Deck Calculator

Work out how many deck boards, joists and screws a rectangular deck needs from its size, board width and length, gap and joist spacing, with extra boards for waste and an optional materials cost. Your inputs stay in your browser.

Your deck

Deck size

Along the house: the direction the deck boards run.

Out from the house: the direction the joists run.

Deck boards

Actual width: 5/4 × 6 and 2 × 6 boards are about 5.5 inches. Check composite boards.

1/8 inch (0.125) is common; follow the board maker's spacing.

Common lengths are 8, 10, 12, 16 and 20 feet.

For cuts and bad boards; 10% is typical for a straight layout.

Joists and screws

16 inches is common. Composite and diagonal decking often need 12.

Screws where each board crosses a joist, usually 2.

Cost (optional)

29 deck boards, 13 joists and 676 screws. See full results

Example: a 16 × 12 ft deck of 16-ft boards 5.5 inches wide with a 0.125-inch gap, joists 16 inches on center and 10% extra. Enter your numbers to update.

Your deck materials

Deck boards
29
16-ft boards: 26 plus 10% extra
Deck area
192 sq ft
16 × 12 ft
Joists
13
12 ft long, 16 in. on center
Deck screws
676
2 per board at each joist

Materials list

ItemQuantityCost
Deck boards (16 ft) 29—
Joists (12 ft) 13—
Deck screws 676—

26 rows of boards, 1 board per row: 416 linear feet of decking before waste. Add prices to estimate the cost of materials. Beams, posts, footings, ledger, rim joists, stairs and railings are not included.

How to calculate deck boards

  1. Rows of boards: convert the depth to inches (1 foot = 12 inches; NIST: tables of units of measurement). With a gap between boards, n boards cover n × width + (n − 1) × gap, so the rows are (depth + gap) ÷ (board width + gap), rounded up: (144 + 0.125) ÷ (5.5 + 0.125) → 26 rows.
  2. Boards per row: the deck width divided by the board length, rounded up: 16 ÷ 16 → 1.
  3. Boards: rows × boards per row = 26, plus 10% extra = 29 boards.
  4. Joists: the width in inches divided by the spacing, rounded up, plus one for the end: 192 ÷ 16 + 1 = 13 joists, each 12 feet long.
  5. Screws: rows × joists × screws per crossing = 26 × 13 × 2 = 676. Hidden fastener systems use one clip per crossing instead; follow the maker's instructions.

Board length and joist spacing

Pick board lengths that fit the width. A 20-foot-wide deck needs 2 sixteen-foot boards per row (58 boards with extra, and most of each second board is cut off), but 1 twenty-foot board per row (29 boards). Where boards meet end to end, the joint has to land on a joist.

Closer joists mean more framing and more screws. At 12 inches on center the example needs 17 joists and 884 screws, against 13 and 676 at 16 inches. Composite and PVC board makers publish the joist spacing their boards need, often closer for diagonal layouts; use the smaller of that and your code's requirement.

What this calculator does not include

  • Structure. Joist size and span, beams, posts, footings, the ledger and its attachment to the house, and rim joists are set by the building code where you live and by the load the deck carries. Many places require a permit for a deck; check with your local building department.
  • Diagonal or picture-frame board layouts, which use more boards and need different joist spacing.
  • Stairs, railings, joist hangers, flashing, stain and labor.

Results are estimates for planning a purchase, not a structural design.

Methodology and testing

Automated tests compare rows, boards, joists and screws with an independent calculation, check the row count against a board-by-board layout, and check that sizes that divide exactly do not get an extra row, board or joist. How we build and test calculators.

Sources

  • Board Foot Calculator

    Board feet for up to four stacks of lumber, quarter-size thickness, waste allowance and cost per board foot.

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  • Home Equity Loan Calculator

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