Joist Span Calculator
Find the maximum allowable span for floor joists — based on standard building code span tables.
Select your joist specs to see the maximum span.
Interactive Joist Layout
Visualization of joist span and spacing.
How Far Can a Floor Joist Span?
Floor joist span depends on 4 factors: wood species, joist size, spacing, and load. The table below shows maximum allowable spans for common joist configurations under standard residential floor loads (40 psf live + 10 psf dead load).
| Joist Size | 12″ OC | 16″ OC | 24″ OC |
|---|---|---|---|
| 2×6 (Douglas Fir) | 10′ 9″ | 9′ 9″ | 8′ 6″ |
| 2×8 (Douglas Fir) | 14′ 2″ | 12′ 10″ | 11′ 0″ |
| 2×10 (Douglas Fir) | 18′ 0″ | 16′ 1″ | 13′ 3″ |
| 2×12 (Douglas Fir) | 21′ 7″ | 19′ 1″ | 15′ 7″ |
💡 Spans shown are for #2 grade lumber. Select or #1 grade allows slightly longer spans. Always check local building codes for your jurisdiction.
Frequently Asked Questions
How far can a 2×8 joist span?
A 2×8 Douglas Fir joist at 16″ OC can span up to 12 feet 10 inches under standard residential floor loads. At 12″ OC spacing, span increases to 14 feet 2 inches.
How far can a 2×10 floor joist span?
A 2×10 Douglas Fir joist at 16″ OC can span about 16 feet 1 inch for standard floor loads. 2×10 joists are recommended for spans over 13 feet.
What is the difference between live load and dead load?
Live load is the weight of people, furniture, and movable objects (typically 40 psf for floors, 30 psf for sleeping areas). Dead load is the weight of the structure itself — joists, subfloor, flooring, ceiling below — typically 10 psf.
Find Your Maximum Joist Span
Code-compliant span tables — size your floor joists correctly the first time.
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