Every floor joist holding up a living room is a beam doing quiet math. When a load presses on a beam, the beam bends — and engineers need to know exactly how much, because a floor that sags half an inch feels bouncy while a floor that sags two inches feels broken. The classic case is a simply supported beam with a load in the middle: maximum deflection equals the load times the span cubed, divided by 48 times the stiffness term (elastic modulus times moment of inertia). In symbols: δ = PL³ / (48EI).

Notice the span is cubed. Doubling the distance between supports makes the beam sag eight times more under the same load, which is why a 2×10 joist that feels rock solid across 10 feet feels like a trampoline across 16. The P × L part — load times span — sits at the heart of every version of this formula, and that is the product this calculator handles.

A homeowner sizing a deck, a contractor checking a header over a garage door, and a student working a statics problem all reach for the same idea. Type the total load in pounds in the Variable A box and the span in feet in the Variable B box. The Result box gives you the load-times-span product — the starting ingredient for bending moment and deflection checks.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Beam Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.