Creating a $1 million portfolio by the time you retire can give you the financial freedom you need to enjoy life in your golden years. If you build up a balance that large, you can slowly withdraw it to fund your lifestyle and also consider investing some of it into bonds or other income-generating investments. Either way, even if you aren't absolutely convinced that will be enough money, it can still be a great goal to strive for, as it can undoubtedly make your retirement much more enjoyable.

By investing $300 per month, you can put yourself on track to building up a portfolio worth at least $1 million. Here's how that's possible, with a simple strategy that can be easy to stick to over the long haul.

Man meeting with financial advisor at home.

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Invest in a top growth fund each month

Investing regularly each month is a good way to build up a habit. The key is to make it simple. Monitoring the market and trying to find the latest hot stocks to buy is a surefire way to make the process a lot more time-consuming and complicated. To ensure it's easy to stick to your plan, simplicity is crucial.

This is where having a place to put money into each month is important. But even if you love Nvidia and believe it's destined to go higher, investing everything in the same stock won't diversify your portfolio. A safer approach is to invest in an exchange-traded fund (ETF) that will give you exposure to dozens and even hundreds of stocks. You can still benefit from the market's long-term growth, but you won't have to manually pick stocks.

A good ETF to consider is the Vanguard S&P 500 Growth Index Fund ETF (VOOG -1.79%). It has a low expense ratio of 0.07% and tracks the best growth stocks within the S&P 500. The broad index already includes the leading 500 companies on the U.S. markets, and with this Vanguard ETF, you're getting a narrower selection of growth stocks, making it an even more attractive option for those who believe that top growth stocks will do well over the long term. In five years, the Vanguard fund has generated total returns (including dividends) of 120%, as it has outperformed the S&P 500 during this time frame.

How $300 per month can turn into $1 million

To turn a $300-per-month investment into $1 million will take time. In all likelihood, it'll be decades before those monthly investments build up your portfolio to anywhere close to $1 million. Here's a breakdown of what your investment might be worth if it grows by an average of 10% per year, which is in line with how the S&P 500 has historically performed.

Year 10% Growth
5 $23,425
10 $61,966
15 $125,377
20 $229,709
25 $401,367
30 $683,798
35 $1,148,483

Calculations by author.

Under these assumptions, it would take approximately 34 years of making monthly investments of $300 into the Vanguard ETF before your portfolio reaches $1 million.

Why you're never too old to start investing

If you don't have 30-plus investing years to go, you may feel discouraged with a strategy that involves investing for decades. But there are ways to accelerate your gains. If you're able to invest more per month or perhaps come into some money due to inheritance or the sale of an asset, putting that money into a top growth ETF can not only increase your portfolio's value but also means more money is compounding each year, positioning you for greater gains along the way.

There are plenty of good reasons to invest in the stock market, even if you are worried you may not have enough investing years left to make a big profit. It's still likely a far better approach for the long term than simply putting your savings in a bank account.