ISO 2909:2002 describes two procedures for calculating the viscosity index (VI) of petroleum products and related materials, such as lubricating oils, from their kinematic viscosities at 40 °C and 100 °C.

Procedure A is applicable to petroleum products of viscosity index up to and including 100.

Procedure B is applicable to petroleum products of viscosity index 100 or greater.

ISO 2909:2002 does not apply to petroleum products with kinematic viscosities less than 2,0 mm²/s at 100 °C. It applies to petroleum products with kinematic viscosities between 2 mm²/s and 70 mm²/s at 100 °C. Equations are provided for calculating the viscosity index of petroleum products having kinematic viscosities above 70 mm²/s at 100 °C.