NEW YORK — These days, it takes a billion — at least.

For the first time, Forbes magazine's list of the 400 richest Americans consists exclusively of people worth $1 billion or more. As a group, the people who made the rankings released yesterday are worth a record $1.25 trillion, compared with $1.13 trillion last year.

In the billionaire-athon, casino magnate Sheldon Adelson vaulted to No. 3 from 15 in last year's ranking, finishing behind the mainstays at Nos. 1 and 2: Microsoft Corp. founder Bill Gates and Warren Buffett of Berkshire Hathaway Inc.

Adelson is now estimated to have $20.5 billion, Buffett $46 billion and Gates $53 billion. Gates has held the No. 1 spot for the last 13 years while Buffett has been No. 2 every year since 1994 except 2000, when Larry Ellison of Oracle Corp. held that spot.

Adelson's expanding net worth is related in no small part to his decision to open a casino two years ago on the island of Macau, an emerging gambling haven off the southeastern coast of China. Profits are growing rapidly thanks to the Las Vegas Sands Corp.'s Macau casino.

Forbes estimates Adelson earned about $1 million an hour over the past two years. In the second quarter alone, the Sands Macau property saw net revenue jump to $310.4 million, up from $205.1 million a year ago. To tap the demand from gamblers in Asia going forward, the Sands Corp. plans a second property on Macau and a casino in Singapore.

The two Google Inc. founders were also big earners. Sergey Brin and Larry Page gained about $13 million a day over the last two years, according to Forbes. That puts them in 12th and 13th place, up from a tie at 16th place last year.

Page and Brin also share the distinction of being, at 33 years old, the two youngest people on the list and two of only eight who are younger than 40.

The list was led off by technologists, such as Gates, Microsoft co-founder Paul Allen, Dell Inc.'s Michael Dell and Ellison, and rounded out by five members of the Walton clan who have fortunes amassed from sales by the world's largest retailer.

Ellison, with $19.5 billion, moved to fourth place from fifth, while Allen, last year's No. 3, was fifth this year with $16 billion. Dell fell to a tie at ninth place from fourth in last year's list; he is worth $15.5 billion

Adelson's ascension knocks Helen Walton, the wife of Wal-Mart Stores Inc. founder Sam Walton, into 11th place with a net worth of $15.3 billion. Her children, Jim, S. Robson and Alice, and Christy Walton, the widow of her son John, ranked in the bottom half of the top 10 this year. Each was worth between $15.5 billion and $15.7 billion, Forbes reported.

Martha Stewart, founder of the eponymous Martha Stewart Living Omnimedia Inc., fell off the list completely, as she lost $395 million over the past year.

The biggest number of people on the list live in California, which houses 90 of the 400, and another 44 live in New York City.

The only person on the list from Hawai'i was Barbara Cox Anthony, 83, of Cox Enterprises. Anthony was listed as the 17th richest person with $12.6 billion.