Joist Span Calculator

Select your joist size, spacing, lumber species, and design load to look up the maximum allowable span from simplified IRC 2021 tables — helping you pick the right joist for your floor framing.

Joist Size

Spacing

Lumber Species / Grade

Live Load

Dead Load (PSF)

Max Allowable Span

15'5"

15.42 decimal ft

Configuration

2×10 @ 16" OC

SPF #2

Total Design Load

50 PSF

40 PSF live + 10 PSF dead

Purchase Option	Quantity	Est. Cost
2×6	9'4"	9.33 ft
2×8	12'3"	12.25 ft
2×10Best value	15'5"	15.42 ft
2×12	17'4"	17.33 ft

Always verify with your local building departmentThese spans are simplified reference values from IRC 2021 Table R502.3.1. Actual allowable spans depend on lumber grade, moisture content, and local amendments to the building code. Always confirm with your local building authority before framing.

This calculator provides reference span values only. It does not replace engineering analysis. Verify all framing decisions with local code officials and/or a licensed structural engineer.