Here's Exactly What You'd Need to Invest Each Month to Earn $1 Million in the Stock Market

The stock market has the power to make you a millionaire, and you don't necessarily need to invest thousands of dollars per month or spend countless hours researching stocks. Buying low-maintenance index funds or exchange-traded funds (ETFs) and making regular contributions can supercharge your net worth with minimal effort on your end.

There are countless funds to choose from, all with unique advantages and disadvantages. Two of the most popular types, though, are the S&P 500 ETF and the growth ETF. Here's exactly what you'd need to invest each month with each type of fund to reach $1 million in total savings.

The S&P 500 ETF: A stable and reliable workhorse

While it's impossible to invest in an index itself, you can invest in an ETF that tracks a particular index -- like the S&P 500 (SNPINDEX: ^GSPC).

An S&P 500 ETF aims to replicate the index's performance by containing all of the stocks within the S&P 500. Buying just one share of an S&P 500 ETF essentially allows you to own a slice of the S&P 500, as you'll instantly gain exposure to all 500 companies within the index.

This type of investment is a fantastic choice for those looking for a stable fund with relatively low long-term risk. The S&P 500 has a flawless track record of recovering from market downturns, even if it does face short-term volatility at times. By investing in an S&P 500 ETF, you're all but guaranteed to see positive long-term returns.

^SPX data by YCharts

Perhaps the biggest downside of an S&P 500-tracking fund, however, is that you can only earn average returns. This index is generally considered a representation of the stock market as a whole, so its performance is essentially the market average. Because this ETF follows the S&P 500, it can't beat the S&P 500.

The growth ETF: An earnings-focused powerhouse

There are numerous growth ETFs available, each with distinct goals and portfolios. But the one thing they all have in common is that they aim to earn above-average returns.

Some growth ETFs focus on large-cap stocks from established companies, while others contain smaller stocks that carry more risk but offer more potential for explosive growth. You could also opt for a growth ETF that only includes stocks from a particular industry, or a fund with only international stocks.

Your earning potential with a growth ETF will depend largely on where, specifically, you invest. For example, even between three popular growth ETFs -- the Vanguard Information Technology ETF (NYSEMKT: VGT), the Schwab U.S. Large-Cap Growth ETF (NYSEMKT: SCHG) and the iShares Russell Mid-Cap Growth ETF (NYSEMKT: IWP) -- performance varies widely from fund to fund.

Data sources: Vanguard, Charles Schwab, and iShares.

All of these funds have earned returns higher than the market's long-term 10% average annual return, but just how much you'll earn depends on where you invest and how that particular fund performs over time.

Growth ETFs are often more volatile than S&P 500 ETFs, so expect more significant short-term fluctuations. Also, be prepared to keep your money in the market for at least a few years to minimize the impact of volatility.

Building a million-dollar portfolio

How much you'll need to invest each month will depend on the returns you're earning on your investments, so let's look at a few different scenarios.

The market itself has earned an average return of around 10% per year over decades, so let's assume that's what your S&P 500 ETF might continue earning over time. Let's also assume that your growth ETF might earn a 12%, 16%, or 20% average annual return.

If you have a goal of accumulating $1 million within, say, 25 years, here's what you'd need to invest each month in all of these scenarios:

Data source: Calculations by author via investor.gov.

Of course, the higher the returns you're earning, the less you'll need to invest each month. But if you have a longer timeline, that can also make it easier to build significant wealth.

Say, for example, that you're only earning 10% average annual returns on your investment, but you have 40 years to save. In this case, you'd only need to contribute around $200 per month to reach $1 million.

Building wealth in the stock market takes time, but patience pays off. By investing in an ETF that fits your goals and comfort with risk and staying invested for decades, you could earn more than you might think.

Katie Brockman has positions in Vanguard Information Technology ETF. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.