Calculating the Cross-Sectional Area of a Pipe

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 d2

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 di2

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

and di = inside diameter (inches)

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

More information related to Calculating the Cross-Sectional Area of a Pipe:


The area of any shape is the amount of space it takes up in one plane of geometry. In simpler terms, the area can be thought of as the footprint left by a shape. Sometimes, as is the case with rectangular shapes, the area is calculated by multiplying the length of the shape by its width. A football field is 120 yards long (including the endzones) by approximately 53 yards wide. The area of a football field is 120 yards x 53 yards = 6,360 square yards. Notice that the units are also multiplied by each other (yards x yards = square yards).

Calculating the area of a triangular object takes our rectangular equation just one step further. Imagine drawing a line from one corner of that same football field to the opposite corner. The two shapes you have created are triangles, each one half of the total area. So, to calculate the area of a triangle, we simply take our same equation for rectangles (Length x Width) and multiply it by 1/2. Just to keep things straight, we often use the word "base" to describe the "width" and "height" to describe "length" when discussing triangles. So, the area of a triangle is: (1/2 base x height).

The area of a circle is a formula that takes a bit of memory. You may recall that the radius is the distance from the center of a circle to its outer edge, which is also the same as 1/2 of the diameter. The area of a circle is calculated as: (Pi x radius2). For example, if we have a circle that has a diameter of 20ft, we would figure out the area by first calculating the radius (1/2 x diameter = 10 feet) and then plugging it into our equation. In this case, the area is Pi (estimated as 3.14) x 10 feet x 10 feet = 314 square feet.

The Calculating the Cross-Sectional Area of a Pipe slide is featured in the following lessons:

Water Mains, Pipes & Valves

Duration: minutes