Torsional Constant I Beam Calculator

Calculator Inputs

Use consistent dimensions. The result automatically stays in the same base unit family.

Overall depth, d

Flange width, b_f

Flange thickness, t_f

Web thickness, t_w

Input unit

Formula Used

This calculator uses a standard open-section approximation for an I beam. It estimates the Saint-Venant torsional constant by summing the web and flange rectangle contributions.

J ≈ (1 / 3) [ 2b_ft_f³ + (d − 2t_f)t_w³ ]

Where:

d = overall beam depth
b_f = flange width
t_f = flange thickness
t_w = web thickness
d − 2t_f = clear web height

Keep every length in the same unit. Then the result becomes that unit raised to the fourth power.

How to Use This Calculator

Enter the overall depth of the I beam.
Enter the flange width for one flange.
Enter the flange thickness.
Enter the web thickness.
Select the unit used for every dimension.
Press the calculate button.
Review the result cards, detail table, and chart.
Download the result table as CSV or PDF.

Example Data Table

These sample cases use millimetres. Fillet effects are ignored in the approximation.

Case	Overall Depth	Flange Width	Flange Thickness	Web Thickness	Estimated J	Unit
Example A	300.00	150.00	12.00	8.00	219,904.00	mm⁴
Example B	450.00	200.00	16.00	10.00	685,466.67	mm⁴
Example C	600.00	250.00	20.00	12.00	1,655,893.33	mm⁴

Frequently Asked Questions

1. What does the torsional constant represent?

The torsional constant, J, measures Saint-Venant torsional stiffness. Larger J values usually mean less twist under the same torque, length, and material stiffness.

2. Is this exact for every I beam?

No. The calculator uses a common open-section approximation. It ignores root fillets, corner radii, and shape-specific refinements. Use manufacturer properties or detailed analysis for final design.

3. Which dimensions do I need?

Enter overall depth, flange width, flange thickness, and web thickness. Keep all dimensions in one consistent unit system so the result stays correct.

4. Which unit should I use?

Any consistent unit works, such as mm, cm, m, or inches. Because J has fourth-power units, the output becomes unit⁴.

5. Why is clear web height used?

The web contribution acts only over the clear web height. That height equals overall depth minus both flange thicknesses.

6. Can I use this for welded I sections?

Yes, when the welded section behaves like an open I shape and plate dimensions are known. Validate critical designs with code-based or finite-element checks.

7. What happens if depth is too small?

The entered geometry is impossible for this approximation. The calculator needs a positive clear web height, so overall depth must exceed twice the flange thickness.

8. Does a larger J always reduce twist?

Usually yes. For the same torque, member length, and shear modulus, a larger J reduces twist angle.