Calculator
Deck Calculator
Estimate decking boards, total linear feet, joists and deck screws for a rectangular deck from its size, board width, gap and joist spacing.
Decking is bought by the board, but a deck is designed in square feet. This calculator bridges the two: it works out how many rows of boards fit across your deck, how many linear feet of decking that adds up to, how many boards to buy at the length you choose, and how many joists and screws the surface needs.
Project dimensions
Enter your measurements below. Results update with a waste allowance you can adjust.
Important disclaimer
This tool estimates decking boards, joist count and screws for a rectangular deck with boards running along its length. It is not a structural design: joist size, beam size, footings, ledger attachment and railing must follow your plans, the product instructions and local code. Cost uses only the price per board you enter and excludes framing, hardware, labor, tax and delivery.
About this calculator
How the deck calculator works
- Rows = deck depth ÷ (board width + gap), rounded up.
- Decking length = rows × deck length.
- Boards = decking length × (1 + waste% ÷ 100) ÷ board length, rounded up.
- Joists = deck length ÷ joist spacing + 1, plus an end joist when the length is not an exact multiple of the spacing.
- Screws = rows × joists × 2.
Use the actual width of the board, not the nominal size. A nominal 6 inch board, such as 5/4×6 or 2×6, is usually 5.5 inches wide.
Worked example
A 16 by 12 foot deck with 5.5 inch boards, a 3/16 inch gap, 16 foot boards and joists 16 inches on center:
- Each board plus gap covers 5.6875 inches, so 144 ÷ 5.6875 = 25.3, which rounds up to 26 rows.
- 26 rows × 16 feet = 416 linear feet of decking.
- With 10 percent waste, 457.6 feet ÷ 16 = 28.6, so 29 boards.
- 192 inches ÷ 16 = 12 spaces, so 13 joists, each about 12 feet long.
- 26 rows × 13 joists × 2 = 676 screws.
A 20 by 14 foot deck with 20 foot boards and joists at 12 inches on center needs 30 rows, 600 linear feet, 33 boards with 10 percent waste, 21 joists and about 1,260 screws.
Joist spacing and board type
Many wood decking boards are installed on joists 16 inches on center. Some composite boards, thinner boards and diagonal layouts require 12 inches. Always follow the decking maker's span table; it decides the spacing, not the calculator.
What this calculator does not include
- Beams, posts, footings, ledger board, rim joists, blocking and joist hangers.
- Stairs and railings. The stair calculator lays out deck stairs.
- Fascia boards, picture-frame borders and hidden fastener clips.
Frequently asked questions
How many deck boards do I need for a 12 by 12 deck?
With 5.5 inch boards and a 3/16 inch gap, a 12 by 12 deck has 26 rows. Using 12 foot boards that is 26 boards, or 29 with a 10 percent allowance.
Should I buy longer boards?
Boards that span the whole deck avoid butt joints and look cleaner. When the deck is longer than your boards, plan the joints so they land on joists and are staggered.
How much gap should I leave?
A gap of about 3/16 inch is common for wood, but wet lumber shrinks after installation and composite boards have their own gap rules. Follow the product instructions.
For a full planning walkthrough, read How Many Deck Boards Do I Need?
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