It depends on whether your are looking for the inner or the outer circumference of the pipe. In both cases you need to use the formula: circumference = pi * diameter. If it is the inner circumference you would use 5", if it is the outer one, you use 5.5". You can use either 3.14 or 3.1416 for pi, depending on the degree of precision you are requiring (using a calculator with a value for pi in memory should give you even greater precision). Since your diameter is already in inches, you won't require to do any further conversions. Check the link, where the formula for the circumference is given.