[Stef]

Help me see the future… if I have 500k in index funds today, and I don’t add to it, what’s the reasonable range of values for it in 10 years? In 15?

I’m looking for some perspective on long-term investing. I don’t plan to add any more to it—just let it sit and grow.

I understand that nothing is guaranteed, and markets can be unpredictable, but I’m just trying to get a ballpark idea based on historical averages, typical returns, or conservative estimates.

I’d appreciate any insights—whether you base it on past performance, personal experience, financial planning tools, or just general market knowledge.

Feel free to include both optimistic and conservative scenarios.

Thanks in advance!

[Mark]

On average will Double every 7 to 8 yrs

[Sarah]

The S&P 500 has historically returned an average of around 10% per year, so it would take roughly 7.2 years to double your investment (using the Rule of 72).

[Ron]

I personally would look at historical performance. I asked Claude (AI). This screen shot represents the high and low ranges in the S&P 500 for 15 year periods.* Example, 1931 to 1946 is one 15 year period, 1932 to 1947 another … 2009 to 2024 is the most recent.

I also asked for 10 year periods. No 15 year period has been negative on average, but the lowest 10 year period is negative.

Personally I plan for and expect the worst, but certainly hope things will work out at least somewhat better than the worst historical average over the next 10-15 years.

If that is the case, it’s all upside for me. You could take a different approach depending on what you are doing.

You could take the long term average of your portfolio and maybe plan or project for at least 50% of that result over the next 10 years, or 75% over 15 years, something like that. So, for 10 years, 10% becomes 5% and for 15 years it becomes 7.5%.

Then of course put it in a tool to project where you think you will be.

*Note, I did not double check Claude, and you should. The ranges of best and worst 10, 15 and 20 year periods in the S&P 500 is something I know that’s out there and I used AI as an illustrative shortcut.

[Nick]

Rule of 72 is a good rule of thumb. Take 72 divided by your expected interest rate and that’s how long in years it will take you to double your money, assuming no additional contributions.

For example, if you expect an 8% return, take 72 divided by 8% and you get 9 years.

[Paul]

Far from exact, on avg the market has 2x every 7 years, figuring inflation and spending power 2x 10 years.

[Matt]

Given it’s an average, it’s more probable to be a multiple of the average over a longer, not shorter time frame.

There are such things as “lost decades” from time to time. But 20+ year averages are more reliable. 30+ year more so…

[Author]

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