Measure the inside diameter and multiply it by 3.14159 (pi)
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Well, calculating the square footage of a pipe is a lot like painting a happy little tree. First, you measure the circumference of the pipe using a tape measure. Then, you use a simple formula to find the area of the pipe's cross-section: Area = πr^2, where r is the radius of the pipe. Just remember, there are no mistakes, only happy accidents in math and painting!
The formula for working out the circumference of a circle (like a pipe) is diameter times pi. (You should find pi on your calculator, if not, it is 3.14159265) Since you know the circumference, you divide the circumference (5inch) by pi (3.14159265), and you get the diameter, i.e. the size pipe. While the above formulas are correct, using this equation example will not tell you what size pipe you have. There are a number of factors involved in determining the correct answer. There are many different materials that pipe is made out of and some of these affect the dimensions of the pipe. As an example, the actual outside diameter of 1" copper pipe and 1" steel pipe are not the same. If you are talking about steel pipe, it is important to understand that all pipe 12 inch and under is referred to as "nominal" pipe size. In this case, the outside diameter of 12 inch pipe is actually 12.75 inches. For the original question asked above, the 5 inch circumference would equal 1-9/16" outside diameter which would be the equivalent of 1-1/4" pipe (nominal).
It is 47.1 inches.
The answer depends on what the "36 inch" measures: inside circumference, radius, diameter?
"4 inch" would usually refer to the diameter in such cases. You can multiply that by pi (which is about 3.1416) to get the circumference.