Water meters have drastically improved through the years. Prior to some of these changes, the industry referred to a meter that could process flow rates of 1-20 GPM with a 13 psig pressure drop as a 5/8 meter. Therefore a meter that can handle 1-20 GPM with a ¾” NPT connection was referred to as a 5/8 X 3/4.
Some manufacturer’s still also provide a ¾ X ¾ meter, which typically can handle a 2-25 GPM flow with ¾” NPT couplings. In addition, this meter is a physically longer meter, 9.00”, compared to the standard 5/8 X 3/4 of 7.5”. It also is a considerably more expensive meter and isn’t used often unless the flow difference is required for the application.
To further confuse matters, the ¾” designation does not even reflect the actual inlet and outlet on the meter. It reflects the line size that the meter will connect to when installed with the fittings that are provided.