About SectionLab, the free section properties calculator

SectionLab computes the geometric and torsional properties of structural cross-sections and the elastic stresses produced by one set of section forces. Choose a shape, enter its dimensions in inches or millimeters, and results appear in your browser. Nothing you enter is uploaded.

What it calculates

• Area, centroid, and the second moments of area I_x, I_y and I_xy about the centroidal axes
• Principal moments of inertia I₁ and I₂ and the principal-axis angle
• Elastic section moduli S_x and S_y (top and bottom, left and right, and governing)
• Radii of gyration r_x and r_y and principal radii r₁ and r₂ (r₂ equals the AISC r_z for angles)
• St. Venant torsion constant J, including the true closed-section J for HSS
• Optional stresses from N, V_x, V_y, M_x, M_y and T: normal, shear and von Mises contours with max and min locations, the neutral axis and the shear center

Shapes

Rectangle and circle, I-section (W-shape) and channel (C/MC) with optional root fillets, equal or unequal angles with root and toe radii, rectangular and square HSS with corner radii, round HSS and pipe, and custom polygons, with vertices you can paste from Excel.

Method and verification

The finite-element engine is a JavaScript port of the open-source sectionproperties package (version 3.10.2). It uses 6-node triangular elements and the Triangle mesher compiled to WebAssembly. On identical meshes, its results match sectionproperties to within 1e-9 relative, and most quantities agree to about 1e-14.

Limitations

• Analysis only: SectionLab makes no design, code or allowable-stress checks.
• Linear-elastic, homogeneous sections with one load combination at a time.
• Shapes are built from the dimensions you enter. There is no built-in steel shape database, so compare with published tables where needed.

Frequently asked questions

How does SectionLab calculate the torsion constant J?

It solves the Saint-Venant warping function over a finite-element mesh of the section. This gives an accurate J for open, closed and custom shapes instead of relying on thin-wall approximations.

Can I use millimeters?

Yes. Choose in or mm, and every input and result is labelled with its units.

Does SectionLab check AISC or other design codes?

No. It reports section properties and elastic stresses only. Design checks must be done separately by the engineer of record.

Can I save the results?

Yes. Export a self-contained HTML report with the inputs, drawing, results, stresses and warnings, then print it to PDF from your browser. The report includes a timestamp and an input SHA-256 for traceability.

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For educational and preliminary design purposes only. Always verify results with a licensed PE.