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Joist Span Calculator
Joist Span Calculator
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Joist Span Calculator

Find the minimum joist size for your span, species, and spacing. Based on IRC deck joist span tables.

DIY difficulty

HardTypical time: 2–4 weekends (as part of deck project)
  • ›Structural member sizing has direct safety implications
  • ›Species, spacing, and load must all match your actual project conditions
  • ›Local codes may require stricter spans than IRC minimums — always verify

Must-have tools

Framing squareLevelCircular sawDrill + impact driver

How to find the right joist size for your span

  1. Measure the clear span — the distance between supports (beam to beam, or beam to ledger), not the total joist length.

  2. Choose your joist spacing: 16″ O.C. is standard for most decks; 12″ O.C. gives more capacity; 24″ O.C. is rarely used for decks.

  3. Select your wood species. Douglas Fir-Larch and Southern Yellow Pine are the most common deck framing species in the US.

  4. Read the minimum required joist size. If your span exceeds all table values, reduce the spacing or add a mid-span beam.

Worked example

You're building a 14-foot deck using Douglas Fir-Larch lumber with joists at 16″ O.C.

  1. Enter 14 feet, 0 inches for the span.
  2. Select 16″ O.C. — standard spacing for most decks.
  3. Select Douglas Fir-Larch #2.
  4. The table shows: 2×6 maxes at 8'10″ (too small), 2×8 at 11'10″ (too small), 2×10 at 14'8″ (passes — this is your minimum size).
→ You need 2×10 joists. A 2×8 won't make it — its max span at 16″ O.C. is 11'10″, nearly 2 feet short.

Common uses

  • Sizing joists for a new deck build before purchasing lumber
  • Checking whether an existing joist size is adequate for a span
  • Comparing species and spacing options to optimize cost

Frequently asked questions

What span tables does this calculator use?
IRC Table R507.6 — the residential deck joist span table for 40 psf live load plus 10 psf dead load, #2 grade lumber.
My local code says something different — which do I follow?
Always follow your local jurisdiction's adopted code. The IRC is a model code; local amendments can be stricter. We show IRC minimums — verify the actual requirement with your building department.
Can I use this for floor joists inside the house?
These are deck-specific span tables. Interior floor joist spans use different load assumptions (typically 40 psf live + 15 psf dead). Use a separate floor joist span table for interior applications.
What does "O.C." mean?
"On center" — the distance from the center of one joist to the center of the next. A 16″ O.C. layout means a joist every 16 inches, giving roughly 14.5 inches of clear space between joists.
What is the minimum joist size for a 12-foot deck span?
For Douglas Fir-Larch at 16″ O.C., a 12-foot span requires a 2×8 (max span 11'10") — actually a 2×10 is needed. At 12″ O.C., a 2×8 (max 13'1") passes comfortably. Always use the calculator with your exact span and spacing.

Common mistakes to avoid

  • ✕Measuring the total joist length instead of the clear span — the span is the distance between support points, not the full board length.
  • ✕Using the wrong species — Southern Yellow Pine spans noticeably farther than Hem-Fir. Substituting species without rechecking the table can result in undersized joists.
  • ✕Ignoring local code amendments — the IRC is a baseline. Some jurisdictions require 50 psf live load for decks, which requires larger joists than shown here.
  • ✕Skipping the mid-span blocking — even correctly sized joists need solid blocking at mid-span and at supports to prevent rotation and bounce.

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Not sure whether to DIY this project?

Read our project-by-project guide on when to hire a contractor vs. do it yourself.

Read the guide →

About this calculator

Enter your span, joist spacing, and wood species to find the minimum required joist size per IRC R507.6 deck joist span tables. Shows pass/fail for all four sizes (2×6 through 2×12) with exact max spans. Covers Douglas Fir-Larch, Southern Yellow Pine, and Hem-Fir at 12, 16, and 24 inch on-center spacing.

All results are estimates. Verify quantities with your local supplier and confirm measurements before purchasing materials. Requirements vary by region and project type.

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