Water is money! The price of water is increasing all over the world; due to this the water industry wants the highest accuracies possible on the methods of flow measurement used. In large distribution networks, flow meters over 48 inches in diameter measure tremendous volumes of water and any errors are magnified. Meters in these considerable diameters are also a challenge to calibrate since there are few suitable facilities. This paper presents several calibration methods used, and their accreditation, to explain how volumetric flow measuring accuracies are achieved and certified.