Deck Calculator
Plan a deck build: decking boards, joists, beams and screws for any deck size, with joist spacing options, board-gap math and a material cost estimate.
Along the joists' direction
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Enter a decking price to estimate cost.
You might also need:Board Foot Calculator·Concrete Calculator
Deck material math
linear feet = rows × deck length × 1.10 waste
joists = (deck length ÷ spacing) + 1
A 16 × 12 ft deck with 5/4 × 6 boards and ¼-inch gaps: 25 rows of decking, 400 linear feet before waste, 440 with. Joists at 16 inches on center across 16 ft means 13 joists, each 12 ft long, plus hangers at each end and roughly 700 screws.
Decking coverage per board width
| Board | Actual width | Coverage with ¼″ gap | Rows per 12 ft width |
|---|---|---|---|
| 2 × 4 | 3½″ | 3.75″ | 39 |
| 5/4 × 6 | 5½″ | 5.75″ | 26 |
| 2 × 6 | 5½″ | 5.75″ | 26 |
| Composite 5¼″ | 5¼″ | 5.5″ | 27 |
Building notes
- Footings come first: below frost line, sized for the load — pour them with the Sonotube calculator or the concrete calculator.
- Ledger flashing is non-negotiable: water getting behind a ledger board rots the house rim joist. It's the most common serious deck failure.
- Check span tables: joist size depends on span and spacing — a 12 ft span typically needs 2×8 or 2×10 at 16 inches on center.
- Total the lumber order with the board foot calculator, and add railing or a privacy screen with the fence calculator.
Frequently asked questions
How many deck boards do I need?+
Divide the deck width by the board coverage (board width plus gap) to get rows, then multiply by deck length for total linear feet. A 5/4 × 6 board covers 5.75 inches with a ¼-inch gap, so a 16 ft wide deck needs about 34 rows.
What spacing should deck joists be?+
16 inches on center is standard for most decking laid perpendicular to joists. Go to 12 inches for diagonal patterns or thin composite boards, and check your decking manufacturer's spec — many composites require 12 or 16 inch spacing to avoid sagging between joists.
How much gap should I leave between deck boards?+
Leave ⅛–¼ inch between boards for drainage and airflow. Kiln-dried and composite boards need the full gap since they won't shrink; wet pressure-treated lumber can be butted tighter because it shrinks as it dries.
How many screws per deck board?+
Two screws per board at every joist crossing. A 16 ft board over joists at 16 inches on center crosses 13 joists, so 26 screws per board. Plan on about 350 screws per 100 square feet of deck.
How much does a deck cost to build?+
Materials run $15–$35 per square foot for pressure-treated, $25–$50 for composite. Professionally built decks cost $30–$60 per square foot for wood and $50–$90 for composite, including framing, footings and railing.