Understanding beam loads before you build

A beam does one job: it carries load across a gap and passes that load down into its supports. Everything about sizing a beam — the steel section, the timber depth, the concrete reinforcement — starts with knowing how big that load is and how the beam responds to it.

The calculator above handles the two beam arrangements you’ll meet most often in early-stage design: a simply supported beam resting on two supports, and a cantilever fixed at one end and free at the other. It covers two load cases too — a single point load and a uniformly distributed load (UDL), such as the self-weight of a floor or a wall running along the span.

The four numbers that matter

Every beam calculation is really answering four questions, and those are the four figures the calculator returns.

Reactions

The reactions are the forces the supports push back with to keep the beam in equilibrium. For a simply supported beam they split between the two ends depending on where the load sits; for a cantilever, the entire reaction is carried at the fixed end.

Shear force

Shear force is the internal force trying to slide one slice of the beam past the next. It’s highest near the supports and drives decisions like web thickness in a steel section or stirrup spacing in a reinforced concrete beam.

Bending moment

Bending moment is the internal force trying to bend the beam, and it’s usually the number that governs how deep or how strong the beam needs to be. A simply supported beam under a central point load sees its peak moment at mid-span; a cantilever sees its peak at the fixed support.

Deflection

Deflection is how far the beam physically sags under load. It depends on the material’s stiffness (Young’s modulus, E) and the cross-section’s stiffness (second moment of area, I) as well as the load and span. Deflection limits are often the deciding factor for long, lightly loaded spans — a floor beam can be strong enough to carry the load and still deflect enough to crack a ceiling below it.

Reading the diagram

The panel above updates its sketch as you change inputs. Triangle supports represent a beam that’s free to lift, spin or slide slightly at its ends — a simply supported beam. A hatched wall represents a cantilever’s fixed end, which resists rotation as well as vertical movement. Watching the load marker move along the span is a fast way to build intuition for why a load near mid-span produces a bigger moment than the same load near a support.

Where this fits in a real design

Hand calculations like these are exactly right for early-stage sizing: comparing timber against steel, checking whether a longer span is realistic, or sanity-checking a number that came out of software. They are not a substitute for the checks a real design has to pass — combined loading, lateral-torsional buckling, bearing at supports, connection design, and the load factors and material safety factors set out in the design code that applies to the project and country.

1. Establish the actual loads: dead load (self-weight, finishes), imposed load (occupancy, furniture, snow), and any point loads from posts or partitions above.
2. Combine and factor those loads according to the relevant design standard before running a serious calculation.
3. Check bending, shear, deflection and bearing against the section’s actual capacity, not just against the raw applied force.
4. Have the final sizing reviewed by a qualified structural engineer before anything is built.

This calculator is provided for preliminary, educational estimating only. It does not apply load factors, safety factors, or the design rules of any specific building code, and it is not a substitute for a calculation prepared or checked by a qualified structural engineer. Always have load-bearing work verified before construction.