By CLAIRE CAIN MILLER

As stores lure shoppers with Black Friday deals as early as Thanksgiving Day, many shoppers are planning to stay on their couches and shop online.

Online sales will increase 15 percent this holiday season to $37.6 billion, according to predictions by comScore, which measures Web use. That is up from 12 percent last year, though still not in the 20 percent growth range that online retailers saw during the holidays before the 2008 financial crisis. Already in the first 20 days of November, online shoppers have spent $9.7 billion, up 14 percent from last year.

Forrester Research, a technology research firm, also predicted that online sales would increase 15 percent over the holidays. Offline stores, meanwhile, are on track to see only a 2.8 percent increase in sales, less than the 5.2 percent increase last year, according to the National Retail Federation.

More shoppers are going online in large part because of convenience and competitive pricing, said Gian Fulgoni, chairman of comScore.

“The fact that this economy isn’t getting better is increasing the shift from in-store to online,” Mr. Fulgoni said. “Basically you can get better prices on the Internet. Price comparison is part of it, a lot of retailers are pricing products lower on the Internet and in many places shoppers are not paying state sales tax.”

Last year, half of shoppers told Forrester they shopped less offline over Thanksgiving weekend in favor of shopping online, and half said they found better deals online. Free shipping is crucial: 40 percent of shoppers told comScore they would abandon an online purchase without it.

Mobile phones and tablets are also driving online spending this year. People are increasingly relying on them for browsing and researching items before going to stores. But once in stores, they are also using them to compare prices and making purchases on their mobile devices if they find a better deal.

“It’s been called a retailer’s worst nightmare,” Mr. Fulgoni said.