Deck Joist Calculator
A deck joist calculator finds how many joists a deck frame needs by dividing the deck width by the joist spacing (12 or 16 inches on center) and adding one. Enter your deck dimensions and spacing below to get the joist count and beam guidance.
Enter deck width to see the joist count.
Estimate only. Confirm against your local building code: stairs, framing, rebar, roofing and retaining walls are structural and code-dependent. This site does not provide engineering judgment.
How Many Deck Joists Do You Need?
To find deck joists, divide the deck width (the direction joists run across) by the joist spacing in feet and add one. A 16 foot wide deck at 16 inch (1.33 foot) spacing needs about 13 joists. The joist length equals the deck depth. The calculator counts joists from your deck size and spacing, and tighter 12 inch spacing for composite decking raises the count.
What Joist Spacing Should You Use?
Use 16 inch on-center spacing for standard wood decking laid straight, and 12 inch spacing for composite boards or any diagonal layout, which need more support to prevent sag. The decking manufacturer sets the requirement, so check it. Closer spacing means more joists and a stiffer deck, which the calculator reflects when you change the spacing value.
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Frequently asked questions
- How many joists do I need for a deck?
- Divide the deck width by the joist spacing and add one. A 16 foot wide deck at 16 inch spacing needs about 13 joists.
- What spacing for deck joists?
- 16 inches on center for standard wood decking, 12 inches for composite boards or diagonal layouts, which need more support.