As the number of PCs in use around the world just passed the one billion mark, a research firm is estimating that the total will double in just another six years.

Gartner, based in Stamford, Conn, released a study Monday noting that the number of PCs in use worldwide is now growing at a rate of just under 12 percent a year. The firm is not counting the number of PCs shipped every year but the number of machines they believe to actually be in use.

Gartner analyst Meike Escherich forecasted in the report that just over 180 million PCs, or approximately 16 percent of the worldwide installed base today, will be replaced this year. A fifth of those being replaced, he noted, are expected to be dumped into landfills.

The double-digit growth in the installed base largely is coming from expanding emerging markets.

"Mature markets, such as the United States, Western Europe, and Japan, currently account for 58 percent of the world's installed PCs, but these markets only account for 15 percent of the world's population," said George Shiffler, a research director at Gartner, in a written statement.

"We expect per capita PC penetration in emerging markets to double by 2013. Rapid penetration in emerging markets is being driven by the explosive expansion of broadband and wireless connectivity in these markets and the continuing fall in PC average selling prices."