Let's say I use dollar cost averaging and indexing with an S&P500 index fund over a 20-year period. Can I expect to double my investment, taking inflation and dividends into account?

impossible to say



the market was flat for ten years from 2000-2010  for example.  to many variables.

About all you can do is track the index fund's rate of return and apply the rule of 72. Take the actual rate of return and divide it into 72. That will tell you HOW LONG it will take to double your money. It only applies if your return rate stays constant for the duration.

Dollar cost averaging is a good basic strategy, as is buying low, or no-load index mutual funds. Especially when you consider that only +/- 15% of ALL Mutual Fund managers ever manage to simply equal the performance of index funds, much less outperform them.

And remember that a diversified portfolio does not mean having 20 or 30 mutual funds, it means having 8 to 10 that cover the 9 basic mutual fund style models that are commonly in use.

As to what taxes and inflation will do to your returns, one can only guess.
ymmv.

Past returns are history, not indicative of future results.

On the other hand, an index fund will not UNDERperform the market and will incur the least amount of fees compared to active funds.