Twitter co-founder Biz Stone told the Reuters Global Technology Summit on Monday that unless Twitter’s growth rate slowed, by the end of the year its user base would exceed the population of Planet Earth.

Break out those calculators, nerds!

Let’s put Biz’s admittedly tongue-in-cheek statement to the test, using some back-of-the envelope math and the much-hyped, newly launched online knowledge compendium Wolfram Alpha.

Estimates of Twitter’s user base vary widely. Let’s take the frequently cited figure of 10 million, and double it to 20 million, since we’re feeling generous and Oprah is involved.

Biz cited a week-by-week compound growth rate that would exceed the Earth’s population by the end of the year.

We’re in week 21 of 2009 — that leaves 31 more weeks. We plugged the compound growth formula into Wolfram Alpha, and voila!

Twitter would have to grow its users by an astounding 20.5 percent a week to exceed Earth’s population by the end of the year.

As Biz himself admits, this is impossible.

“Obviously there’s going to have to be some changes in growth,” he said before unleashing another metaphor:

“When there’s a baby growing in the womb, if it grew as fast in the next months, it would be as tall as Mt. Everest. Sign-ups and that sort of thing will have to taper off, but that enables us to look at other areas of opportunity and growth.”