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Deck Board Calculator: How Many Boards and Joists

Enter the deck size and the decking board you plan to use. The calculator counts the rows of boards across the width, converts them to whole boards of your chosen length with a waste allowance, and counts the joists needed along the deck. After switching units, re-enter your measurements.

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Formula

  • rows = round up ((deck width + gap) / (board width + gap))
  • boards before waste = rows x deck length / board length
  • boards to buy = round up (boards before waste x (1 + waste %))
  • joists = round up (deck length / joist spacing) + 1

Metric and imperial inputs are converted to millimetres with 1 ft = 304.8 mm and 1 in = 25.4 mm.

Worked example

A 16 ft by 12 ft deck, 5.5 in boards, 1/8 in gap, 12 ft boards, 10% waste, joists at 16 in:

  1. Width 144 in: (144 + 0.125) / (5.5 + 0.125) = 25.62, so 26 rows.
  2. Boards before waste: 26 x 16 / 12 = 34.67.
  3. With waste: 34.67 x 1.10 = 38.13, so 39 boards.
  4. Joists: 192 / 16 = 12, plus 1 = 13 joists.
  5. Area: 16 x 12 = 192 sq ft (17.84 m2).

Assumptions

  • Boards run along the deck length in parallel rows, with joists running across the width.
  • Boards are cut from stock lengths and offcuts are reused only through the waste allowance.
  • Rim joists, beams, posts, fasteners and railings are not counted.

Frequently asked questions

How many deck boards do I need for a 16 x 12 ft deck?

With 5.5 in boards and a 1/8 in gap, 144.125 / 5.625 gives 25.6, so 26 rows. Using 12 ft boards that is 26 x 16 / 12 = 34.7 boards, and with 10 percent waste you buy 39.

How much waste should I allow for decking?

5 to 10 percent suits straight runs. Use 15 percent or more for diagonal patterns, picture-frame borders or when you want to pick the best boards. Remember the offcuts from joins and the end of each run.

How many joists does a deck need?

Joists run across the width, spaced along the length. The count is the length divided by the spacing, rounded up, plus one. For 16 ft at 16 in centres, 192 / 16 = 12, so 13 joists. Doubled outer joists and beams are not counted.

How wide should the gap between deck boards be?

Common gaps are 1/8 to 1/4 in (3 to 6 mm) for drainage and expansion, but wood moisture and the manufacturer's instructions decide the right value. This tool only counts boards from the gap you enter, and span limits for joists depend on your local code.

How we calculate

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