Beam Load Capacity Calculator
Calculate the allowable bending moment a beam or I-beam can carry from its section modulus, material yield strength, and safety factor.
How this calculator works
A beam can fail two different ways: it can sag too much, or it can bend past what its material can take. This calculator checks the second one — the bending-strength limit — by comparing a section’s resistance to bending stress against the material’s yield strength, divided down by a safety factor.
It works for steel, wood, or any other material, as long as you supply the section modulus and the material’s yield strength.
Formula: Allowable moment = section modulus (S) × yield strength (Fy) ÷ safety factor. This is the bending-strength side of beam capacity — compare it against the Beam Span Calculator’s deflection side, since whichever governs is the limiting case.
Worked example
S = 21.4 in³ (a common wide-flange shape), Fy = 50,000 psi (50 ksi steel), safety factor 1.67 (standard ASD):
- Allowable moment = 21.4 × 50,000 ÷ 1.67 ≈ 53,393 ft-lb (after converting from in-lb to ft-lb)
Notes
This calculator uses a generic section modulus, so it works for a specific steel W-shape — read S off its spec sheet, since the number is publicly listed for every standard AISC shape — a built-up wood beam, or any custom section computed with the Moment of Inertia Calculator.
50 ksi (Fy = 50,000 psi) is a common structural steel grade, covering A992 and A572-50 steel, while A36 steel uses a lower yield strength of Fy = 36,000 psi.
How to use
Enter the section modulus for your beam shape, the material’s yield strength, and your safety factor. The calculator returns the allowable bending moment, which you can compare against the actual moment your beam will carry, or against the Beam Span Calculator’s deflection-governed span to find which limit controls your design.
This is a planning estimate using standard mechanics formulas, not a substitute for engineering design. Confirm actual member sizing and code compliance with a licensed structural engineer.
Frequently asked questions
What is allowable bending moment?
Allowable bending moment is the maximum bending load a beam can safely carry before its material stress reaches an unsafe fraction of its yield strength. It's the bending-strength side of beam capacity, distinct from the deflection (sag) side — a beam has to pass both checks, and whichever produces the lower limit governs the design.
What is a safety factor and why is 1.67 common?
A safety factor divides down the material's raw yield strength to leave a margin for uncertainty in loads, material variation, and modeling error. 1.67 is the standard factor used in Allowable Strength Design (ASD) for structural steel, meaning the allowable stress is the yield strength divided by 1.67.
Where do I find the section modulus for a steel beam?
Section modulus (S) is published for every standard AISC steel shape, such as W-beams, in manufacturer and industry reference tables, listed by shape designation. For a custom or built-up section, including wood beams, you can compute S from its width and height with the Moment of Inertia Calculator.
What's the difference between A992/A572-50 and A36 steel?
They're different structural steel grades with different yield strengths. A992 and A572-50 steel have a yield strength (Fy) of 50,000 psi, commonly called 50 ksi steel. A36 steel has a lower yield strength of 36,000 psi. A higher yield strength means a higher allowable bending moment for the same section modulus.
Estimates only. Verify quantities with your supplier before purchasing.