Can saving/investing 15% of your income starting age 25, likely make you a millionaire?
The article links to William Bernstein’s plan that he outlined for Business Insider, which says: Modelling this investment strategy Picking three funds from Google and running some numbers. The international stock index only goes back to April 29th 1996, so a run of 21 years was modelled.  Based on 15% of a salary of $550 per month with various annual raises: Broadly speaking, this investment doubles the value of the contributions over two decades. Note:  Rebalancing fees are not included in the simulation. Below is the code used to run the simulation.  If you have Mathematica you can try with different funds.    Notice above how the bond index (VBMFX) preserves value during the 2008 crash.  This illustrates the rationale for diversifying across different fund types.