U.S. e-commerce sales totaled $50.27 billion in the first quarter, up 15.4% from $43.58 billion in the first quarter of 2011, according to an estimate released today by the U.S. Commerce Department. It is the first time e-commerce sales have topped $50 billion outside the fourth quarter, the busiest quarter for online sales because of holiday shopping.

The estimate, not adjusted for seasonality, says e-commerce accounted for 4.9% of total retail spending in the quarter, up from 4.6% a year ago. When excluding sales in categories that cannot be bought online—automobiles, fuel and foodservice sales—Internet Retailer calculates that e-commerce accounted for 7.3% of total retail sales during the quarter, up from 6.8% in the first quarter of 2011. Effective with this report, Internet Retailer has changed the equation it uses to calculate the percentage of sales e-commerce accounts for compared with total retail sales. Internet Retailer now excludes only retail categories that cannot be bought online as outlined above. Previous reports excluded sales for categories not commonly bought online, notably groceries.

This change means sales attributed to food and beverage stores are now included in the calculation, as grocery shopping can be done online in many areas. Consumers last year spent an estimated $479 million with online grocer Peapod LLC, No. 52 in Internet Retailer’s 2012 Top 500 Guide, for example.

On a quarter-over-quarter basis, e-commerce sales declined 18.6% from Q4 2011, according to the Commerce Department. That decline is in line with previous years’ decreases as sales slow after the holiday sales season. 

On a seasonally adjusted basis, the Commerce Department says e-commerce accounted for $53.16 billion of the $1.08 trillion of total retail spending during the first quarter; that’s equal to 4.9% of total retail spending. Q1 2012 e-commerce sales increased 15.4% year over year on the seasonally adjusted basis, whereas total retail sales grew 6.5%.

Web measurement firm comScore Inc. earlier this month said online retail sales totaled nearly $44.3 billion during Q1, up about 16.6% from $38.0 billion it reported for the same period in 2011. Commerce Department estimates are based on a quarterly survey of more than 11,000 U.S. merchants. ComScore makes estimates of e-commerce sales based on monitoring the online behavior of some 2 million consumers who agree to be tracked, about half of them in the United States.