Industrial Lubricant ISO Viscosity Grades

What an ISO VG number means

ISO viscosity grades classify industrial liquid lubricants by kinematic viscosity at 40 °C. The grade number is the nominal midpoint in mm²/s (equivalent to cSt), while the standard class has an allowed range around that midpoint. The table shows the ASHRAE-published midpoint and limits.

For example, ISO VG 32 has a 32 mm²/s midpoint at 40 °C and a listed range of 28.8–35.2 mm²/s. The range describes viscosity at the reference temperature, not the lubricant’s behavior across all operating temperatures.

Do not confuse kinematic and dynamic viscosity

Kinematic viscosity is reported in mm²/s. Dynamic viscosity is reported in mPa·s and depends on density for conversion. A product sheet may show either value, so check the property name, units and test temperature before comparing products.

The ISO VG system does not identify grease consistency, viscosity index, base oil, additives or compatibility with a machine. A matching grade by itself does not confirm suitability.

Choose a grade for the equipment

Equipment makers specify viscosity based on operating temperature, load, speed, lubrication method and other conditions. Follow the machine’s current recommendation and lubricant supplier data. If the oil runs far from 40 °C, use a viscosity-temperature curve rather than assuming the ISO VG number remains the operating viscosity.

ISO 3448 remains the classification reference shown by ISO; the linked status page marks the 1992 edition as confirmed. Always verify the applicable edition and machine specification for a controlled application.

Read the tolerance range

Most listed grades use a band of ±10% around the nominal midpoint at 40 °C. For ISO VG 46, that gives 41.4 to 50.6 mm²/s. This tolerance applies to the standardized grade at its reference temperature; it is not an allowed variation at every operating temperature.

Adjacent grades have distinct ranges. If a product sits between two catalogue labels, use its measured viscosity and the equipment maker’s required grade rather than rounding to the nearest value.

Understand dynamic viscosity

Kinematic viscosity is related to dynamic viscosity through density. A product data sheet that reports dynamic viscosity in mPa·s cannot be compared directly with this table without density and a consistent temperature. The reference does not provide density or a conversion for individual oils.

ASHRAE describes ISO VG by the kinematic viscosity at 40 °C. A viscosity index or viscosity-temperature chart is needed to estimate how the oil changes as temperature changes.

Keep product chemistry and equipment in scope

Two oils with the same ISO VG can use different base stocks and additive packages. They may not be interchangeable where a machine requires a specific hydraulic, turbine, gear, compressor or circulating-oil specification.

Check compatibility, seals, contamination control and the equipment maker’s fluid approval. The grade table describes viscosity class only and does not recommend a brand or lubricant formulation.

Use a viscosity report correctly

When checking a lab result, confirm whether it is measured at 40 °C and reported as kinematic viscosity. A value at 100 °C, a dynamic-viscosity result or a viscosity index is a different property and should not be compared directly with the ISO VG midpoint.

This table lists the standardized grade classes only. Product-specific acceptance, oxidation condition, contamination limits and change intervals remain part of the equipment maintenance specification.

Source and scope

Source: ISO 3448 and ASHRAE Handbook, lubricant viscosity grades. Use the source edition and conditions stated in the table.