Deck Calculator
Estimate decking boards, joists, and screws for any deck size. Enter your dimensions and lumber specs to get a complete material list in seconds.
Planning a fence too? Use the Fence Calculator →
This deck calculator takes the guesswork out of lumber orders. Whether you're building a ground-level platform or an elevated backyard deck, knowing your board count, joist count, and screw quantity before you head to the lumber yard saves time and reduces expensive waste. Enter your deck dimensions, choose your board size and joist spacing, and get a complete material estimate instantly — including a 10% waste allowance already built in.
How to Use the Deck Calculator
- Enter your deck length and width in feet.
- Select your board width — 3.5 inches for 4-inch nominal boards, or 5.5 inches for the more common 6-inch nominal boards.
- Choose the board length you plan to buy: 8, 10, 12, or 16 feet.
- Set the gap between boards (0.25 inches is the standard default for most climates).
- Select your joist spacing — 16 inches OC is standard for most residential decks.
- Click "Calculate Deck" to see your full material estimate.
Deck Material Example: 12×16 Deck
A 12-foot by 16-foot deck using 6-inch nominal (5.5 in) boards, 12-foot board lengths, 0.25-inch gaps, and 16-inch OC joist spacing:
| Measurement | Value |
|---|---|
| Deck area | 192 sq ft |
| Boards (with 10% waste) | ~59 boards (12 ft) |
| Joists | 13 |
| Screws | ~1,534 |
At this scale, buying deck screws by the 5-pound box is far more economical than buying smaller quantities. A 5-pound box of 2.5-inch coated deck screws typically contains around 350–400 screws, so plan for four to five boxes for a project this size.
Frequently Asked Questions
How much wood do I need to build a 12x12 deck?
A 12×12 deck is 144 sq ft. Using 6-inch nominal (5.5 in) boards at 12 ft length with 0.25-inch gaps and 10% waste, you need approximately 37 boards. Joist count depends on spacing — at 16 inches OC you need 10 joists spanning the 12-foot width.
What is the standard spacing between deck boards?
The standard gap is 1/8 to 1/4 inch — about 0.125 to 0.25 inches. This allows water to drain and wood to expand in humid conditions. Use 0.25 inches as a safe default for most climates.
How far apart should deck joists be?
Residential decks typically use 16-inch on-center joist spacing, which works for most standard decking boards. 12-inch OC is used for softer woods or longer spans; 24-inch OC is acceptable only for thicker composite boards. Check your local building code for requirements.
What size deck can I build without a permit?
Rules vary by location, but many jurisdictions allow decks under 200 square feet that are not attached to the house and are less than 30 inches above grade without a permit. Always check with your local building department before starting — permits protect you during resale.
How many deck screws do I need per board?
A common estimate is 2 screws per board per joist crossing. For a 12-foot board over 10 joists, that's about 20 screws per board. This calculator estimates total screws as 2 × boards × joists, which is a reliable planning figure. Buy in bulk — 5-lb boxes of 2.5-inch deck screws are standard.