I-Beam Calculator
An I-Beam's Cross-Section Is Three Rectangles, Not One
An I-beam (or W-shape) gets its efficiency from putting most of its material in the two flanges, far from the bending axis, connected by a comparatively thin web. Working out its weight means adding up three separate rectangular areas — two flanges and one web — rather than treating it as a single solid shape.
The Formula
Weight = Area × Length × Density
The calculator doubles the flange area (since there's an identical flange top and bottom) and adds the web's area — its height times its thickness — to get the total cross-sectional area of the "I" shape. That area, converted to square feet or square meters, is multiplied by the beam's length and by steel's standard density (490 lb/cu ft, or 7,850 kg/m³) to get the total weight.
Where This Calculation Matters
- Structural steel takeoffs — estimating the weight of I-beams and W-shapes from their dimensions lets you cross-check a mill certificate or supplier quote against the beam's actual geometry.
- Custom or built-up sections — non-catalog beam dimensions, common in retrofit or repair work, don't appear in a standard steel shapes table, so calculating weight directly from measured dimensions is the only option.
- Load and lifting planning — crane and rigging capacity for setting a structural beam depends directly on its weight, which needs to be known before the lift is planned.
Worked Example
A 20 ft beam with a 5.25 in flange width, 0.33 in flange thickness, 7.34 in web height, and 0.23 in web thickness — representative proportions for a small structural I-beam:
| Step | Calculation | Result |
|---|---|---|
| Cross-sectional area | 2 × (5.25 × 0.33) + (7.34 × 0.23) | 5.15 in² |
| Area in sq ft | 5.15 ÷ 144 | 0.0358 sq ft |
| Weight | 0.0358 × 20 × 490 | 350.7 lb |
How to Use This Calculator
- Select Imperial (in, ft, lb) or Metric (mm, m, kg) units.
- Enter the Flange Width and Flange Thickness.
- Enter the Web Height and Web Thickness.
- Enter the beam's Length.
- Select Calculate to see the total beam weight.
Related Calculations
Checking how much that beam will deflect under load? The Structural Beam Calculator covers deflection, and the Steel Weight Calculator handles simpler bar and pipe stock.