How to use the Beam Load
- Enter the span between supports and the load, marking it point or distributed.
- For a point load, give its distance from one support.
- Read the reactions, maximum moment and deflection estimate.
- Compare the moment against the section's capacity from the manufacturer's tables.
How the calculation works
Reactions come from static equilibrium: the sum of vertical forces and the sum of moments about a support are both zero. For a uniformly distributed load on a simple span each support carries half the total, and the maximum moment is wL²/8 at midspan. For a central point load each support carries half and the maximum moment is PL/4 — which is twice the moment of a distributed load of the same total weight, a key reason concentrated loads govern many designs.
Deflection depends on the section's stiffness, EI, and on span to the fourth power for distributed loads. That fourth power is why small increases in span cause disproportionate sag, and why serviceability limits — commonly span/360 for floors carrying plaster ceilings — frequently govern the design before strength does.
UDL: R = wL/2, M_max = wL²/8, δ = 5wL⁴/384EI | Central point load: R = P/2, M_max = PL/4, δ = PL³/48EISource: Classical beam theory (Euler-Bernoulli) for simply supported spans; deflection limits per common building code serviceability criteria.
Worked example
A 4.2 m simply supported timber beam carrying a uniform load of 3.6 kN/m.
- Total load = 3.6 × 4.2 = 15.12 kN.
- Each reaction = 15.12 / 2 = 7.56 kN.
- M_max = wL²/8 = 3.6 × 4.2² / 8 = 7.94 kN·m.
- Serviceability limit at span/360 = 4,200 / 360 = 11.7 mm.
Reactions of 7.56 kN each and a maximum moment of about 7.94 kN·m, with deflection to be held under roughly 11.7 mm.
Frequently asked questions
Can I use this to size a structural beam in my house?+
Use it to understand the loads and sanity-check a proposal. Structural members in a building must be designed to the applicable code and, in most jurisdictions, certified by a qualified engineer.
Why is a point load worse than a distributed load of equal weight?+
The moment from a central point load is PL/4 versus wL²/8 for a distributed load. For the same total weight that is twice the bending moment, because none of the load is carried close to the supports.
Is a safety factor included?+
No. The output is the nominal working value. Codes apply partial factors to loads and material strengths, and those factors differ by material and by design standard.
What deflection limit should I use?+
Span/360 is common for floors with brittle finishes, span/240 for more tolerant construction, and stricter limits apply where large glazing or tiling is supported. Confirm the limit in your governing code.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.