Floor Joist Span Calculator — Size, Spacing & Species
Use a simplified IRC-style lookup to compare 2×8, 2×10, and 2×12 floor joists. The output shows the smallest size that works, the allowable span, and basic quantity planning.
Simplified allowable spans used in this tool
| Species | 2×8 | 2×10 | 2×12 |
|---|---|---|---|
| SPF | 11.7 ft | 14.0 ft | 16.5 ft |
| Doug Fir | 12.3 ft | 14.8 ft | 17.3 ft |
| Southern Yellow Pine | 13.0 ft | 15.3 ft | 17.9 ft |
| Hem-Fir | 11.5 ft | 13.7 ft | 16.1 ft |
Spacing, live load, and grade adjust the base values slightly to create a field-friendly planning table. Always verify the final design against local code and actual engineered tables.
Frequently Asked Questions
What does L/360 mean?
It is a common live-load deflection limit for floors. Divide the span in inches by 360 to get the maximum acceptable live-load deflection.
Why only 2×8, 2×10, and 2×12?
These are the most common dimensional floor joists for quick planning. Larger spans, unusual loads, and engineered lumber need project-specific design.
Does this replace the IRC span table?
No. It is a simplified estimator inspired by IRC guidance, not a substitute for the full code table or engineering review.
Keep the project moving
Use the free calculator for fast takeoffs, then plug the result into a reusable field template.
Need templates? Browse our professional template store — contracts, checklists, and planners from $17.