If swimsuit season hasn't gotten you exercising, maybe this will: Some airline-industry analysts predict that the cost of flying eventually could depend on your weight.

It's likely, they say, because of simple physics: The more weight you bring on the plane, the more fuel is needed to get you to your destination. And when fuel prices rise again, it might be time to hit the gym.

Robert Mann, an industry analyst and consultant in Port Washington, N.Y., envisions passengers being charged based on body and baggage weight, rather than simply by the seat.

"It's the way cargo flies; if something is twice the weight, you pay twice as much," Mann said. "It might be widely pooh-poohed, but if fuel goes back to being as expensive as it was, there will be some more of these public articulations."

How exactly such a plan would be implemented is unclear. One scenario would have people estimating their weight when buying a ticket on the Internet, then weighing in with their bags at the airport, presumably with a degree of privacy. Or airlines simply could go with the estimate.

Zeke Adkins, cofounder of Boston-based Luggage Forward, a luggage-delivery service, said a pay-by-weight system long has seemed inevitable, which is part of the reason he launched his company in 2004. It seemed even more likely when fuel prices rose and airlines began putting less water on planes, reduced the number of magazines they carried, and washed planes twice as often to reduce friction.

"I don't know it will happen in the next four or five months, but in the next year or 18 months, airlines will have to start taking this into account," Adkins said. "Remember, two years ago, charging for a bag seemed pretty far out there."

Rebecca Puhl, director of research at Yale University's Rudd Center for Food Policy and Obesity, warned that such a pricing model could stigmatize heavy passengers.

"If some kind of increased fee is going to be inevitable, they need to find ways to charge those fees without humiliating people," she said. "I just don't see how all this can be implemented in a way that is effective and fair."