Joist Span Calculator — Max Floor Joist Span by Size & Spacing
Calculate the maximum allowable span for floor joists based on lumber size, spacing, and species. Includes IRC span tables for 2×6, 2×8, 2×10, and 2×12 joists.
How to Use This Calculator
- Select your joist size (nominal dimension)
- Choose the on-center spacing between joists
- Select the wood species and lumber grade
- Optionally enter your actual span to check code compliance
- Click "Calculate Max Joist Span" to see the IRC maximum and compliance status
How the Calculation Works
max_span = spanTable[species][joistSize][spacing] // IRC 2021 Table R502.3.1(2)
compliant = actualSpan <= maxSpan // PASS if actual span does not exceed max Maximum joist spans are determined by IRC span tables based on lumber species, size, and on-center spacing. The table values assume standard residential floor loading of 40 psf live load plus 10 psf dead load. Enter your actual span to get a PASS or FAIL compliance check against the code maximum.
Variables:
- species: Wood species group — affects strength and stiffness (Fb and E values)
- joistSize: Nominal joist size: 2×6, 2×8, 2×10, or 2×12
- spacing: On-center spacing between joists: 12", 16", or 24"
- actualSpan: Your measured clear span in feet — compared against the code maximum
- grade: Lumber grade affects allowable stress values (Select Structural > No.1 > No.2)
Pro Tips
- Always check local building codes — spans vary by jurisdiction
- Use #2 grade or better for structural floor joists
- Reduce span by 20% for wet locations like decks
- 16" OC spacing is the most common for residential floors
- Engineered lumber (LVL, I-joist) can achieve longer spans than dimensional lumber
Recommended Products
Frequently Asked Questions
What is the maximum span for a 2×8 floor joist at 16" OC?
For Southern Pine No.2, approximately 14 feet 7 inches. Douglas Fir-Larch allows about 15 feet 0 inches at the same spacing. Always verify with your local IRC adoption and any local amendments.
Can I use 2×6 joists for a floor?
Yes, for shorter spans. At 16" OC with Southern Pine, the maximum span is about 11 feet 1 inch. For most living areas wider than a hallway, 2×8 or larger joists are required.
Does joist spacing affect maximum span?
Yes, significantly. Closer spacing (12" OC) allows longer spans than 16" or 24" OC because each joist carries less load. Tighter spacing also reduces floor bounce and deflection.
What is the best wood species for floor joists?
Southern Pine and Douglas Fir-Larch have the highest strength values and longest allowable spans. They are also widely available at lumber yards. Spruce-Pine-Fir is commonly used in northern climates.
Do I need a permit for floor joist work?
Most structural framing work — including replacing or adding floor joists — requires a building permit. Check with your local building department before starting. Unpermitted structural work can cause issues when selling your home.