How many women millionaires? Depends on the study

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Sara Blakely, the founder of Spanx shapewear, is considered to be the youngest, self-made female billionaire in the world.

The new craze in the wealth-management world is wealthy women.

Every major private bank and trust company seems to have a special women's initiative or women's outreach program. A recent report warned of a severe shortage of female financial advisors given the vast amount of female wealth that's expected to pour into the purses of Americans in the coming years.

How much exactly? Well, that's the trouble. Determining the current and future number of wealthy women in the U.S.—and their wealth—has become a wild guessing game.

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The most commonly cited numbers come from the Internal Revenue Service, which said that in 2005 about half of the Americans with assets of more than $675,000 were women and they had a total fortune of $5.8 trillion.

But a 2009 study in the Harvard Business Review said women controlled 51.3 percent of wealth in America, which amounts to about $14 trillion in personal wealth. It projected their assets to grow to $22 trillion by 2019.

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.

—By CNBC's Robert Frank. Follow him on Twitter @robtfrank.