Floor Joist Span Calculator
A floor joist must carry the combined dead load (the structure itself) and live load (people and furniture) over its span without excessive deflection. This calculator computes the tributary area, the uniformly distributed load per foot, the total load on the joist, and the end reaction. Use these numbers alongside an official span table, such as those in the International Residential Code, to confirm the joist size and species you need. Always have structural work checked by a qualified professional.
Floor joist load formula
Tributary width = spacing (in) / 12
Total design load = live load + dead load (psf)
Load per foot (plf) = total psf * tributary width
Total load on joist = load per foot * span
End reaction = total load / 2
The IRC sets a typical residential floor live load of 40 psf for general living areas (30 psf for sleeping rooms), with a common dead load assumption of 10 to 15 psf. A uniformly loaded simple-span joist carries half the total load at each end support.
Worked example
A 12 foot joist at 16 inch spacing carries 40 psf live plus 10 psf dead. Tributary width = 16 / 12 = 1.33 ft. Load per foot = 50 times 1.33 = 66.67 plf. Total load = 66.67 times 12 = 800.00 lb. Each end reaction = 400.00 lb. Compare the span and load to an IRC span table to confirm, for example, that 2x10 No. 2 lumber is adequate.
Joist span notes
- IRC residential floor live load is generally 40 psf for living areas and 30 psf for sleeping rooms.
- Dead load for a typical wood floor is usually taken as 10 to 15 psf.
- Span depends on lumber species and grade, joist depth, and spacing; always check an official span table.
- Deflection limits (commonly span over 360 for live load) often govern joist size before strength does.
- This tool estimates loads only; it does not size the joist. Have all structural framing verified by a professional.
Floor Joist Span Calculator: frequently asked questions
How far can a floor joist span?
It depends on the lumber size, species, grade, spacing, and load. For example, 2x10 Southern Pine No. 2 joists at 16 inch spacing can span well over 15 feet at 40 psf live load, but you must confirm against an official IRC span table for your exact lumber.
What live load should I use for a floor?
The International Residential Code uses 40 psf live load for general living areas and 30 psf for sleeping rooms. Decks and other uses have different values. These are user-editable inputs in this calculator.
What is tributary width?
Tributary width is the strip of floor that a single joist supports, equal to the joist spacing. At 16 inch spacing, each joist carries a 16 inch (1.33 foot) wide strip of the total floor load.
Does this calculator size the joist for me?
No. It computes the loads and reactions so you can compare them to an official span table. Joist sizing depends on deflection and bending limits for your specific lumber and must be verified by a qualified professional.
Sources and methodology
- U.S. Department of Energy / Building America span guidance and the International Residential Code floor live-load values (40 psf living, 30 psf sleeping) are the basis; loads are user-editable. See DOE energy-efficient home design.
- Load arithmetic (tributary width, plf, reactions) follows standard statics for a simply supported beam.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.