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. One beam near the outer edge sits on a row of posts.
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
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.
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 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. 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, because many decking products need joists 12 inches on center when the boards run at 45 degrees.
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.