Maximum permissible error (MPE)

The largest error a meter is allowed to show, positive or negative, under ISO 4064 and OIML R 49. For accuracy class 2 meters on water up to 30 °C, it is ±5 % in the lower zone (Q1 ≤ Q < Q2) and ±2 % in the upper zone (Q2 ≤ Q ≤ Q4).

MPE is the yardstick for every meter test, on the bench or in a dispute. A meter can be within its MPE and still under-record a little, which adds up across thousands of connections.