck needs 26 rows of 16 ft, about 458 linear feet with 10% waste. That is about 39 boards of 12 ft or 29 boards of 16 ft.
How far apart should deck joists be?
16 inches on center is standard for most wood decking. Composite boards and diagonal layouts often need 12 inches. Always follow the decking manufacturer and local code.
How many screws do I need for a deck?
Plan on two screws per board at every joist. That is roughly 400 screws per 100 square feet with 5.5 inch boards and joists at 16 inches on center.
What size are deck boards really?
A nominal 5/4 × 6 or 2 × 6 board is about 5.5 inches wide. Composite boards are often 5.36 or 5.5 inches. Measure the actual board before calculating.
Should deck boards have a gap?
Yes. A gap of 1/8 to 1/4 inch lets water drain and allows wood to swell. Wet pressure-treated boards are often laid tight because they shrink as they dry.
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Last updated October 2026.