Garden & Outdoor
Deck Board Calculator
Enter your deck size, board width, gap spacing, and board length to get an exact board count — including a 10% buffer for waste and cuts.
Calculate Deck Boards Needed
Boards span the length — counted across the width.
Match to your deck length to minimize joints
How the Deck Board Calculator Works
The calculator counts the number of board runs across your deck width, then figures out how many boards of your chosen length are needed to span the deck length — and multiplies them together.
The Formula
- Board runs = deck width (in) ÷ (face width + gap), rounded up
- Boards per run = deck length (ft) ÷ board length (ft), rounded up
- Total boards = board runs × boards per run
- Buy quantity = total boards × 1.10 (10% waste buffer), rounded up
Coverage by Board Size and Gap
| Board | Face Width | + 3/16" gap | Boards per 10 ft width |
|---|---|---|---|
| 5/4×6 / 2×6 | 5.5" | 5.69" | 22 boards |
| 2×4 | 3.5" | 3.69" | 33 boards |
| Composite 6" | 5.5" | 5.63" (1/8" gap) | 22 boards |
Board counts are before waste factor, for a 10 ft wide deck.
Worked Examples
Example 1: 12×20 Deck
Size: 12 ft wide × 20 ft long
Boards: 5/4×6 (5.5" face)
Gap: 3/16", 20 ft boards
Board runs: (12×12) ÷ 5.69 = 26 runs
Boards per run: 20 ÷ 20 = 1
Total: 26 × 1 = 26 boards
+10% buffer = 29 boards to buy
Example 2: 16×24 Deck
Size: 16 ft wide × 24 ft long
Boards: 2×6 (5.5" face)
Gap: 3/16", 16 ft boards
Board runs: (16×12) ÷ 5.69 = 34 runs
Boards per run: 24 ÷ 16 = 2
Total: 34 × 2 = 68 boards
+10% buffer = 75 boards to buy
Example 3: 10×10 Small Deck
Size: 10 ft wide × 10 ft long
Boards: 2×4 (3.5" face)
Gap: 3/16", 10 ft boards
Board runs: (10×12) ÷ 3.69 = 33 runs
Boards per run: 10 ÷ 10 = 1
Total: 33 × 1 = 33 boards
+10% buffer = 37 boards to buy
Pro Tips
Expert advice to get better results on your project.
Crown up, always
Every board has a slight curve (crown) along its length. Install them crown-side up so the center bows upward — it sheds water and flattens over time as the board dries. Crown down traps water in the center and speeds up rot.
Leave a 1/2-inch gap at the house
The gap between your first board and the house (or ledger flashing) should be at least 1/2 inch to prevent water from wicking into the structure. Use a spacer or a piece of plywood as a consistent guide.
Let pressure-treated lumber acclimate before installing
Freshly treated lumber has high moisture content and will shrink as it dries. Stack boards flat with stickers between them for 2–4 weeks before installation, or install with tighter gaps knowing they will open up. Dry lumber installed with 3/16-inch gaps tends to stay at 3/16 inch.
Use a consistent gap spacer on every board
A Kreg deck jig or even a set of 3/16-inch drill bits used as spacers makes uniform gaps fast. Eyeballing gaps leads to wavy lines that are obvious and permanent. Consistent gaps are what separate a professional-looking deck from a DIY one.
Snap a chalk line for the final trim cut
Run boards a few inches long past the edge, then snap one chalk line and trim all boards at once with a circular saw. This gives you a perfectly straight, clean edge with one pass — much better than cutting each board to length individually before installing.
Sort boards at the lumber yard, not at home
Pull boards from multiple units in the stack and inspect each one. Reject boards with large knots near the ends, heavy warp, or deep checks. The time spent sorting at the store saves hours of frustration on the job. Most yards allow culling — you're paying for quality lumber, so take it.
Frequently Asked Questions
Common questions about estimating and buying deck boards.