Steel Beam Size Calculator

Formula used

This calculator uses standard beam theory for preliminary sizing.

Maximum bending moment

Simply supported, uniform load: Mmax = wL²/8

Simply supported, point load at distance a: Mmax = P·a·(L−a)/L

Cantilever, uniform load: Mmax = wL²/2

Cantilever, point load at distance a from fixed: Mmax = P·a

Required section modulus

LRFD: Sreq = Mu·kb / (phi·Fy)

ASD: Sreq = Ma·kb·Omega / Fy

kb is the bracing factor. Use 1.00 when fully braced.

Deflection check

The tool computes maximum deflection by numerically integrating y″ = M(x)/(E·I). Deflection limit is delta_allow = L/(deflection ratio).

How to use this calculator

1. Select a unit system and support condition.
2. Choose uniform load or point load.
3. Enter span, load magnitude, and load position if needed.
4. Set material properties, method, and deflection ratio.
5. Use bracing factor to reflect unbraced conditions.
6. Press Calculate to see required S and I above.
7. Review suggested sample sections and export results.

Steel Beam Sizing Article

1) What this calculator sizes

This tool estimates the minimum flexural section modulus and second moment of area for a steel beam under idealized loading. It is intended for rapid comparison during planning, takeoffs, and early design, before detailed code checks and connection work.

2) Loads and span drive demand

Bending demand increases sharply with span. For a simply supported uniform load, the peak moment scales with L² (Mmax = wL²/8). Doubling span roughly quadruples moment, so modest span changes can dominate the required beam size.

3) Strength basis: Fy and design method

Steel grade matters through yield strength (Fy). Typical structural values range from about 250 to 350 MPa (or 36 to 50 ksi). With LRFD, the calculator uses ϕ and checks Sreq = Mu·kb/(ϕ·Fy). With ASD, it uses Ω: Sreq = Ma·kb·Ω/Fy.

4) Serviceability: deflection limits

Many projects limit deflection to L/240, L/360, or L/480 depending on finishes and occupancy. Because deflection depends on stiffness, the calculator estimates Ireq by numerically integrating curvature, using E ≈ 200,000 MPa (29,000 ksi) unless you override it.

5) Bracing and lateral stability

Real beams can lose capacity through lateral torsional buckling when unbraced. To reflect reduced bracing, the bracing factor kb increases required S. Use kb = 1.00 for fully braced beams; use higher values (for example 1.10–1.50) for less favorable bracing assumptions.

6) Reading the recommended section list

The sample section shortlist compares each candidate’s Sx and Ix against your required values. “Meets S” indicates bending adequacy under the selected method. “Meets I” indicates deflection adequacy against your chosen L/… limit. Prefer sections that pass both with reasonable reserve, not extreme oversizing.

7) Practical workflow for field decisions

Start with realistic load takeoff values, then run a conservative deflection ratio such as L/360. Tighten assumptions (higher loads, higher kb, or stricter L/480) when sensitive finishes are expected. Export the CSV or PDF to document assumptions and compare alternatives with consistent inputs.

8) What to verify before final approval

Final design should confirm shear capacity, bearing, web crippling, vibration, camber needs, and connection forces. Also validate section properties from official steel tables and check stability requirements per your governing standard. Use this calculator as a fast screening step, not a stamped final design.

Use this estimator to shortlist beams, then complete checks.

FAQs

1) Does the tool include beam self‑weight?

No. Add self‑weight to your uniform load if it matters. You can approximate self‑weight after a first pass, then rerun with the updated w value for a tighter selection.

2) Which deflection ratio should I choose?

Common starting points are L/240 for basic members, L/360 for typical floors, and L/480 for sensitive finishes. Project specifications and occupancy often control, so follow the stricter requirement when uncertain.

3) What does the bracing factor change?

It increases the required section modulus to reflect reduced lateral restraint. If bracing is uncertain, use a higher factor to stay conservative, then refine once bracing spacing and detailing are confirmed.

4) Can I model multiple point loads?

Not directly. For preliminary sizing, you can convert several point loads into an equivalent uniform load, or size for the critical point load case and then verify a full loading diagram in a dedicated analysis tool.

5) Why are Fy and E separate inputs?

Fy affects strength (required S), while E affects stiffness (required I). Higher Fy can reduce Sreq, but it does not improve deflection much because E changes little across common structural steels.

6) Are the listed W‑sections complete?

No. The table is a small sample to demonstrate screening. For procurement and final design, use your regional steel manual or supplier tables and confirm the exact designation, properties, and availability.

7) Is this acceptable for permitting?

Typically no. Permitting usually requires a full code‑compliant design, sealed calculations, and detailed drawings. Use this for quick comparison, then finalize with a licensed engineer and verified section properties.