Enter the viscosity of the oil at 40°C, the viscosity of the oil at 100°C, and the viscosity of a reference oil at 100°C into the calculator to determine the Viscosity Index.

Base Oil Viscosity Index Formula

The following formula is used to calculate the Viscosity Index (VI) of a base oil. 

VI = (L - U) / (L - H) * 100

Variables:

  • VI is the Viscosity Index
  • L is the viscosity of the oil at 40°C
  • U is the viscosity of the oil at 100°C
  • H is the viscosity of a reference oil at 100°C

To calculate the Viscosity Index, subtract the viscosity of the oil at 100°C from the viscosity of the oil at 40°C. Then, subtract the viscosity of the reference oil at 100°C from the viscosity of the oil at 40°C. Divide the first result by the second result, and multiply the quotient by 100.

What is a Base Oil Viscosity Index?

The Base Oil Viscosity Index is a measure used to indicate the change in viscosity of lubricating oil with changes in temperature. It is a dimensionless number that represents how much the oil’s viscosity changes as temperature changes. A higher viscosity index indicates a smaller decrease in viscosity with increasing temperature, meaning the oil will maintain its lubricating properties better under high temperatures. This is crucial in applications like engines and machinery where temperature conditions can vary greatly.

How to Calculate Base Oil Viscosity Index?

The following steps outline how to calculate the Base Oil Viscosity Index:

  1. First, determine the viscosity of the oil at 40°C (L).
  2. Next, determine the viscosity of the oil at 100°C (U).
  3. Next, determine the viscosity of a reference oil at 100°C (H).
  4. Next, gather the formula from above = VI = (L – U) / (L – H) * 100.
  5. Finally, calculate the Base Oil Viscosity Index.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

viscosity of the oil at 40°C (L) = 50

viscosity of the oil at 100°C (U) = 20

viscosity of a reference oil at 100°C (H) = 10