How to Calculate Plastic Modulus

Written by brendan conuel
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How to Calculate Plastic Modulus
The plastic modulus is used in beam theory as a theoretical tool designed to prevent building dilapidation. (collapsing farmhouse image by Adrian Hillman from

The plastic modulus (also known as the "plastic section modulus") is a theoretical tool used in structural engineering to quantify the strength of beams and how those beams deform under stress. It is based strictly on two-dimensional beam cross sections. The "plastic" in the name refers to the type of deformation to which the beams in question are prone -- in this case, deformation through irreversible ("plastic") processes. Different beam geometries exhibit different characteristic plastic modulus formulas. The higher the plastic modulus, the more reserve strength the beam has after stress-induced deformation has begun.

Skill level:

Things you need

  • Tape measure or beam documentation

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  1. 1

    Determine the geometry of the beam whose plastic modulus you are interested in. Common geometries you will often encounter include rectangular (including square), solid circular, hollow circular and I-beam.

  2. 2

    Measure the dimensions of your beam using a tape measure. Alternatively, you can look up these values if you have the beam's documentation handy. The most important values for calculating the plastic modulus of a beam are generally the width and height of the beam's cross section. If your beam has any unusual geometry (such as flanges in an I-beam), though, measure those dimensions as well.

  3. 3

    Apply one of the following formulas for the plastic modulus Z based on the geometry of the beam you're dealing with:

    Rectangular: Z = (b x h^2)/4

    where b is the width (or base) of the beam cross section and h is its height

    Solid circular: Z = (d^3)/6

    where d is the diameter of the beam cross section

    Hollow circular: Z = (d_2^3 - d_1^3)/6

    where d_2 is the outer diameter of the beam cross section and d_1 is the inner diameter

    I-beam: Z = (b_1 x t_1 x y_1) + (b_2 x t_2 x y_2)

    where b is the width (or base) of each respective flange in the beam cross section, t is the thickness of each respective flange, and y is the distance between the centre of mass of each respective flange and the centre of mass of the beam as a whole

    Plug the indicated values into the appropriate formula, and your plastic modulus will be given as Z.

Tips and warnings

  • You are only interested in the two-dimensional cross section of the beam (as if you were looking at the beam head on). Length is not a useful variable in calculating the plastic modulus.

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