Q3 (permanent flow rate)

The highest flow rate at which a meter can run continuously in normal use while staying within its maximum permissible error. Q3, in cubic metres per hour, is the main figure used to name and size a meter, as in Q3 2.5.

Sizing by Q3 against real demand, rather than by pipe size alone, avoids meters that sit mostly at low flows where error is wider, or that wear early from constant overload.