Deck Board Calculator
Find how many deck boards, joists and screws you need to build or resurface a deck.
How to use this deck calculator
Enter the deck's length in the direction the boards will run, and its width across the boards. Pick your board width, the gap between boards and the board length sold at your lumberyard. The calculator counts rows of boards, how many boards each row needs, the joists underneath at your chosen spacing, and the screws to fasten everything (two per board at each joist).
If the deck is longer than one board, the boards are joined end to end over a joist. Stagger those joints from row to row so they don't line up.
Joist spacing and board type
| Decking | Max joist spacing (straight lay) |
|---|---|
| 5/4 pressure-treated wood | 16 in |
| 2× pressure-treated wood | 16–24 in |
| Most composite boards | 16 in (12 in for diagonal) |
Always check the decking maker's instructions. Composite brands set their own spacing rules, and the warranty can depend on following them.
The math behind it
- Rows = width in inches ÷ (board width + gap), rounded up.
- Boards = rows × (length ÷ board length, rounded up).
- Joists = (length in inches ÷ spacing, rounded up) + 1.
- Screws = rows × joists × 2.
Example: a 16 × 12 ft deck with 5.5-inch boards and 3/16-inch gaps needs 26 rows of 16-foot boards, 13 joists at 16 inches apart, and about 676 screws.
Easy to forget
- Fascia boards to cover the rim joists.
- Joist tape on top of the joists, which helps them last longer.
- Stair stringers and treads if the deck sits more than a step off the ground.
Often used together
Quick answers
How many deck boards do I need for a 12 × 16 deck?
With 5.5-inch boards and small gaps, about 26 rows. Using 16-foot boards running the long way, that's 26 boards, or about 29 with 10% extra.
How many screws do I need per deck board?
Two at every joist the board crosses. With joists every 16 inches, that's about 300–350 screws per 100 square feet.
Should deck boards be gapped?
Yes. Leave about 1/8 to 1/4 inch for dry wood or composite boards. Wet pressure-treated boards are often laid tight, because they shrink as they dry.