Rectangle section properties
About the axis parallel to the breadth.
Method
I = bd³/12, Z = bd²/6. Worked case: 100 mm by 200 mm gives I = 66,666,667 mm⁴ and Z = 666,667 mm³.
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Logic: how this figure is built.
How to read a rectangle
Second moment about the axis parallel to the breadth is bd³/12. Elastic modulus is bd²/6. A 100 mm by 200 mm rectangle, bending about the strong axis, is 66.7×10⁶ mm⁴ and 6.67×10⁵ mm³. Swap breadth and depth and both numbers change, because depth is cubed.
The axis matters. A flat bar bent the wrong way is a different section. The page is a solid rectangle. A hole, a flange and a steel-table shape are not this product.
Units should stay in millimetres if you intend to compare with a stress in N/mm². An I in m⁴ and a moment in kN·m need a conversion before they become a stress.
Use it to check a modelled rectangular member. It is not a utilisation.
- Related: circle section
- structural logic
Last checked 3 October 2026. See the common-calculations note and the disclaimer.