Deck joist calculator
Work out how many joists a deck needs and how many boards it takes to frame them.
Deck joist calculator
Work out how many joists a deck needs and how many boards it takes to frame them.
Enter your deck dimensions to see the joist count.
Joists needed
Project summary
How it works
These are material estimates, not span or structural design. Joist size, spacing and doubling requirements depend on load and local code; check span tables before building.
Deck joist calculator
Spacing sets the count, width sets the length
Joist count comes from the deck length divided by joist spacing, the same one-more-than-the-sections logic post spacing uses for a fence. Each joist then spans the deck width, which is what determines how long a board that joist needs, and whether a single stock length covers it or two boards are needed end to end.
This is a material count, not a span design
This calculator counts joists and lumber from a spacing you choose; it does not verify that spacing is adequate for the load, species and grade of lumber you are using. Joist size and maximum span are set by span tables and local code, so check those before finalizing spacing, especially for anything beyond a typical residential deck.
Frequently asked questions
How many joists does a 16 by 12 foot deck need?
At the common 16 inch joist spacing, a 16 foot long, 12 foot wide deck needs 13 joists, each spanning the 12 foot width. Doubling the rim joists at each end adds 2 more, for 15 total. Enter your own dimensions above for the exact count.
What is the standard joist spacing for a deck?
Sixteen inches on center is the most common residential deck joist spacing, though 12 inch spacing is sometimes used for heavier loads or thinner decking. Check the span table for your specific joist size, species and decking material, since spacing and maximum span are linked.
How many joists do I need for a 12 by 16 foot deck?
At 16 inch on centre spacing, you need 13 joists for a 16 foot run. Add one extra for the ledger board connection and you get 14 joists. The calculator gives you the exact count based on your deck dimensions, joist spacing and the direction you are running the joists.
What size joists do I need for a deck?
Most residential decks use 2 by 8 or 2 by 10 joists. The size depends on the span between beams, the spacing between joists and the load they carry. A span table from your local building code tells you the minimum joist size for your specific dimensions. The calculator focuses on the count, and you select the size that matches your span.
Should joists be 12 or 16 inches on centre?
16 inches on centre is the standard for most residential decking. 12 inch spacing is stronger and required for thinner decking materials or higher load applications. It uses more joists, which increases material cost. The calculator lets you enter either spacing and shows how the count changes.
What is the difference between joists and beams?
Joists are the horizontal members that span between beams and support the decking boards directly. Beams are the larger horizontal members that support the joists and transfer the load to the posts and footings. Joists run perpendicular to the decking boards. Beams run perpendicular to the joists. The calculator counts joists, beams and posts separately.
Do deck joists need blocking?
Blocking between joists adds rigidity and prevents twisting, especially at beam locations and where joists are long. Most building codes require blocking at midspan for joists over 12 feet. The calculator does not count blocking, but your local code will specify the requirement. Blocking uses short pieces of the same lumber as the joists.
How far can a 2 by 8 joist span?
A 2 by 8 pressure treated joist at 16 inch on centre spacing can span roughly 12 to 14 feet depending on the species and grade of lumber. A 2 by 10 spans further, around 15 to 17 feet. Always check your local building code span tables, which account for live load, dead load and lumber species. The calculator gives you the joist count for whatever span you are working with.