Process Mass Intensity (PMI) is the total mass of material used to produce a product of a given mass, determined in units of mass of input material (e.g., kg) per mass of product (e.g., kg). The PMI calculation starts with commonly available materials. 
PMI = [Total mass of material used in the entire process or step (kg)] / [Mass of product (kg)]
Relationship to E factor
E factor = [Total mass of waste produced in the entire process or step (kg)] / [Mass of the product (kg)]
= PMI – 1
In 2011, the Pharmaceutical Roundtable of the American Chemical Society (ACS) Green Chemistry Institute (GCI) selected PMI as the standard metric. The reasons for choosing PMI over other metrics such as E factor and atom economy are discussed in the paper.
Jimenez-Gonzalez, C. et al. Org. Process Res. Dev. 2011, 15, 4, 912–917. DOI: 10.1021/op200097d