PHILADELPHIA — 438 million.

That's the estimated number of vacation days Americans failed to take in 2007. Psychologists, demographers and others say we pass over time off for many reasons — everything from an entrenched Puritan work ethic, to fear of being seen as reckless slackers relaxing while the economy burns, to simply squirreling away time for days when we're stuck at home awaiting a repair call.

The figure for time left behind comes from Harris Interactive, the polling and research group, which for seven years has examined trends in unused vacation days for Expedia.com, the Internet travel booker. Expedia, along with just about every other travel-related business, overlays the findings onto the bottom line: If people aren't taking vacation days, they're not taking vacations.

Others also keep an eye on unused vacation. It's an issue for human resources managers, responsible for overseeing employee time off — especially in companies where workers can roll days over to a new year or bank them for a windfall on leaving the firm. Researchers on work-life issues, psychologists, and labor lawyers also deal in the subject. And, of course, employers grapple with it, even as they fail to take their own time off.

"I ask people at meetings all over the United States how many have had unused vacation time in the last year. Generally about a third of the hands will go up," Judy Randall says. Her North Carolina-based Randall Travel Marketing tracks and forecasts trends, and she takes her vacation, frequently in Philadelphia to visit a daughter.

"I think it's an epidemic. We take less of the pitiful amount of vacation time we have than anybody else on the planet. We are genuine to-the-bone workaholics, and even if it's killing us, we're still doing it."

The 438 million days are worth about $60 billion, Harris Interactive says, using average hourly wages for the tabulation. As with many extrapolations, you can argue with the figure — the Harris/Expedia poll also says that 35 percent of all American workers, or about 51 million people, are the people who scrap vacation days, on average three per person. By that accounting, the total number of unused days would be about 153 million.

The study then multiplies the entire American work force by three unused days, arriving at 438 million and change, and some time-management experts say they believe that's closer to reality. Even at the lower 153 million figure, we bypass enough time to fill more than 5,500 lifetimes of 75 years each.

At least we're doing better than in 2006, when the figure was four unused days per worker. The Harris/Expedia research last spring, among a nationwide cross-section of more than 4,000 American workers, asked about vacation plans for the year.