## We use the inside diameter measurement of a pipe to calculate its cross-sectional area.

In Lesson 3, we learned how to calculate the area of a circle. Now, we'll use that equation to calculate the cross-sectional area of a pipe.

As you know, A = 0.785 d^{2}

Remember that pipes have both inside and outside diameters, which vary by material, pipe wall thickness, and size. To solve the cross-sectional area of a pipe, you'll need the inside diameter measurement.

Then, use the following equation:

A = 0.785 d_{i}^{2}

where: A = cross-sectional area of pipe (this is usually expressed in inches)

and d_{i} = inside diameter (inches)

Once we know the cross-sectional area, we can calculate the volume and flow of water in a pipe.