The World Bank said women control only 1 percent of the world's wealth. But the Boston Consulting Group put the number at 27 percent.

A study from Mary Quist-Newins of the American College of Financial Services in Byrn Mawr, Pa., estimates that $25 trillion will be passed to women through 2030 through inheritances and spouses. By that time, women will control 66 percent of the nation's wealth, the study said. The American College found that 45 percent of American millionaires are women—about 1.5 million.

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But other studies reveal rich women to be a smaller group. A recent study from BMO Private Bank found that women made up only a third of the nation's millionaires. That would be closer to 1 million women millionaires, based on the Capgemini estimates.

About 15 percent of American millionaires are self-made women, BMO said, while the rest got their fortune from marriage or inheritance.

Women represent an even smaller slice of billionaires. Less than 10 percent of the billionaires in the world are women—and a small minority of them are self-made.

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Of course, measuring wealth is always a guessing game of sorts—whether it's tracking the millionaire population or the net worth of individuals.

But measuring women's wealth is made tougher by the nature of households and wealth creation. For married couples, what is the "woman's wealth" versus that of her spouse? How does anyone determine the "primary wealth holder?" And determining how much of a woman's wealth is self-made, family-made or spouse-made is often subjective.

Whatever the numbers, women are clearly gaining share of both income and wealth. The question of how much wealth—and what they do with it—will hopefully become more clear with time.