The National Lubricating Grease Institute (NLGI) is a not-for-profit organization that serves the lubricating grease industry. The NLGI consistency number, sometimes referred to as NLGI grade, refers to the relative hardness of a grease used for lubrication as specified by the institute's standards. NLGI 2 grade grease has a consistency similar to peanut butter.
Grease consistency is a major contributor (in combination with additive technology, base fluids, and types of thickeners) to the performance of a grease and how well it will execute lubrication tasks in specific applications.
NLGI consistency numbers do not provide enough information to determine whether a specific grease is applicable to any given undertaking - instead, these numbers should serve as a general guideline and should be taken in consideration with the structural and mechanical stability, viscosity, and specific additive technology of an exact grease.
The NLGI number system categorizes greases according to their hardness, from an NLGI No. 000 (very fluid) through No. 6 (very hard), with an NLGI No. 2 being the most common.
NLGI consistency should not be confused with viscosity. The viscosity and compatibility of the base oil is a major factor in what NLGI grease type to select, as well as additive and thickener compatibility.