Chaining the two calculations together
A 50 m² residential floor with a 1.5 kPa dead load and a 2 kPa live load has a total design load of 3.5 kPa. If a beam supports a 3-metre-wide strip of that floor, the beam's uniformly distributed load (UDL) is 3.5 kPa × 3 m = 10.5 kN/m — a real number, fed directly into the beam load calculator, not a separate estimate.
What "tributary width" means
A beam doesn't support the whole floor — it supports the strip of floor load that transfers to it, called the tributary width, typically half the distance to the next beam or wall on each side. Multiplying the floor's load-per-square-metre figure by that strip's width converts an area load (kPa, i.e. kN per m²) into a line load along the beam's length (kN/m) — the exact unit the beam load calculator's UDL input expects.
Running the chained number through the beam calculation
Feeding that 10.5 kN/m UDL into a 4-metre steel beam (elastic modulus 200 GPa, moment of inertia 8000 cm⁴) gives a maximum bending moment of 21 kN·m, a maximum shear force of 21 kN, and a maximum deflection of 2.1875 mm — each number higher than the calculator's default 10 kN/m example, exactly in proportion to the 10.5 kN/m load being 5% higher.
Why skipping this step understates the real load
Typing a beam's UDL as a guess, rather than deriving it from the floor's actual design load and the beam's real tributary width, risks silently under-sizing the beam — the beam calculator only checks the number it's given, so an understated UDL produces an understated (and wrong) bending moment, shear, and deflection. Deriving the UDL from the floor load calculation first keeps the two checks consistent with each other.