E-commerce spending in the United States will increase about 15.1% year over year in November and December—prime holiday shopping time—says eMarketer in a projection released today. Spending during those two months will total $61.8 billion, up from $53.7 billion in 2012.

During the entire fourth quarter of 2013, U.S. online shoppers will spend $83.2 billion, up 15.5% over the same period last year, the research firm says. eMarketer says it crafts its projections from some 60 sources that include U.S. Department of Commerce figures, estimates from other research companies, reported revenue from major online retailers and interviews with e-commerce vendors and e-retail executives.

The new eMarketer projection is roughly in line with a recent e-commerce spending estimate from comScore Inc., which monitors the online behavior of some 2 million consumers. Last month, it said second quarter 2012 e-commerce spending increased 15.5% year over year; for the fourth quarter of 2012, the most recent holiday shopping period, comScore says online spending increased 14% year over year, compared with 15% for the entire year 2012. The U.S. Commerce Department, which makes its estimates based on quarterly surveys of U.S. retailers, says Q2 online spending increased 18.4% year over year in the second quarter; for the fourth quarter of 2012, the department said e-commerce spending increased 15.8% year over year and 15.8% for the entire year.

The holiday e-commerce growth forecast by eMarketer would represent the smallest quarterly gain for online spending in 2013, according to the firm’s projections. Year-over-year e-commerce growth in the first quarter of 2013 reached about 16.3%; in Q2, 18.4%; and in Q3, 16.0%, (projected). For 2013 as a whole, the firm projects a 16.4% year-over-year increase in e-commerce spending. “Previous quarters of the year are expected to have higher growth rates than the fourth quarter, which account for the higher overall annual growth rate for 2013,” says eMarketer vice president Clark Fredricksen.

Mobile commerce stands to gain a larger share of U.S. e-commerce sales this year, eMarketer also predicts. M-commerce will account for nearly 16%, or $41.68 billion, of the $262.30 billion that online shoppers are expected to spend this year in the United States, the market research firm says. Earlier this year, eMarketer pegged m-commerce’s share at 15%.

“The revision comes as data sets from multiple research sources showed m-commerce as a percent of retail e-commerce sales reached record highs earlier this year,” the report says. “In addition, large retailers, such as Best Buy and Home Depot, continue to invest heavily in smartphones and tablets, building out their mobile app and web site offerings, and increasing spending on digital advertising.” Best Buy Co. is No. 10 in the Internet Retailer Top 500 Guide. The Home Depot Inc. is No. 46.

Consumers will do most of their mobile buying on tablets, the report adds. The devices will account for 62.5%, or $26.05 billion, of U.S. m-commerce this year, up from 56.2% last year. By 2017, tablets’ share will increase to 73%.