The surface area of a pipe represents the area of the pipe material that is exposed. You may want to find the surface area if you are painting a pipe so you can estimate how much paint you will need. To find the surface area of a pipe, you need to know the length of the pipe and the radius.
Measure the length of the pipe with the ruler and call the result L.
Measure the diameter of the pipe and call the result D. The diameter is the distance from one side of the pipe, through the center, to the other side of the pipe.
Plug in L and D into the following equation to calculate the surface area of the pipe: 3.14 x L x D. For example, if you had a pipe with a length of 20 feet and a diameter of 2 feet, you would get 3.14 x 20 x 2 and find that the surface area of the pipe equals 125.6 square feet.
TL;DR (Too Long; Didn't Read)
Make sure you are using the same units to measure the length and the diameter. For example, if you measure the diameter in inches, you must also measure the length in inches.