What the calculator assumes
A rectangular deck attached to a house. Deck boards run along the house. Joists run out from the house, from a ledger board on the wall to a rim board at the outer edge. A beam near the outer edge sits on a row of posts. Deeper decks get a second beam near the middle.
How it works
- Rows of boards = deck width ÷ (board width + gap), rounded up
- Decking = rows × deck length, plus waste
- Boards to buy = decking ÷ board length, rounded up
- Joists = deck length ÷ joist spacing, rounded up, plus 1
- Screws = rows × joists × 2, plus waste
- Posts = deck length ÷ post spacing, rounded up, plus 1, for each beam
- Beam boards = 2 × deck length, for each beam
Example: a 16 by 12 foot deck with 5.5 inch boards and a 1/8 inch gap. Rows = 144 ÷ 5.625 = 25.6, so 26 rows. Decking = 26 × 16 = 416 feet, or 458 feet with 10 percent waste, which is 29 boards at 16 feet. Joists at 16 inches on center = 192 ÷ 16 + 1 = 13, each 12 feet long. Screws = 26 × 13 × 2 = 676, or 744 with waste.
Materials lists for common deck sizes
Each of these pages has a full list for that size, with the framing, railing and footings worked out.
8x1010x1010x1212x1212x1412x1612x2014x1614x2016x1616x2020x20
Joist spacing depends on board thickness
The International Residential Code limits how far apart joists can be under wood decking. Boards 5/4 thick need joists no more than 16 inches on center, or 12 inches when the boards run diagonally. Boards 2 inches thick can span 24 inches. Composite decking follows the maker instructions, which are often 16 inches or less.
This is a materials list, not a structural design
Joist size, beam size, post size, footing size and how far apart they can be all depend on your loads, lumber and local code. The beam count here assumes a typical beam made of two boards nailed together. The American Wood Council publishes a free guide, DCA 6, with span tables for wood decks. Most building departments will also tell you what they require when you apply for a permit.
Common questions
How many deck boards do I need for a 12x16 deck?
About 29 boards at 16 feet long, using 5.5 inch wide boards, a 1/8 inch gap and 10 percent waste. The boards run the 16 foot direction.
How many screws do I need for a deck?
Two screws at every point where a board crosses a joist, which is also the building code minimum for wood decking. That works out to about 350 screws per 100 square feet with 5.5 inch boards and joists 16 inches on center.
What gap should I leave between deck boards?
Between 1/8 and 1/4 inch is typical for dry lumber. Wet pressure treated boards shrink as they dry, so they are often installed tight. Composite decking has its own required gap, so follow the maker instructions.
How much extra should I buy for diagonal decking?
Plan on about 15 percent waste instead of 10. Also check joist spacing. Under the International Residential Code, 5/4 wood decking laid at 45 degrees needs joists no more than 12 inches on center.
Does this calculator size my joists and beams?
No. It counts pieces. Sizes and spans come from your local building code or a span table such as the American Wood Council DCA 6 guide.
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Sources
- International Residential Code 2021, Chapter 5: decks (R507)
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide
Facts on this page were last checked against these sources in October 2026.