Modern flow meters used for natural gas custody transfer offer very high accuracy and repeatability — typically 1% of mass flow for orifice meters, and often better for linear meters. These specifications are usually verified in a flow laboratory. Unfortunately, even with a wellinstalled and maintained meter, every flow technology provides worse performance in the “real world.” This paper presents the key reasons for this deviation between laboratory and real world performance, and tools that allow the user to quantify expected deviations prior to installation. Finally, “best practices” are detailed which will allow the user to minimize the difference between laboratory and real-world performance