On this page you will find several calculators to calculate viscosity index or viscosity depending on the temperature. You can also find a tool to interpolate the viscosity of lubricants based on the given viscosities at 2 temperatures. Read more about viscosity index here.
Here is a simple VI calculator to calculate viscosity index from the temperatures at 40°C and 100°C:
This calculator allows calculating kinematic viscosity at 100°C from the known VI and kinematic viscosity of the oil at 40°C:
This calculator allows calculating kinematic viscosity at 40°C from the known VI and kinematic viscosity of the oil at 100°C:
The common equation used to calculate the viscosity by interpolation between two reference points is with the Ubbelohde-Walther equation, which is adopted in ASTM D341. Here is a simple calculator to interpolate viscosity: