That's in terms of sales, not screen size. The firm says shipments of the phone-tablet hybrids will reach 318 million in 2015, more than tablet and portable PC shipments.
Emerging markets are still showing interest in new tablets, but that isn't enough to sustain IDC's prior forecast for this year.
Market researcher revises its prediction for global PC shipments in 2014, but it still expects a decline.
The number of personal computers worldwide is expected to drop 6 percent this year, according to a study published Tuesday.
Increasing demand for big-screen smartphones means tablet sales could be lower than expected, says research firm IDC.
Microsoft's mobile OS will surge, after a fashion, amid slower growth for the smartphone industry as a whole, predicts research firm IDC.
Apple will hold onto to a "value edge" in 2014, but Android's ecosystem is poised to make big gains, according to IDC.
Apple is still the No. 1 tablet vendor, but its Android counterparts are really the ones pushing market growth, a new study shows.
But analysts warn that their report shouldn't be construed as a comeback for the global PC industry.
The research firm says tablet shipments should remain weak in the current quarter but pick up in the last three months of the year.