Want to Be a Multimillionaire? Buy This ETF and Never Sell.

Many people have the financial goal of saving $1 million (or more) for their retirement. But it's difficult to know where to get started or how to achieve such a large sum of money.

The good news that the path toward a multimillionaire portfolio isn't complicated. While accumulating that much money isn't easy, the investment strategy is straightforward: Contribute substantial sums of money every month, for many years, into an exchange-traded fund (ETF).

Here's why the Vanguard Mid-Cap ETF (NYSEMKT: VO) is one of the best places to invest your money if you want to retire a multimillionaire without needing to pick individual stocks.

Image source: Getty Images.

You don't have to be an expert

One of the great advantages of buying the Vanguard Mid-Cap ETF is that you don't need to be an investing expert. The fund tracks the CRSP US Mid Cap Index, which consists of about 290 mid-capitalization stocks across many sectors, including energy, consumer goods, technology, finance, and others.

This means that you don't have to follow investing trends, like picking artificial intelligence stocks, shifting your investments if the economy slows down, or changing course if some new company becomes a must-have stock in your portfolio.

With the Vanguard Mid-Cap ETF, you'll have exposure to well-established companies that are in the sweet spot between generating cash flow and still being small enough for significant growth opportunities. A few examples of the top holdings in the fund are Robinhood Markets, Constellation Energy, and Roblox.

In short, if you don't have any interest in following what specific companies are doing regularly and instead want your portfolio to benefit from established companies in the market, the Vanguard Mid-Cap ETF is the way to do it.

You can become a multimillionaire with enough time

Becoming a multimillionaire isn't easy, so I won't promise anything. But you can become a multimillionaire by investing in the Vanguard Mid-Cap ETF if you put enough money into the fund on a regular basis and invest for decades.

That's because the fund has had an average annual return of 9.95% since its inception in 2004, which is nearly on par with the average historic annual return of the S&P 500. That's an impressive return, and it can do wonders for your portfolio if you let it sit there for a long time.

Here's how much you'll need to invest on a monthly basis and for how many years to reach at least $2 million, assuming a 9.95% average annual return:

Calculations by author via Investor.gov.

There's no guarantee the fund will earn 9.95% each year -- sometimes it will be more and sometimes less -- but with more than two decades of those average returns, it's a pretty good indicator of the fund's long-term potential for returns.

And considering the Vanguard Mid-Cap ETF has a very low expense ratio of just 0.04% -- which equals $4 annually for every $10,000 invested -- you'll pay next to nothing in fees to own the fund.

Just remember, the key to reaching $1 million, $2 million, or whatever your personal financial goal may be is to invest regularly, stay invested through both the good times and the bad, and let your money grow over time.

Chris Neiger has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends Constellation Energy, Roblox, and Vanguard Index Funds-Vanguard Mid-Cap ETF. The Motley Fool has a disclosure policy.