Abstract: A method and system for determining the viscosity index of a petroleum liquid. The determination may be made on a single, discrete sample or on a continuous sample stream. A sample is fed into a first pressure drop device where a first pressure drop is measured. A first sample temperature and mass flow are measured at that point. A first density of the sample is determined from a sample mass flow meter so that a first kinematic viscosity can then be determined using the first sample pressure drop, temperature, and density. Then the sample temperature is changed to a second sample temperature and the sample is passed through a second pressure drop device. The second sample temperature, sample density and pressure drop are used to determine a second kinematic viscosity at the second sample temperature. The viscosity index of the sample is determined using the first kinematic viscosity and the second kinematic viscosity. The viscosity index of the sample is presented in digital form to system operators.