Fluid Properties · DWG LS-007

Viscosity index calculator & VI comparator

Viscosity–temperature curves for two fluids using the Walther / ASTM D341 double-log model. Enter base-oil viscosity at 40 °C and 100 °C (or pick from inventory); VI is auto-calculated and both curves are plotted with the operating-temperature point highlighted.

Published standard (ASTM D2270, ASTM D341)Last reviewed

Source: ASTM D2270 (astm.org) · ASTM D341 (astm.org)

Curve fits stand in for the D2270 reference tables.

Validation
  • Checked against independent ASTM D2270 and D341 calculations: VI 89, 143, 94, 102 and 55 for five test oils match D2270, and the curves match the D341 reference at 72, 104, 150 and 212 °F for four oils. The calculation code has not changed since.

How calculators are checked: editorial standards.

VI Comparator
Fluid 1 — choose oil or grease

Stocked mill products sort first. Clear selection to type KV40 / KV100 manually.

Fluid 2 — choose oil or grease

Stocked mill products sort first. Clear selection to type KV40 / KV100 manually.

Method, worked example & FAQ

How this calculator works

Viscosity index comes from the kinematic viscosities at 40 °C and 100 °C using the ASTM D2270 method. Curve fits stand in for the standard's reference L and H tables. VI is only calculated when KV40 is greater than KV100 and both are at least 2 cSt.

Each curve is the ASTM D341 (Walther) relation log₁₀(log₁₀(ν + 0.7)) = A − B·log₁₀(T), with T in kelvin, fitted through your two points. The chart spans the operating temperature ±80 °F on a log viscosity scale and marks each fluid's viscosity at the operating temperature. If no operating temperature is entered, 100 °F is used.

Picking an oil from the catalog fills in its KV40, KV100 and VI. When the catalog has no KV100, it is estimated from KV40 using the listed VI, or VI 95 if none is listed. Picking a grease uses its base-oil KV40 and estimates KV100 assuming VI 140 for a synthetic base or VI 95 otherwise. The curve then shows the base oil only.

Inputs explained

  • Fluid 1 and Fluid 2: choose oil or grease, pick a product from the catalog (stocked products first) or clear the selection to type values by hand.
  • Base oil viscosity at 40 °C and 100 °C (cSt): the two points that set VI and the curve.
  • VI: calculated automatically.
  • Operating temperature (°F): where the chart marks each fluid's viscosity. Defaults to 100 °F when blank.

Worked example

Two ISO VG 220 oils compared at 150 °F. Fluid 1 has 220 cSt at 40 °C and 19 cSt at 100 °C. Fluid 2 has 220 cSt at 40 °C and 25 cSt at 100 °C.

Calculator output: Fluid 1 VI 97, 62.0 cSt at 150 °F · Fluid 2 VI 143, 72.9 cSt at 150 °F. The higher-VI oil holds more viscosity as it warms.

Frequently asked questions

How is viscosity index calculated?

This calculator applies the ASTM D2270 viscosity index method to the kinematic viscosities at 40 °C and 100 °C, using curve fits for the standard's reference L and H values. KV40 must be greater than KV100, and both must be at least 2 cSt. For example, 220 cSt at 40 °C and 19 cSt at 100 °C gives VI 97.

How do I find oil viscosity at another temperature?

Enter KV40, KV100 and an operating temperature in °F. The calculator fits the ASTM D341 (Walther) relation log10(log10(ν + 0.7)) = A − B·log10(T), with T in kelvin, through your two points and reports the viscosity at that temperature. If the temperature is blank, 100 °F is used.

Can I compare grease base oils?

Yes, but only the base oil. Picking a grease uses its base-oil KV40 and estimates KV100 assuming VI 140 for a synthetic base or VI 95 otherwise. Grease apparent viscosity also depends on shear rate and thickener, so the chart does not predict in-service grease flow.