How to use it
- Enter the span between the supports.
- Enter the uniform load along the beam and any point load at midspan. Include the beam's own weight in the uniform load.
- Enter the width and depth of the section, and choose the material.
Worked example
A 75 × 225 mm softwood joist spans 4 m and carries 2.5 kN/m. The maximum moment is 2.5 × 4² ÷ 8 = 5.0 kN·m, and each support carries 5.0 kN. The moment of inertia is 0.075 × 0.225³ ÷ 12 = 7.12 × 10⁻⁵ m⁴, so the midspan deflection is 10.6 mm, just inside the L/360 limit of 11.1 mm. The bending stress is 7.9 MPa, which you then compare with the design strength of the timber grade you are using.
Reading the results
The bending moment and reactions depend only on the span and loads. Deflection also depends on stiffness, which is the material's E multiplied by the section's moment of inertia. Bending stress tells you how hard the material is working; the calculator does not pick a grade, so check it against the allowable or design stress for your material.
Frequently asked questions
What does L/360 mean?
It is a deflection limit: the span divided by 360. A 12 ft span has a limit of 0.40 in. It is a common limit for floors under live load, while L/240 is often used for roofs and total load.
Does the calculator include the weight of the beam?
No. Add the beam's self-weight to the uniform load. A softwood beam weighs roughly 5 kN/m³, and steel about 77 kN/m³.
Why does beam depth matter more than width?
Stiffness grows with the cube of the depth. Doubling the depth makes a beam eight times stiffer, while doubling the width only doubles it.
Can I use it for cantilevers or continuous beams?
No. The formulas are for a single span resting on two simple supports. Cantilevers, fixed ends and continuous beams need different equations.