ATLANTIC CITY, N.J. >> As legal sports gambling proliferates, the number of Americans betting on the Super Bowl and the total amount they’re wagering is surging — although most of the action is still off the books.

An estimated 1 in 5 American adults will make some sort of bet, laying out a whopping $16 billion, or twice as much as last year, according to an industry trade group.

Even as legal gambling has spread to two-thirds of U.S. states, independent analysts say only about $1 billion of the total being wagered on today’s game will happen through casinos, racetracks or companies such as FanDuel and DraftKings.

The vast majority of people, in other words, are still betting with friends and family, participating in office pools or taking their chances with a bookie.

More than 50 million American adults are expected to bet on the game between the Eagles and Chiefs, according to the American Gaming Association, whose estimates are based on a nationwide online survey of 2,199 adults. That’s an increase of 61% from last year.

Experts in addiction say aggressive advertising is contributing to a rise in problem gambling.

“As sports betting expands, the risk of gambling problems expands,” said Keith Whyte, executive director of the National Council on Problem Gambling.

Thirty-three states, plus Washington, D.C., now offer legal sports betting, and more than half of all American adults live in one of those markets.

But legal sports betting still represents just a small piece of the pie.

Eilers & Krejcik Gaming Research, an independent analytics firm in California, estimates that just over $1 billion of this year’s Super Bowl bets will be made legally. The leading states are: Nevada ($155 million); New York ($111 million); Pennsylvania ($91 million); Ohio ($85 million) and New Jersey ($84 million.)

The research firm estimates 10% to 15% of that total would be wagered live after the game begins. Another 15% to 20% would come in the form of same-game parlays, or a combination of bets involving the same game, such as betting on the winner, the total points scored and how many passing yards Eagles quarterback Jalen Hurts will accumulate.

As legal sports betting grows, so too has concern about its effect on people with gambling problems.

The National Council on Problem Gambling has conducted nationwide surveys since 2018, when New Jersey won a U.S. Supreme Court case clearing the way for all 50 states to offer legal sports betting. They ask questions like, “Do you ever borrow money to gamble?”

Between 2018 and 2021, the number of people whose answers indicated they were at risk of a gambling problem increased by 30%, said Whyte, the council’s executive director.

He added that the Super Bowl presents an opportunity to see how well responsible gambling messaging and campaigns by sports books and professional sports leagues are working.