Proposed and proven is a correction for filer equation for hydraulic machinery accounting for viscosity influence. It is shown, that for existing centrifugal pump types design parameters of impeller can change from 1 up to 4 mm, which, considering the width of impeller＇s channels of 3 - 33 mm, amounts to significant value, which should be accounted for during calculations. Presented are calculation results of theoretical head for different thickness of displaced boundary layer.