How Many Deck Boards Do I Need?
Short answer: a 12 × 12 ft deck with standard 5.5 inch boards needs about 29 boards of 12 ft (26 rows plus 10% waste), 10 joists at 16 inches on center and about 550 screws.
How the count works
Deck boards run along the length of the deck and lie side by side across its width. So the number of rows depends on the width, and the length of each board depends on the length of the deck.
Boards = rows × deck length ÷ board length × 1.10 (waste), rounded up
Joists = deck length (in) ÷ joist spacing + 1
A "6 inch" wood deck board is actually 5.5 inches wide. With a 3/16 inch gap between boards, each row takes 5.69 inches.
Deck boards for common deck sizes
5.5 inch boards, 3/16 inch gaps, joists 16 inches on center, 10% waste, board length matching the deck length:
| Deck | Rows | Boards | Joists | Screws (2 per joist) |
|---|---|---|---|---|
| 8 × 8 ft | 17 | 19 × 8 ft | 7 | 250 |
| 10 × 10 ft | 22 | 25 × 10 ft | 9 | 400 |
| 12 × 10 ft | 22 | 25 × 12 ft | 10 | 450 |
| 12 × 12 ft | 26 | 29 × 12 ft | 10 | 550 |
| 16 × 12 ft | 26 | 29 × 16 ft | 13 | 700 |
| 16 × 16 ft | 34 | 38 × 16 ft | 13 | 900 |
| 20 × 12 ft | 26 | 29 × 20 ft | 16 | 850 |
The first number is the length the boards run. A 12 × 16 deck with boards running the 12 ft way needs 34 rows of 12 ft boards instead (38 boards).
Picking the board length
- Match the deck length when you can. One board per row means no butt joints and almost no waste.
- Longer decks need joints. Where two boards meet, the joint must land on a joist, ideally with a doubled joist or blocking. Stagger the joints from row to row.
- Overhang: many builders let boards run 1 to 1.5 inches past the rim joist and trim them straight after installing, so buy boards a little longer than the frame.
Composite and narrow boards
Composite boards are often about 5.36 inches wide and the manufacturer specifies the gap, so check the installation guide. Narrow 3.5 inch boards (nominal 4 inch) need about 50% more rows: a 12 × 12 deck takes 40 rows instead of 26.
Worked example
A 16 × 12 ft deck with the boards running the 16 ft length:
- Width 12 ft = 144 inches. 144 ÷ 5.69 = 25.3, so 26 rows.
- 26 rows × 16 ft = 416 linear feet, plus 10% = 457.6 ft. 457.6 ÷ 16 = 28.6, so 29 boards of 16 ft.
- Joists: 16 ft = 192 inches ÷ 16 + 1 = 13 joists, each 12 ft long.
- Screws: 26 rows × 13 joists × 2 = 676, so buy a 700 count box.
Before you buy
- Check local code. Joist size and span, footings, ledger attachment and railing height are set by your building department, and most attached decks need a permit.
- Footings: work out concrete for the piers with the concrete calculator (see also how many bags of concrete you need).
- Finish: one gallon of deck stain covers roughly 150 to 300 sq ft depending on the wood, so a 12 × 16 deck often takes 1 to 2 gallons per coat.
Try the deck calculator
Enter your own numbers below, or open the full deck calculator with the formula and more examples.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
About 29 boards of 12 ft (26 rows plus 10% waste) using standard 5.5 inch boards with 3/16 inch gaps.
How many deck boards for a 10x10 deck?
About 25 boards of 10 ft, with 22 rows and 10% waste.
How many deck boards for a 16x16 deck?
About 38 boards of 16 ft (34 rows plus waste), 13 joists at 16 inches on center and around 900 screws.
How many screws do I need per deck board?
Two screws at every joist the board crosses. At 16 inches on center, a 12 ft board crosses 10 joists, so 20 screws per board.