If you want to grow your portfolio to $1 million or more within 20 years, you don't need a huge lump sum right now. One way to get around that is by investing regularly every month. That way, you benefit from the effects of compounding over time.

As your income grows, you can also invest more money to help accelerate your gains. And by investing in a growth-focused exchange-traded fund (ETF), you can minimize just how big a monthly investment you need to make.

How much would you need to invest every month to get to $1 million in 20 years?

Ideally, you would start investing for your future in your 20s. By doing so, the monthly payment you would need to grow your portfolio to $1 million will be lower. Unfortunately, it doesn't often work out that way.

The good news is that if you invest later in life, you might not necessarily be in a much worse position because you could make larger contributions as you advance in your career and your earnings increase.

Ultimately, the monthly payment you will need to make to get to $1 million will not only depend on the number of investing years you have left, but also on the average annual growth rate you expect from your investments. Here's a look at what those monthly payments would need to be, based on different growth rates, and assuming you make them for 20 years.

Calculations by author.

As you can see, it's a lot more challenging to get to $1 million during a 20-year time frame when you're only aiming for a 10% growth rate, which is roughly around the S&P 500's long-term average of 9.7%.

This is where finding the ideal ETF can be a key part of your long-term strategy.

A diversified growth ETF can keep your risk low

There are many thematic ETFs focused on different niches and growth opportunities. But the problem can be that your investment strategy might become too narrow, leaving you exposed to changing market dynamics.

For example, while artificial intelligence stocks (AI) are hot buys this year, it might not be the case in the future. High valuations, insufficient payoffs for companies' AI investments, and concerns relating to energy consumption are just a few reasons these types of investments could struggle down the road.

The safer option for investors is to look at a more balanced ETF, such as the Invesco QQQ Trust (NASDAQ: QQQ). It tracks the top 100 nonfinancial stocks in the Nasdaq, effectively giving you exposure to the top tech and growth stocks in the world. You'll get AI stocks, but also many other types of investments. By taking a broader approach, you can keep your risk lower versus investing in thematic ETFs with a narrow scope.

Historically, the Invesco QQQ Trust has generated some impressive returns. In the past 10 years, the fund has produced total returns (including dividends) of around 440%. That averages out to a compound annual growth rate of 18.4%.

If the fund were to average that kind of growth rate over a 20-year period, you would need to invest around $408 per month to get to $1 million.

Gains are never a guarantee, but growth ETFs can increase the odds that you beat the market

The past doesn't predict the future, of course, and just because the Invesco QQQ fund has performed exceptionally well in the past 10 years doesn't mean the next 20 are a sure thing. And given the sharp gains that tech stocks have achieved in recent years, a case can be made that they have been outperforming of late and that a slowdown could be inevitable.

Expecting an oversize return for decades would be a best-case scenario, and it could lead to investors' disappointment if it ends up falling short.

The conclusion remains the same, however: By focusing on a growth ETF like the Invesco QQQ Trust, you can still set yourself up for market-beating gains and increase your chances of getting to a $1 million portfolio within 20 years. There are other growth ETFs you can invest in, but the Invesco fund might be one of the better ones if you're looking for a high return.

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Want to Get Your Portfolio to $1 Million in 20 Years? Here's How Much You Should Invest Every Month. was originally published by The Motley Fool