I work for a utility with ~260km of gas pipe, ranging between 60 year old cast iron to brand new polyethelene pipe, and everything in between. We aim for <2% UAFG (unaccounted for gas) and our regulatory requirement is <2-4% depending on the network classification.

This is leakage from between our recieving gas and it being metered to the customer. On the customer side, there's another estimated 2-4% loss.

Get your appliances serviced regularly!