The Masonic Service Association requests membership numbers from each Grand Jurisdiction.  Each Grand Lodge uses different metrics to determine who should be counted. 

For example, some Grand Lodges include Entered Apprentices in their overall numbers, some Grand Lodges count only the number of Master Masons in their jurisdiction, while others count each Master Mason in every lodge (plural memberships included).

While these metrics may not be exact or predictive, they are helpful in making educated generalizations about the state of our Fraternity.