Answer:
There isn’t one “magic number” because the term “fair share” can be defined in several ways—whether that means taxing all forms of income (including unrealized capital gains) at ordinary income tax rates or also taxing wealth directly. However, multiple analyses suggest that if the ultra‐rich, including billionaires, paid taxes on all their wealth and income at rates comparable to those paid by typical high‐income earners, the additional tax revenue could easily run into the hundreds of billions of dollars every year.

For example, one influential line of evidence comes from proposals and analyses that indicate if unrealized gains (which currently are minimally taxed) were taxed at rates near the top ordinary income rate of about 37%, then just the capital gains of the ultra‐rich would yield roughly $120 billion each year. In another assessment, experts at Americans for Tax Fairness have shown that a modest 3% wealth tax on individuals with fortunes above $1 billion—which would affect just the top 10 or so wealthiest Americans—could raise more than $50 billion annually. Proponents of further taxing billionaire “income” have estimated that coordinated efforts (such as a global minimum tax on billionaires’ effective income or wealth) might allow governments to collect an additional $250 billion per year from this very small group, thereby addressing growing inequality and funding major public priorities [1] [2].

Complementing these figures, a Treasury study noted that when accounting for federal, state, local, and even foreign taxes (especially for those with substantial international business interests), the effective tax burden on some of the ultra-wealthiest individuals could approach levels as high as 50–60% of their “income” once all taxes (including corporate income and estate taxes) are considered. In contrast, many billionaires have historically paid effective rates as low as 3–8% on their reported individual income, largely due to tax rules that favor returns on wealth (like capital gains) over wages [3] [4].

In summary, if billionaires were required to pay what many policy experts argue is their “fair share,” the extra revenue—when aggregated across the wealthiest few thousand Americans—could be in the several-hundred–billion–dollar range per year. This extra tax revenue might then be used to fund significant public investments in areas such as healthcare, infrastructure, climate action, and social safety net programs.

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