U.S. online sales from smartphones will account for 15% of total e-commerce sales by 2020 and tablets will account for 33%. That means mobile commerce will account for nearly half of total U.S. online sales five years from now.

Sales from smartphones and tablets will top $252 billion by 2020—up from $115 billion this year—representing a 17% compound annual growth rate over the next five years, Forrester says. For 2015, Forrester estimates U.S. sales from smartphones will total $35 million, sales from tablets will reach $80 million and  mobile sales will account for 35% of total U.S. e-commerce.

The report, entitled “U.S. mobile phone and tablet commerce 2015 to 2020”, also says that 47% of traffic to retailer websites comes from mobile devices but only 28% of sales. Other research from comScore Inc. finds that in 2015’s first quarter, 59% of U.S. digital shopping time occurred on smartphones and tablets. Additionally, Google Inc. said in May that more than half of searches in 10 countries, including the United States and Japan, now occur on smartphones.

So, why aren’t Forrester’s mobile commerce projections higher, given mobile’s shift toward becoming U.S. consumers’ primary computing device? One explanation is that the vast majority of mobile sales occur in three categories: apparel, consumer electronics and media. Those three categories make up two-thirds of mobile shopping, Forrester says. This, Forrester says, is leaving most shopping categories under-penetrated.

The Forrester report also finds:

• 77% of retailers have sites optimized for smartphones.
• Retailers are spending more on smartphone investments than tablet investments. Retailers on average invested $1.2 million in optimizing websites and apps for smartphones last year compared with $550,000 for tablets.
• 60% of smartphone owners  say they shop and buy on their smartphones and 71% of tablets owners  do the same on tablets. That’s compared to 52% for smartphone owners and 47% for tablet owners in early 2014.
•  Only 20% of smartphone owners and 30% of tablet owners say they have not been frustrated with shopping on their mobile devices.
• 38% percent of phone owners and 41% of tablet owners say they prefer using their desktop or laptop for shopping.
• Retailers continue to struggle with matching customers across devices in spite of better technology designed to enable cross-device tracking. Only 22% of retailers surveyed said they were able to match or track customer activity across devices. Companies like Datalogix and even Facebook can help businesses understand customer behavior across devices and channels, Forrester says.

To drive more mobile sales, Forrester suggests retailers and other companies marketing to mobile shoppers borrow what it calls mobile moments from other dominant players. For example, while consumers spend 85% of their time on phones in apps, they spend the majority of that time on only five apps, Forrester says.

“Retailers need to engage customers where they are, whether it’s on a mapping site, a search engine, or a retail aggregator,” the report says. Eat24, for example, offers takeout and delivery options with the Yelp app, Forrester says.

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