Deck Board & Joist Calculator
Calculate exactly how many decking boards, linear feet of material, and floor joists you need to build your wood or composite deck.
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Deck Dimensions
Measure the Length of the deck (typically parallel to the house) and the Width (the distance it projects outward). We automatically account for standard board gaps and a 5% waste factor.
How to Use the Deck Calculator
Buying deck materials can be incredibly expensive, especially if you are using premium composite decking. Measuring correctly ensures you don't overspend or fall short on a Sunday afternoon.
- Measure the Frame: Input the length (usually the side parallel to your house) and the width (how far the deck extends into the yard) of your planned deck frame.
- Select Board Width: Choose whether you are using standard 6-inch boards or narrower 4-inch boards.
- Select Joist Spacing: Use 16-inch spacing for standard wood, or 12-inch for composite/diagonal decking.
Decking Material Formula
We calculate the linear feet of decking required by dividing the total surface area by the true coverage width of a single board.
Step 1: Board Coverage Width
Coverage (feet) = (Actual Board Width in inches + 0.125 inch gap) ÷ 12
Step 2: Linear Feet Needed
Linear Feet = Total Deck Area (sq ft) ÷ Coverage (feet)
Step 3: Add Waste & Convert to Boards
Total Linear Feet = Linear Feet × 1.05 (for 5% waste)
Number of 16' Boards = Total Linear Feet ÷ 16
Deck Example Calculation
You are building a 20-foot long by 12-foot wide deck using standard 5.5-inch wide boards.
- Area: 20 × 12 = 240 sq ft.
- Board Coverage: (5.5" + 0.125") ÷ 12 = 0.46875 feet.
- Linear Feet: 240 ÷ 0.46875 = 512 linear feet.
- Add 5% Waste: 512 × 1.05 = 537.6 linear feet.
- 16ft Boards: 537.6 ÷ 16 = 33.6 (Round up to 34 boards).
Conclusion: You need to purchase exactly 34 sixteen-foot decking boards.
Deck Building Tips
- Crown your joists: When installing floor joists, look down the edge of the board. Every board has a slight natural curve (the "crown"). Make sure the crown faces UP. If you place them crown down, the deck will sag under its own weight over time.
- Leave a drainage gap: Never butt wood decking boards tightly against each other. Always use a 16d nail as a spacer between boards to leave a 1/8-inch gap. This allows rainwater to drain and leaves room for the wood to expand in high humidity.
- Use joist tape: Before screwing down your decking boards, apply waterproof butyl joist tape to the top edges of your joists. This prevents water from rotting the joists around the screw holes and adds years to your deck's lifespan.
Frequently Asked Questions
What is the standard joist spacing for a deck?
For standard pressure-treated wood decking, joists are typically spaced 16 inches on center. However, if you are using composite decking (like Trex) or running your boards at a 45-degree diagonal angle, you must use 12-inch joist spacing to prevent the boards from sagging.
How wide is a standard decking board?
A standard '5/4x6' decking board has a nominal width of 6 inches, but its actual physical width is 5.5 inches. Our calculator automatically accounts for this actual width plus an industry-standard 1/8-inch gap between boards for drainage.
How much waste should I account for?
A 5% waste factor is standard for decking. This covers the small cut-offs at the ends of your boards that are too short to use elsewhere. If your deck has angled cuts or octagonal shapes, increase this to 10-15%.
How many screws do I need per board?
You will need 2 decking screws at every point where a decking board crosses a joist. As a rule of thumb, you will need roughly 3.5 screws per square foot of deck surface.