What the RSJ Beam Size Calculator Does
Choosing the correct RSJ beam size is the foundation of any safe structural alteration. Our free Beam Size Calculator helps you narrow down the most suitable universal beam section for your opening by combining your clear span, load width and the type of structure the beam must support. It translates basic site measurements into a recommended steel section such as 203×133×25 or 254×146×31, giving you a confident starting point when talking to suppliers or a structural engineer.
How to Use the Calculator
- Measure the clear span – Record the distance between the inside faces of the supports (walls or columns). Remember to add at least 150 mm bearing on each side when ordering the physical beam.
- Enter the load width – This is the width of floor or roof that transfers its weight onto the beam. If joists span 4 m and meet the beam at their midpoint, the load width is 2 m.
- Select the load type – Choose from typical domestic floor, roof or light commercial loads. The calculator applies sensible dead-load and live-load values aligned with UK Building Regulations.
- Review the recommendation – The tool suggests a beam size and shows whether it is likely to satisfy deflection limits for your span.
Why Beam Size Matters for Safety and Building Control
An undersized beam can sag, crack plasterwork, or—in the worst case—fail structurally. An oversized beam wastes money and may create installation headaches due to weight. More importantly, UK Building Control expects structural calculations for any load-bearing alteration. While this calculator gives you a realistic preliminary size, it is not a substitute for stamped drawings from a chartered structural engineer. Always obtain professional verification and Building Control approval before ordering steel or removing walls.