As the television upfront market gets underway, media buyers are busy putting together ratings estimates for the coming season, but there will be a big and growing chunk of TV viewing they won't be factoring in.

That's a goodly part of the viewing of 113 million adults who are exposed to television in locations outside their homes each week, whether at a tavern or a hotel or increasingly at work.

That's up from 86 million people only four years ago, according to the latest study from research company Total TV Audience Monitor.

Most of this viewing is not picked up by Nielsen Media Research, meaning many of these viewers are not included in the ratings media buyers and sellers use in negotiations.

T-TAM found that 26 million people in the 18-49 demographic are watching TV outside the home each week, and of that figure 4 million only watch TV outside the home, never at home.

“I think advertisers realize there is more frequency to their message because there are additional viewers outside the home. But they don’t realize that so many of these are additional viewers," says Lynnae Psaras, vice president of T-TAM.

"In the 18-49 demographic, for example, 16 percent of the out-of-home viewers are new viewers available only outside the home in an average week.”

In its study T-TAM had 4,000 adults keep track of their viewing in a diary log last October, and it found that 20 percent of adults 18-49 watched some TV outside the home in an average week and that 17 percent of all viewers watched at locations other than their homes.

These findings are in line with early test results from radio ratings service Arbitron’s multimedia measurement tool the Portable People Meter, which found that as much as 15 percent of TV viewing takes place outside the home.

Daytime television sees the biggest boost in viewing among adults 18-49 from OOH TV viewing, according to T-TAM, with a 9 percent increase over the in-home-only audience.

The overall increase in 18-49s on an all-day basis is 5 percent, with primetime up 4 percent and early morning up 6 percent.

About 45 percent of OOH TV viewing tracked by T-TAM takes place at colleges, while 27 percent is watched at work, and 10 percent takes place in bars and restaurants. Hotels account for 7 percent of this viewing and vacation homes account for another 5 percent.

“The workplace is becoming more important as an out-of-home TV location,” says Psaras. “We’re seeing that 27 percent of the time that 18-49s are watching outside the home that they’re watching in the workplace, but that was only 19 percent last year.”

Meanwhile, Nielsen will soon begin measuring OOH TV viewing, which will come on top of new DVR ratings that media buyers and sellers only two weeks ago agreed to use for the first time in upfront negotiations, and average-commercial-minute ratings that will start coming out later this month.

The only OOH TV viewing Nielsen currently measures is college students living away from home, which it began including in its TV ratings in January.

But in September Nielsen will begin tracking OOH TV viewing in a move that will likely have broadcast and cable networks charging advertisers for at least some of these viewers in next year’s upfront.

Nielsen in April partnered with Integrated Media Measurement to measure OOH TV viewing though a cell phone that will be carried by about 3,000 people. These ratings will start coming out in September as a supplement to Nielsen’s TV ratings.

Nielsen is also testing two of its own devices called Go Meters to measure OOH TV viewing, one a cell phone and the other an MP3 player-type device. This will be part of Nielsen’s Anytime Anywhere Media Measurement initiative.