Joist Span Calculator

When it comes to the anatomy of a deck, joists are part of a deck’s framing system. Each joist is one of a series of long, horizontal structures that support the decking itself. While posts are vertical, joists lay horizontally on top of each beam to provide a sturdy, level surface to lay your decking materials across. Joists can be made from wood or steel.

Ready to start calculating how much lumber you’ll need for your joists? Use the joist span and spacing calculator below to plug in your desired type of wood, the size of your joists, and the required spacing between each joist.

This calculator will tell you:

  • Maximum Joist Span
  • Maximum Joist Span

Type of Wood


Joist Size


Joist Spacing

Your Joist Span Calculator Results


MAXIMUM JOIST SPAN: {{ Result.JoistSpan }}

Ready To Build Resized 2

Ready to Build Your Deck?

Do it with the brand that invented composite decking, backed by 25-, 35- and 50-year limited warranties.

Still Planning Your Deck Min

Explore More Deck Plans

Choose from hundreds of free downloadable deck plans based on the International Residential Code

  • Ad Zone: Calculator Feature Page
  • Instances Rendered: 3


Learn More About Joists

As a critical part to a deck that will last you for years to come, it’s important to find out just how many joists you’ll need to safely support your decking — and that depends on the size of your deck and approved building permit plans

If you just asked yourself “do I need a permit to build a deck?” remember that the answer is almost always yes. As a rule, the larger the deck, the larger the joists. For example, joists spaced 16 inches from the center of the joist next to it can span 1.5 times in feet the depth of their inches. To learn more about deck joist sizing, spacing, and allowable span, reference this chart.

Choosing Wood for Your Joists

The location where you live plays an important role in choosing wood for your joists. Not every species of wood is widely available in every region. For instance, California Redwood, Hemlock, or Douglas Fir are less common in the Southeast. Similarly, Southern Yellow Pine is less common on the West coast.

There are also strength differences between Redwood, Fir, Pine, and other types of wood. Check your local building codes for find out which species of wood are available in your market and -- most importantly -- check into the strength and spanning capabilities of a species.

Even if you like the look of Redwood, it may not be the strongest material available to you. Treated Hemlock or Pine may be a better solution. You can always stain a particular type of wood to match your aesthetic. It’s the strength and span that counts!

What is the Standard Deck Joist Span?

The standard deck joist span can vary due to the size of your deck and the type of material used to construct it. Depending on the size of the deck itself, whether your joists are spaced either 12 inches or 16 inches from the center, and the framing materials and size of deck boards used, deck joist spans may vary. With this in mind, the joist span can range between 6 ft, 10 inches, to 18 ft.

Use our calculator above to determine the proper joist span for your deck based on the type of wood or materials used, joist size, and spacing.