To measure the size of an exhaust pipe, measure the outside circumference of the pipe and divide it by pi, then subtract the pipe's thickness to determine the inside diameter. Exhaust pipes are sold based off of their inside diameter.Continue Reading
When measuring an exhaust pipe without a micrometer or caliper, it is important to make sure that the pipe is measured at a straight section that is at least 4 inches away from any bends.
Find a straight section of the exhaust pipe that will be measured.
To find the outside diameter of the pipe, first use a tape measure and measure the circumference of the pipe by wrapping the tape measure around the pipe. Take this number and divide it by pi, or 3.14, to find the outer diameter of the pipe.
Finally, measure the thickness of the exhaust pipe. Multiply this number by two, because the thickness of the pipe was previously taken into account in two places, and subtract this number from the outside diameter that was found above. The resulting number will be the inside diameter of the pipe.