Tributary Width Calculator
Convert a floor, roof, wind, or snow area load into an equivalent line load on a beam, joist, rafter, or girder. Enter a direct tributary width or calculate it from the spacing to adjacent beams.
Quick answer: Tributary width is the width of floor or roof area that transfers load to a supporting member. An interior beam usually receives half the spacing from each side; an edge beam receives load only from its loaded side.
Interior beam
Edge beam
Tributary Width Calculator
For an edge beam in spacing mode, enter the spacing on the loaded side and leave the other side blank. For a tapered tributary strip, use direct width mode with different start and end widths.
Area Load to Line Load Formula
A surface load acts over an area. A beam analysis model usually needs a line load, so the area pressure is multiplied by the width of floor or roof that feeds into the beam:
In metric units, kPa is the same as kN/m², so multiplying by metres gives kN/m. In imperial units, psf multiplied by feet gives lb/ft.
Interior and Edge Beams
For an interior beam supporting one-way floor load from both sides, the tributary width is usually half the spacing to the beam on the left plus half the spacing to the beam on the right. For an edge beam, only the loaded side contributes. Real slab behavior, openings, cantilevers, transfer beams, and two-way action can change the load path.
Tapered Tributary Widths
If the loaded strip widens or narrows along the member, the equivalent line load is trapezoidal. Use the start and end tributary widths to calculate the start line load, end line load, average line load, and total load.
Engineering Note
Tributary loading is a simplified load-path method. It is appropriate for many one-way framing checks, but it does not replace analysis of complex framing, two-way slabs, transfer systems, diaphragm behavior, or members with unusual stiffness. Use professional judgement before relying on the result.