What Size Steel Beam Do I Need? A Quick Span Guide

"What beam do I need for a 4-meter span?" is probably the most common question in small steel construction — and the honest answer is always "it depends on the load." But it depends is not a useful starting point. This guide gives you defensible first estimates, shows the math behind them, and tells you exactly what to check before ordering steel.

Everything below is preliminary sizing for a simply supported beam. Final design must be verified to Eurocode 3 (or your local code) by a qualified engineer.

Rule of thumb #1: depth from span

For ordinary floor beams, the section height ends up around span/20 to span/25:

3 m span → roughly 120–160 mm deep
4 m span → roughly 160–200 mm
6 m span → roughly 240–300 mm
8 m span → roughly 330–400 mm

This works because deflection — not strength — governs most floor beams. Long before steel is at risk of failing, the beam is too bouncy and cracks the ceiling below. Codes cap deflection at L/360 for that reason, and depth is what controls deflection.

A worked sizing table

Assume a simply supported beam carrying a uniform load of 10 kN/m (about right for a light residential floor: one meter of joist spacing, dead plus live load), steel S235, deflection limit L/360. The lightest IPE that passes:

3 m span → IPE 160 (15.8 kg/m) — required Ix ≈ 600 cm⁴, IPE 160 has 869 cm⁴
4 m span → IPE 200 (22.4 kg/m) — required ≈ 1,430 cm⁴, available 1,943 cm⁴
5 m span → IPE 240 (30.7 kg/m) — required ≈ 2,790 cm⁴, available 3,892 cm⁴
6 m span → IPE 270 (36.1 kg/m) — required ≈ 4,820 cm⁴, available 5,790 cm⁴
8 m span → IPE 330 (49.1 kg/m) — required ≈ 11,430 cm⁴, available 11,770 cm⁴

Bending stress in every case lands around 95–115 MPa — comfortably under the S235 design limit — which confirms that deflection, not strength, made the decision.

The math, if you want to reproduce it: required Ix = 5 × w × L³ × 360 / (384 × E), with E = 210,000 MPa. Note the cube on span: doubling the span needs eight times the stiffness. This is why "one size bigger, to be safe" works for load uncertainty but never fixes an underestimated span.

What changes these numbers

More load — point loads (a column landing on the beam), partition walls, storage: required Ix scales directly with load.
Wider joist spacing — 2 m of tributary width doubles the line load. Scale accordingly.
Cantilevers — deflect about 16× more than the same simply supported span. Rules of thumb do not transfer; calculate.
Height limits — if an IPE 270 does not fit, an HEB 220 gives similar stiffness in 50 mm less height (at +98% weight). See our IPE vs HEA vs HEB guide.
Long unbraced spans — lateral-torsional buckling can govern before deflection does; brace the compression flange or check it explicitly.

Verify your exact case in two minutes

Open the free beam deflection calculator
Pick the profile (I-beam) and enter its dimensions from the profile tables
Enter span, support type, and your real load
Read deflection, stress, and the L/360 verdict instantly

Try the size from the table above, then the one below it — if the smaller one passes with margin, you just saved money; if it fails, you know why.

Preliminary estimates only. Confirm the final section with a structural engineer — including shear, buckling, bearing, and connection checks that no quick guide can cover.