How to Calculate the Centroid

centroid

The centroid of an area is the geometrical center, i.e. the average of all of the points in an area.  It is always the same, regardless of how you turn the shape.

It is also the center of gravity of a three dimensional object.

Basic Shapes

The centroids of common shapes are shown below:

 

Shape Centroid
Square/Rectangle centroid of a rectangle
Right-Angle Triangle  centroid of a right angle triangle
Triangle  centroid of a triangle
Semi-circle  centroid of a semicircle
Quarter-circle  centroid of a quarter circle

Multiple Shapes

Most complex real-world geometry can be estimated by approximating from a hybrid of the basic shapes.  For two shapes:

$$\bar{x} = \frac{\bar{x_1}A_1 \cdot \bar{x_1}A_2}{A_1 + A_2}\newline\newline

\bar{y} = \frac{\bar{y_1}A_1 \cdot \bar{y_1}A_2}{A_1 + A_2}$$

For example, with an area that looks like this:

centroid of multiple shapes

The centroid of the shape, from the bottom left corner, is:

$$\bar{x} = \frac{\frac{b}{2}A_1 + (b + \frac{4r}{3\pi})A_2}{A_1 + A_2}\newline\newline

\bar{y} = \frac{\frac{b}{2}A_1 + (b + \frac{4r}{3\pi})A_2}{A_1 + A_2}$$

About Bernie Roseke, P.Eng., PMP

Bernie Roseke, P.Eng., PMP, is the president of Roseke Engineering. As a bridge engineer and project manager, he manages projects ranging from small, local bridges to multi-million dollar projects. He is also the technical brains behind ProjectEngineer, the online project management system for engineers. He is a licensed professional engineer, certified project manager, and six sigma black belt. He lives in Lethbridge, Alberta, Canada, with his wife and two kids.

View all posts by Bernie Roseke, P.Eng., PMP

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