NLGI Grease Consistency Grades

NLGI grades classify grease consistency using worked cone penetration. The table gives the grade bands measured at 25 °C (77 °F), with penetration expressed in tenths of a millimeter. A higher penetration number means the cone sank farther into the sample, so the grease is softer.

Worked penetration by NLGI grade

What worked penetration measures

In the ASTM D217 method, a standardized cone penetrates a grease sample under specified conditions. “Worked” describes the sample after it has been conditioned in a grease worker; the NLGI ranges shown here are referenced to 25 °C. The result is reported in tenths of a millimeter (dmm), not whole millimeters.

The bands run from grade 000, which is very soft or fluid, through grade 6, which is extremely firm. The grade boundaries are classification bands. A grade is not a viscosity value and does not tell you the full performance of the lubricant.

Consistency is only one lubricant property

Greases with the same NLGI grade can use different base oils, thickener systems, additives, and manufacturing processes. Those differences affect temperature range, water resistance, oxidation stability, load carrying behavior, pumpability, and compatibility. NLGI 2 is common in many applications, but that does not make it the right choice for every bearing or machine.

Do not use the grade alone to select grease. Check the equipment manufacturer’s specification for base-oil viscosity, thickener chemistry, relubrication interval, operating speed, load, sealing, temperature, and compatibility with existing lubricant. Mixing incompatible greases can degrade performance or damage equipment.

Reading an example

NLGI grade 2 corresponds to worked penetration from 265 to 295 dmm at 25 °C. Grade 3 is firmer, with a range of 220 to 250 dmm. The penetration values themselves come from the test method and grade table; descriptive words such as “soft” or “firm” are only helpful comparisons.

Temperature and sample preparation affect the measurement. Do not compare a field observation or a different penetration test directly with these grade bands. For a product specification, use the grease manufacturer’s current data sheet and the applicable ASTM procedure.

Sources

The grade ranges are listed by the National Lubricating Grease Institute. The measurement method is ASTM D217-21, Cone Penetration of Lubricating Grease. The table describes consistency classification only; it does not recommend a lubricant for any particular machine.