Exhaust pipe diameter unknowm

I bought one straight piped but the pipe is rusted and to attach the muffler I need a new one but I can't seem to find the diameter of the pipe and since it's angle cut and rusted I can't really measure without removing it but I can't remove it then put it back on to go buy the new pipe because I'll have to cut off the rusted hangers as well and get new of those. Please help.