Retiring with a million dollars is a good goal. A larger or smaller sum might work well for you, but for many, if not most, of us, a million dollars will go far. If you apply the flawed-but-still-useful 4% rule to it, withdraw 4% of that million dollars in your first year of retirement, and adjust the withdrawal for inflation in each successive year, the money stands a good chance of supporting you for several decades. And that first 4% withdrawal will be... $40,000. So how can you amass $1 million by retirement? Well, there are many paths to get there, and one of them is investing in an exchange-traded fund (ETF) (or two) that can grow effectively for you over the long run. Here's a look at some such ETFs. Image source: Getty Images. The lower-risk ETF First, one of the best investment strategies for most people is simply to stick with low-cost index funds. They require little of your brainpower and will deliver roughly the same return as the overall market, less those low fees. The stock market has an average annual growth rate of close to 10% over long periods, which can grow your wealth at a decent clip. Over your investing time frame, though, it might average less (or more!) than 10%. So to be a bit conservative, here's how your money might grow at 8%: Growing at 8% for $5,000 invested annually $10,000 invested annually $15,000 invested annually 5 years $31,680 $63,359 $95,039 10 years $78,227 $156,455 $234,682 15 years $146,621 $293,243 $439,864 20 years $247,115 $494,229 $741,344 25 years $394,772 $789,544 $1.2 million 30 years $611,729 $1.2 million $1.8 million 35 years $930,511 $1.9 million $2.8 million 40 years $1.4 million $2.8 million $4.2 million Calculations by author. Here's one such ETF to consider: the SPDR S&P 500 ETF (NYSEMKT: SPY). It follows the S&P 500 index, which is often used as a proxy for the overall U.S. market, since its components together make up about 80% of the total value of the U.S. stock market. Image source: Getty Images. The higher-risk ETF You can do very well investing regularly in a low-risk, low-fee index fund for many years. But if you want to aim for even better returns and are willing to take on a little more risk, you might consider an ETF focused on a faster-growing segment of the market. The Invesco NASDAQ Next Gen 100 ETF (NASDAQ: QQQJ) is one candidate to consider. Here's how the folks at Invesco describe the fund: The Invesco NASDAQ Next Gen 100 Fund (Fund) is based on the NASDAQ Next Generation 100 Index (Index). The Fund will invest at least 90% of its total assets in the securities that comprise the Index by investing in the 101st to the 200th largest companies on the NASDAQ. As a result, the portfolio may be concentrated in mid-capitalization stocks. The Index is comprised of securities of the next generation of Nasdaq-listed non-financial companies; that is, the largest 100 Nasdaq-listed companies outside of the NASDAQ-100 Index®. Got it? It essentially skips the biggest and most well-known Nasdaq companies, such as Apple (NASDAQ: AAPL), Microsoft (NASDAQ: MSFT), and Amazon.com (NASDAQ: AMZN), which together make up roughly 30% of the Nasdaq 100. By doing so, it can focus on relatively smaller companies, such as mid-caps, many of which may be on their way to becoming huge and powerful, but aren't quite there yet. Its top holdings recently featured: Fortinet Zscaler Roku Datadog Old Dominion Freight Line The Trade Desk MongoDB Garmin Etsy Zebra Technologies The fund is very new, so there isn't much of a track record to look at, but you'll probably notice a few familiar growth stocks among its top 10 -- as strong performers in recent years. TV streaming service Roku, for example, has surged around 1,366% over the past five years -- nearly 94% annually, on average. For cybersecurity concern Fortinet, those numbers are 950% and 60%, respectively, and for database specialist Mongo DB, they're 1,471% and 99%. The fund's future performance is unknown, and a market crash will likely hit recent high flyers hard, but over the long run, there's a decent chance that this group of 100 companies will reward shareholders. This isn't an either-or situation, so you can always invest in both a broad-market ETF and one or more specialized ones. Perhaps park much of your portfolio in the broad market and aim to juice your returns with one or more carefully selected focused ETFs. 10 stocks we like better than MongoDBWhen our award-winning analyst team has a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed what they believe are the ten best stocks for investors to buy right now... and MongoDB wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of September 17, 2021 John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Teresa Kersten, an employee of LinkedIn, a Microsoft subsidiary, is a member of The Motley Fool's board of directors. Selena Maranjian owns shares of Amazon, Apple, Datadog, Microsoft, MongoDB, and The Trade Desk. The Motley Fool owns shares of and recommends Amazon, Apple, Datadog, Etsy, Microsoft, MongoDB, Old Dominion Freight Line, Roku, The Trade Desk, Zebra Technologies, and Zscaler. The Motley Fool recommends Fortinet and Garmin and recommends the following options: long January 2022 $1,920 calls on Amazon, long March 2023 $120 calls on Apple, short January 2022 $1,940 calls on Amazon, and short March 2023 $130 calls on Apple. The Motley Fool has a disclosure policy.Source