Deck Calculator
Calculate how many deck boards, joists and screws you need for a deck. Works for wood and composite decking with adjustable board width, length and gap.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Deck boards to buy
- 29 boards
- 16 ft boards, 26 rows x 1 per row plus 10% waste
- Total decking length
- 458 ft
- 192 sq ft deck surface
- Joists
- 13 joists
- 12 ft long at 16 in on center (2x8 or 2x10)
- Deck screws
- 672 screws
- About 2 x 5 lb (2.3 kg) boxes at 350 per box
- Rim / beam lumber
- 56 ft
- Perimeter of the frame
Deck formula
Rows = Deck width (in) / (board width + gap), rounded up
Boards = Rows x (Deck length / board length, rounded up) x (1 + waste %)
Joists = Deck length (in) / joist spacing + 1
Screws = about 3.5 per sq ft (2 per board at every joist)Worked example
A 20 x 14 ft deck with 5.5 in composite boards, a 1/8 in gap, 12 ft boards and 16 in joist spacing: 168 / 5.625 = 29.9, so 30 rows. Each row needs 2 boards (20 / 12 rounded up), so 60 boards plus 10% = 66 boards. Joists: 240 / 16 + 1 = 16. Screws: 280 sq ft x 3.5 = 980.
Quick reference
| Deck (length x width) | Rows | Boards to buy |
|---|---|---|
| 10 x 10 ft | 21 | 24 |
| 12 x 12 ft | 26 | 29 |
| 16 x 12 ft | 26 | 29 |
| 20 x 12 ft | 26 | 58 |
| 16 x 16 ft | 34 | 38 |
| 20 x 16 ft | 34 | 75 |
How to calculate deck boards
Boards run the length of the deck. Divide the deck width in inches by the board width plus gap to get the number of rows. Then divide the deck length by the board length you plan to buy and round up. Multiply rows by boards per row and add 10 percent for waste.
A 16 by 12 foot deck with 5.5 inch boards and a 1/4 inch gap needs 26 rows. With 16 foot boards that is one board per row, so 26 boards plus waste, or about 29 boards.
Joists, spacing and fasteners
Joists run perpendicular to the deck boards, usually 16 inches on center. Divide the deck length in inches by 16 and add one for the end joist. Composite decking laid diagonally usually needs 12 inch spacing, so check the manufacturer's span table.
Plan on two screws per board at every joist, which works out to roughly 350 screws per 100 square feet. Use coated or stainless deck screws rated for treated lumber.
Key facts
- A 5/4 x 6 deck board is actually 1 in thick and 5.5 in wide.
- Standard joist spacing is 16 in on center; composite decking laid diagonally usually needs 12 in.
- Plan on about 350 deck screws per 100 sq ft of decking.
- Leave a 1/4 in gap between dry boards; wet pressure-treated lumber can be butted tight because it shrinks as it dries.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
About 29 boards of 16 foot 5/4x6 decking with a 1/4 inch gap, including 10% waste. That is 26 rows of one board each plus extras for cuts.
How many joists for a 16 foot deck?
13 joists at 16 inches on center: 16 ft times 12 divided by 16 equals 12, plus one for the end.
How many screws per deck board?
Two screws at each joist. On 16 inch centers a 16 foot board crosses 13 joists, so 26 screws per board. That is about 350 screws per 100 square feet.
What gap should I leave between deck boards?
1/4 inch for kiln-dried lumber. Wet pressure-treated lumber can be butted tight since it shrinks as it dries. Composite is usually 1/8 to 1/4 inch per the manufacturer.
Sources and references
Next steps for this project
The usual order for a framing, decks & fences project. See all framing, decks & fences calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.