Deck Calculator
Enter the length and width of your deck, the size of the boards and the joist spacing to see how many deck boards and joists you need. Board gap and waste are editable.
How to use this deck calculator
- Enter the deck length and width in feet. Boards run along the length and joists run along the width.
- Enter the actual width of one deck board and the gap you plan to leave between boards.
- Enter the length of the boards you will buy. If the deck is longer than the board, the calculator counts the extra pieces for each row.
- Enter the joist spacing, center to center. Check the decking maker for the spacing they allow.
- Keep the waste allowance or change it, add a price per board if you like, and press Calculate.
How to calculate how many deck boards and joists you need
The calculator counts rows of boards across the width, then the pieces needed for each row.
- Rows of boards = deck width (in) / (board width + gap), rounded up
- Pieces per row = deck length / board length, rounded up
- Boards = rows x pieces per row x (1 + waste % / 100), rounded up
- Joists = deck length (in) / joist spacing, rounded down, plus 1
- Deck area = length x width
Worked example
A deck is 16 ft long and 12 ft wide. The deck width is 144 inches. Each board is 5.5 inches wide with a 0.25 inch gap, so each row takes 5.75 inches. Rows = 144 / 5.75 = 25.04, which rounds up to 26 rows. With 16 ft boards, one board spans each row, so you need 26 boards. With 10% waste that is 28.6, so 29 boards.
Joists at 16 inches on center: 192 / 16 = 12, plus 1 gives 13 joists, each 12 ft long, or 156 linear feet.
Assumptions & limits
- The deck is a simple rectangle with boards laid straight, parallel to the length. Diagonal and picture-frame layouts need more boards.
- Only decking and joists are counted. Beams, posts, footings, ledger boards, hardware and railings are separate.
- The joist spacing you enter is not a recommendation. Allowed spacing depends on the decking and local code, so check the product instructions.
- End joints between boards and the placement of joint seams are not planned. Extra boards are covered by the waste percentage.
- Building codes and permit rules apply to decks. Check with your local building department.
Frequently asked questions
How do I calculate how many deck boards I need?
Divide the deck width by the board width plus the gap, round up to get the number of rows, then multiply by the number of boards in each row. Add waste. A 12 ft wide deck with 5.5 inch boards and a 0.25 inch gap needs 26 rows.
How many joists do I need?
Divide the deck length in inches by the joist spacing, round down and add one. A 16 ft deck with joists 16 inches apart needs 13 joists.
How far apart should deck joists be?
It depends on the decking material, how the boards are laid and local code. Spacing of 16 inches is common, but follow the decking maker and local rules.
How much waste should I allow?
Ten percent is a common starting point. Diagonal patterns and decks with many cuts need more.
Does the calculator include posts, beams and railings?
No. It counts deck boards and joists only. Beams, posts, footings, fasteners and railings need their own estimates.
What gap should I leave between boards?
It depends on the board material and the maker instructions. The calculator uses the gap you enter.
Related calculators
Related articles
Last updated: November 2026