If you design pipes, you have a few options to create it in Inventor depending on what you want as an output. Do you just want the aesthetic look of the pipe because the pipe engineer has already established the length of the pipe before asking you are asked to design it in 3D or do you need to design it in 3D after the configurations have been established and then get the length of the pipe.
If looking at a simple design of pipes running along a length and then having a u bend to return the pipe to the other side for a cooling effect, you could add up all the lengths of the pipe (which would probably be of equal length) and then work out what the the length of the u bend is and then add it up. Manual calculations for the win! but really tedious. we have inventor so why do we not use the technology at hand.
Inventor has a few piping options. The tube and pipe module where you could have bent pipe and then you run it along and you will be able to get the length automatically.
The sweep function is also a useful tool but you cannot get the length out..or can you? In comes iLogic. By utilising the power of the APU and iLogic you can output the length of the sweep that you created.
Like I said earlier, use the technology at hand to its fullest. Life is too short for manual calculations.