A method for determining the viscosity of a conveying fluid conveyed by a pump, in which an operating value is detected and is fed to an evaluation unit. The method includes providing a reference fluid, recording a reference performance curve resulting from the reference fluid during test operation of the pump at a predetermined operating parameter of the pump, recording an operating performance curve resulting from the conveying fluid during conveying operation of the pump at the predetermined operating parameter of the pump, determining the viscosity of the conveying fluid from a deviation of the operating performance curve from the reference performance curve by a viscosity correction algorithm stored in the evaluation unit.