Single-number rating of how much viscosity changes with temperature; high VI oils (≥ 100) stay fluid at low temperature and film-protective at high temperature.
Formula
$$ VI = \frac{L - U}{L - H} \cdot 100 $$
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $L$ | Low-VI reference 40 °C viscosity | cSt | ASTM D2270 table |
| $H$ | High-VI reference 40 °C viscosity | cSt | ASTM D2270 table |
| $U$ | Sample viscosity at 40 °C | cSt | input |
| $VI$ | Viscosity index | - | result |
Sources
- ASTM D2270.