FitAndFillDeck Calculator: Boards, Joists and Fasteners
Estimate the deck boards, joists and fasteners for a rectangular deck from its size, the board width and gap, the joist spacing and an allowance for waste.
How the calculation works
rows of boards = width (in) ÷ (board width + gap), rounded up boards = rows × length × (1 + extra) ÷ board length, rounded up joists = length (in) ÷ joist spacing, rounded up, + 1 fasteners = rows × joists × fasteners per crossing × (1 + extra), rounded up The boards run along the length and the joists run across the width, so every board crosses every joist once. Each joist is as long as the deck is wide.
Worked example
A 16 × 12 ft deck is 192 sq ft. A 5½ in board plus a ⅛ in gap takes 5.625 in, so the 144 in width needs 144 ÷ 5.625 = 25.6, which rounds up to 26 rows.
That is 26 × 16 = 416 ft of boards, or 457.6 ft with 10 % extra. In 12 ft boards that is 39 boards.
Joists 16 in on center: 192 in ÷ 16 = 12 spaces, so 13 joists, each 12 ft long. With 2 fasteners at every crossing that is 26 × 13 × 2 = 676, or 744 with 10 % extra.
Assumptions and limits
- The deck is a rectangle. The boards run along the length and the joists run across the width, so every board crosses every joist once.
- Board width, gap, board length, joist spacing and fasteners per crossing are inputs you take from your plan and your decking maker; the tool does not recommend them.
- Board counts add up the feet of boards and divide by the board length, which assumes you can reuse offcuts. Board ends must land on a joist, so the real waste can be higher; the extra is an allowance you choose.
- Rim (band) joists, blocking, the ledger, beams, posts, footings, stairs, railings and flashing are not included. Structural design depends on your plan and local code.
- Many places require a permit for a deck. Check with your local building department before you buy materials.
Frequently asked questions
Does this design the structure of my deck?
No. It counts deck boards, joists and fasteners. Footings, posts, beams, the ledger and the allowed joist span depend on your plan, the lumber and your local building code. The American Wood Council publishes a prescriptive deck construction guide (DCA 6) for that part; your local building department has the final say.
What spacing and gap should I use?
Use the joist spacing in your plan and the board spacing in the instructions of your decking maker, because both depend on the product. The tool takes your numbers; it does not recommend them.
Why does the tool assume I can reuse offcuts?
It adds up the feet of boards and divides by the board length, which assumes the piece cut off one row starts the next. Board ends must land on a joist, so you may waste more. Raise the extra to 15 % or choose a board length that fits your deck.
What is not included?
Rim (band) joists, blocking, the ledger, beams, posts, footings, stairs, railings, flashing and fascia boards. Add them from your plan.
Do I need a permit?
Many places require a permit for a deck, especially one attached to a house or raised above the ground. Ask your local building department before you buy materials. Before you dig for footings, know where your buried utilities are; in the U.S. you can call 811.
What board width should I enter?
Enter the real width of the board you will buy. Nominal sizes, such as a "6-inch" board, are not the actual width, so measure one or read the product page.
Sources
- Geometry: rows = width ÷ (board width + gap); joists = length ÷ spacing, rounded up, plus one; fasteners = rows × joists × fasteners per crossing
- Board width, gap, board length, joist spacing and fasteners per crossing: entered by the user from their plan and the product they plan to buy (the tool does not assume a product or brand)
- American Wood Council, Decks (lists its "DCA 6 – 2015 Prescriptive Residential Wood Deck Construction Guide", the reference for the structural parts this tool does not calculate)
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