We couldn't find the exact figure published. But Forbes's September data puts all U.S. billionaires between $8.6 trillion and $10.6 trillion, and one June tally hit $9.24 trillion.
Americans for Tax Fairness counted 977 U.S. billionaires worth $9.24 trillion on June 16, using Forbes data3. Forbes's September list put the 400 richest at a record $8 trillion, with 590 more U.S. billionaires below its $4.4 billion cutoff11. Each of those 590 is worth between $1 billion and $4.4 billion, so all U.S. billionaires together held between about $8.6 trillion and $10.6 trillion as of Sept. 4, by our calculation. Sanders' office used an all-billionaires count in March, too: 938 billionaires worth $8.2 trillion12.
So the post most likely compares 400 people in 2009 with all billionaires in 2026. In 2009 those were nearly the same group. Like for like, the Forbes 400 alone went from $1.27 trillion to $8 trillion, about 6.3-fold, or about fourfold after inflation, by our calculation2,11,6,7.
After inflation, the post's own figures show about 4.7-fold growth, using annual-average prices, by our calculation6,7.