Joist Span Calculator
Find maximum floor and ceiling joist spans by size, spacing, species, and grade. Based on IRC building code tables.
How to Use This Calculator
Joist span tables are based on engineering calculations that consider wood strength, deflection limits, and code requirements. This calculator helps you find maximum allowable spans and select appropriate joist sizes.
- Select joist size (2x6, 2x8, 2x10, or 2x12).
- Choose spacing (12", 16", or 24" on center).
- Select wood species (SYP is strongest, SPF is most common).
- Choose lumber grade (#1, #2, or #3).
- Optionally enter your desired span to check feasibility.
Why This Matters
Properly sized floor joists prevent bouncy floors, excessive deflection, and structural failure. Building codes specify maximum spans based on extensive engineering analysis. Using undersized joists leads to floor problems; oversizing wastes money. Span tables provide the guidance needed for code-compliant construction.
- Find maximum spans for any joist configuration
- Compare different joist sizes and spacings
- Verify if desired spans are code-compliant
- Select optimal joist size for your application
- Understand how species and grade affect spans
Worked Examples
Bedroom Floor
14-foot span for bedroom using #2 SYP at 16" spacing.
Large Living Room
18-foot span with no center support, using Douglas Fir #2.
Ceiling Joists Only
20-foot garage ceiling span with SPF #2 at 24" spacing.
Common Mistakes to Avoid
- Using the wrong load type - floor loads are higher than ceiling loads.
- Ignoring lumber grade - #3 spans significantly less than #1.
- Assuming all species are equal - SPF is weaker than SYP or Douglas Fir.
- Not accounting for point loads - concentrated loads require engineering.
- Forgetting cantilevers - cantilever spans have different rules.