Amazon rarely discloses important business performance details beyond what it has to. But over the past two years, it has disclosed some high-level information on the size of its Amazon Prime member base, the centerpiece of its online retail machine. And with that information, we can start to get a little bit closer to guessing how many Prime customers Amazon has — a riddle that analysts and reporters have been trying to solve for some time.

Here’s the not-so-difficult math. In late 2013, Amazon said “tens of millions” of people had a Prime membership — so, at a minimum, there were 20 million of them at the time. Last year, Amazon said Prime membership grew 53 percent, giving us a minimum of 30,600,000 members worldwide. And on Thursday, Amazon reported that paid Prime membership grew 51 percent in 2015, giving us a minimum of 46,206,000 Prime members worldwide.

Okay, so what? Well, this information is the closest thing we have to real information about Amazon’s most important retail revenue driver. Prime customers shop more frequently and spend more than non-Prime members do, and Amazon is spending a ton of money to make Prime more attractive. What started out as a program that simply offered unlimited two-day shipping in the U.S. has expanded to include a wide array of perks such as two-hour delivery in some cities and free music and video streaming.

(Note: It is quite possible that 46 million is significantly smaller than Amazon’s actual numbers. For instance: Last September, RBC analyst Mark Mahaney, extrapolating from a survey his team conducted, estimated that Amazon could have 60 million to 80 million Prime subs worldwide.)

It’s safe to say Amazon also generates billions in revenue alone from Prime membership fees each year, though the varied cost of the program in different regions makes a more definitive calculation difficult. In the U.S., the program costs $99 a year for adults and half of that for students. In the U.K., Prime costs 79 British pounds, or $112, and members get unlimited one-day delivery, compared to two-day delivery in the U.S. In Germany, the fee is 49 euro a year, or $53 at current exchange rates. You get the idea.

One more way we can use the 46 million number: We can do that annoying thing where people compare the size of a tech company’s user base to the population of a country. Like so:

Kenya: 47,251,000

Spain: 46,423,064

Amazon Prime: 46,206,000

Argentina: 43,590,400

Ukraine: 42,774,605