Stand back — who knows how big this thing is going to get? The Mac’s market share keeps on rising, according to reports from two different research group.

Apple experienced the strongest growth among the top five vendors in the U.S. PC market, according to the Gartner research group (http://www.gartner.com). Mac shipments increased 21.5% in the third quarter of 2011. The robust growth of the MacBook Air continued to lead Apple’s overall growth in the U.S. market, says Gartner.

Per the research group, Apple is now the third largest computer vendor in the US with 12.9% market share based on sales of 2.3 million Macs in the third fiscal quarter of 2011. That compares to approximately 10.8% market share on approximately 1.9 million sales in the year-ago quarter.

Ahead of Apple in the US rankings are HP (5.2 million sales, 28.9% market share) and Dell (3.8 million sales, 21.9% market share). Trailing Apple in the top five are Toshiba (1.5 million sales, 8.4% market share) and Acer (1.4 million sales, 7.8% market share).

According to the IDC research group (http://www.idc.com) total shipments increased more than 20% in 3Q11, recovering from a dip to 15% growth in the second quarter but otherwise continuing a trend of more than two years with over 20% growth. “The MacBook Air continues to boost volumes, and Apple’s position in driving changes in consumer expectations for devices also positions it favorably relative to other players and tight consumer spending,” says IDC.

Per this research group, Apple is also the third largest computer vendor in the US, though IDC says the company has 11.3% market share based on sales of 2.1 million Macs in the third fiscal quarter of 2011. That compares to approximately 10.5% market share on approximately 1.9 million sales in the year-ago quarter.

Ahead of Apple in the US rankings, according to IDC, are HP (5.3 million sales, 28.6% market share) and Dell (4.1 million sales, 21.9% market share). Trailing Apple in the top five, per IDC, are Toshiba (1.5 million sales, 8% market share) and Acer (1.4 million sales, 6.8% market share).

And keep in mind those totals don’t include the iPad, which some researchers think should be included in the computer total.

— Dennis Sellers