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A calculator that takes into account portfolio rebalancing?
Note that if 1)  The stock prices are continuously differentiable (they aren't) 2)  You rebalance continuously in the absence of trading fees and taxes then the return fraction (future price / original price) will be the geometric mean of the return fractions for each investment. If you don't rebalance then the return fraction will be the arithmetic mean. But the arithmetic mean is ALWAYS greater than or equal to the geometric mean, so continuous rebalancing in the case of continuously differentiable prices will always hurt you, even abscent trading costs/taxes. Any argument in favor of blind rebalancing which does not somehow fail in the continuously differentiable case is simply wrong. See https://dl.dropboxusercontent.com/u/38536036/to%20karim.pdf -JT