Hammer CalculatorCalculators / Outdoor / Deck Joist Span Calculator
Deck Joist Span Calculator
Enter your joist span, spacing, and lumber species to find the smallest adequate joist size. Includes pass/fail checks against standard span tables and L/360 deflection limits.
Deck Joist Span Calculator
Pricing per LF (edit to match local rates)
Recommended Joist
2x10
Max span: 14.5ft for SPF (Spruce-Pine-Fir)
Your Span
12 ft
16" OC, 40 psf live load
Deflection Limit (L/360)
0.40″ max
Max allowable mid-span deflection for deck joists
Most Economical
2x10 @ $2.30/LF
Smallest passing joist = least cost
| Joist Size | Max Span | Status | Cost/LF | L/360 Deflection |
|---|---|---|---|---|
| 2x6 | 8.5 ft | FAIL | $1.10 | 0.40″ |
| 2x8 | 11 ft | FAIL | $1.70 | 0.40″ |
| 2x10 | 14.5 ft | PASS | $2.30 | 0.40″ |
| 2x12 | 17.5 ft | PASS | $3.20 | 0.40″ |
Use 2x10 SPF (Spruce-Pine-Fir) joists at 16" OC2x10 is the smallest (most economical) joist size that spans 12ft at 16" OC spacing with 40 psf live load in SPF (Spruce-Pine-Fir). Max allowable span is 14.5ft, giving you 21% margin.
About L/360 deflection limitThe L/360 deflection standard means the maximum allowable mid-span deflection is the span length divided by 360. For a 12ft span, that's 0.40″. This limit prevents noticeable bounce and ensures the deck feels solid underfoot. Most building codes require L/360 for deck joists under live load.
Span values are based on standard #2 grade lumber span tables at 40 psf live / 10 psf dead load, adjusted for your specified load. Always verify with your local building code and consult a structural engineer for critical applications.