Viscosity Index Calculator (ISO 2909 / ASTM D2270)

Use at your own risk. Verify inputs, assumptions and results independently before applying them.

Formula notes and assumptions

The simplified v + 0.7 double-log relation requires reference viscosities above 0.3 cSt for real logarithms. This mathematical bound is not a guarantee of physical validity at low viscosity. Temperatures use T = t + 273.15 in kelvin. Extreme extrapolation may be rejected when the estimate cannot be represented.

The VI calculator follows ISO 2909 / ASTM D2270-style calculation and keeps the current useful range warning of VI 60 to 350.
The interpolation calculator uses the ASTM D341 Ubbelohde-Walther relationship with v + 0.7.
For formulas and concept background, see the TriboNet viscosity index article.

Input definitions

Kinematic viscosity at 40 C: Kinematic viscosity of the lubricant measured at 40 C. Unit: cSt or mm2/s.
Kinematic viscosity at 100 C: Kinematic viscosity of the lubricant measured at 100 C. Unit: cSt or mm2/s.
Viscosity index: Known viscosity index for the lubricant. Unit: dimensionless.
Kinematic viscosity at 40 C: Known kinematic viscosity of the lubricant at 40 C. Unit: cSt.
Kinematic viscosity at 100 C: Known kinematic viscosity of the lubricant at 100 C. Unit: cSt.
Reference temperature T1: First known reference temperature. Unit: deg C.
Viscosity at T1: Known kinematic viscosity at the first reference temperature. Unit: cSt or mm2/s.
Reference temperature T2: Second known reference temperature; must differ from T1. Unit: deg C.
Viscosity at T2: Known kinematic viscosity at the second reference temperature. Unit: cSt or mm2/s.
Target temperature T3: Temperature at which viscosity should be interpolated. Unit: deg C.

Worked example

Enter kinematic viscosity 68 cSt at 40 °C and 8.6 cSt at 100 °C: the displayed viscosity index is 97. The inverse KV100 mode gives 8.6 cSt for VI = 97 and KV40 = 68 cSt; the inverse KV40 mode gives 67.9 cSt for VI = 97 and KV100 = 8.6 cSt. The small round-trip difference reflects the implemented approximations and rounding. VI describes viscosity–temperature sensitivity, not lubricant quality or viscosity at the bearing’s operating temperature; use measured values and the applicable standard procedure for specification work.