Deck Joist Span Calculator
Enter your joist span and wood species to see which joist sizes and spacings pass the 2021 IRC deck span table, and how many joists you need.
How this calculator works
- Spans come from IRC 2021 Table R507.6: No. 2 grade lumber, wet service, 40 psf live load and 10 psf dead load, with L/360 deflection.
- The calculator picks the smallest joist size whose tabulated span meets or beats the span you need at the chosen spacing.
Joist span questions
How far can a 2x8 deck joist span?
Under IRC Table R507.6, a southern pine 2x8 at 16 inches on center spans 11 ft 10 in; Douglas fir-larch, hem-fir or SPF spans 11 ft 1 in.
How far can a 2x10 span?
Southern pine 2x10 at 16 inches on center spans 14 ft 0 in; Douglas fir-larch, hem-fir or SPF spans 13 ft 7 in; redwood or cedar 13 ft 0 in.
Can joists cantilever past the beam?
Yes, within limits set by the IRC (commonly up to one quarter of the back span, and less for heavier loads). Check Table R507.6 notes and local amendments.