pre_text: results of operations year ended december 31 , 2006 compared to year ended december 31 , 2005 the historical results of operations of pca for the years ended december 31 , 2006 and 2005 are set forth below : for the year ended december 31 , ( in millions ) 2006 2005 change .
post_text: net sales net sales increased by $ 193.4 million , or 9.7% ( 9.7 % ) , for the year ended december 31 , 2006 from the year ended december 31 , 2005 . net sales increased primarily due to increased sales prices and volumes of corrugated products and containerboard compared to 2005 . total corrugated products volume sold increased 0.4% ( 0.4 % ) to 31.3 billion square feet in 2006 compared to 31.2 billion square feet in 2005 . on a comparable shipment-per-workday basis , corrugated products sales volume increased 0.8% ( 0.8 % ) in 2006 from 2005 . shipments-per-workday is calculated by dividing our total corrugated products volume during the year by the number of workdays within the year . the larger percentage increase on a shipment-per-workday basis was due to the fact that 2006 had one less workday ( 249 days ) , those days not falling on a weekend or holiday , than 2005 ( 250 days ) . containerboard sales volume to external domestic and export customers increased 15.6% ( 15.6 % ) to 482000 tons for the year ended december 31 , 2006 from 417000 tons in 2005 . income from operations income from operations increased by $ 109.8 million , or 94.6% ( 94.6 % ) , for the year ended december 31 , 2006 compared to 2005 . included in income from operations for the year ended december 31 , 2005 is income of $ 14.0 million , net of expenses , consisting of two dividends paid to pca by southern timber venture , llc ( stv ) , the timberlands joint venture in which pca owns a 311 20443% ( 20443 % ) ownership interest . excluding the dividends from stv , income from operations increased $ 123.8 million in 2006 compared to 2005 . the $ 123.8 million increase in income from operations was primarily attributable to higher sales prices and volume as well as improved mix of business ( $ 195.6 million ) , partially offset by increased costs related to transportation ( $ 18.9 million ) , energy , primarily purchased fuels and electricity ( $ 18.3 million ) , wage increases for hourly and salaried personnel ( $ 16.9 million ) , medical , pension and other benefit costs ( $ 9.9 million ) , and incentive compensation ( $ 6.5 million ) . gross profit increased $ 137.1 million , or 44.7% ( 44.7 % ) , for the year ended december 31 , 2006 from the year ended december 31 , 2005 . gross profit as a percentage of net sales increased from 15.4% ( 15.4 % ) of net sales in 2005 to 20.3% ( 20.3 % ) of net sales in the current year primarily due to the increased sales prices described previously . selling and administrative expenses increased $ 12.3 million , or 8.4% ( 8.4 % ) , for the year ended december 31 , 2006 from the comparable period in 2005 . the increase was primarily the result of increased salary and .
table: For the Year Ended December 31,
table: <i>(In millions)</i> 2006 2005 Change
table: Net sales $2,187.1 $1,993.7 $193.4
table: Income from operations $225.9 $116.1 $109.8
table: Interest expense, net (31.2) (28.1) (3.1)
table: Income before taxes 194.7 88.0 106.7
table: Provision for income taxes (69.7) (35.4) (34.3)
table: Net income $125.0 $52.6 $72.4
question: Analyse this data from a financial earnings document. total corrugated products volume sold increased by how many billion square feet in 2006 compared to 2005?