By forfeiting more than 200 million vacation days that cannot be rolled over, American workers are giving up about $66.4 billion in lost benefits each year. For the average worker, that comes out to $604, the project said.

While most employees at least want to take full advantage of those hard-earned days off, less than half take the time to plan out their vacations each year, according to Project: Time Off , which is sponsored by the U.S. Travel Association. As a result, they end up burning valuable time.

It's only January, and workers are already on track to miss out on much of their vacation time this year.

Even now, people are still scrambling to put together a winter trip, said Samantha Brown, Project: Time Off's travel expert and host of the PBS "Places to Love" series.

"By the time we realize we want to go on vacation, it's either too late or really expensive," she said.

Uncertainty about work or personal schedules, in addition to difficulty coordinating child care, were most often to blame, according to the group's latest data.

Brown offers these tips to make the most of your 2018 vacation days:

Block your calendar, even if you are not sure of your plans just yet. Keeping the calendar clear creates the opportunity to take vacation.

Talk to your manager about your plans as far in advance as possible. The longer the lead time, the better chance of having your time off approved and coordinating coverage in your absence.

Plan for your bucket list — not just your to-do list. A long weekend has its place, but careful planning allows for longer breaks, making the most of your time away from work.

American workers use only 54 percent of their eligible vacation time on average, according to a separate report by jobs and recruitment website Glassdoor.

Last year, only 23 percent of employees who get paid time off took all of the time they were entitled to — and nearly 10 percent took no paid time off at all, Glassdoor said. The job site polled more than 2,000 adults in March and April.