Most of us would like to become millionaires, but it can seem impossible unless you are a stock market genius or lucky casino frequenter. That's not the case, though. As long as you have enough time before you retire, it's possible to amass surprisingly large sums -- even a million dollars or more. Better still, this can be done very simply, such as by only investing in a broad market index fund. Since many exchange-traded funds (ETFs) exist that track many different stock market indexes, you can achieve millionairehood via ETFs. Image source: Getty Images. How money grows The proof of how much money you can amass over time is in the math -- reflected in the table below. Much depends on how long you invest and on how much money you invest regularly. The bolded numbers below show the points at which you might cross the millionaire line when you're saving various sums for various periods. Growing at 8% for $10,000 invested annually $15,000 invested annually $20,000 invested annually 5 years $63,359 $95,039 $126,718 10 years $156,455 $234,683 $312,910 15 years $293,243 $439,865 $586,486 20 years $494,229 $741,344 $988,458 20.5 years $518,914 $778,371 $1,037,828 23.5 years $688,744 $1,033,115 $1,377,487 25 years $789,544 $1,184,316 $1,579,088 28 years $1,029,659 $1,544,489 $2,059,319 30 years $1,223,459 $1,835,189 $2,446,918 Data source: Calculations by author. The table uses an 8% annual growth rate, and that's an attempt to be somewhat conservative. Over long periods, the average stock market return has been roughly 10%, but an investor might average a little or a lot more or less depending on the specific period in which they invest. Image source: Getty Images. Some ETFs to consider So which specific ETFs can help you amass significant wealth? Well, there are lots of great low-cost ETFs that are passively managed index funds, and fund companies such as Vanguard, Fidelity Investments, and Schwab. Here are a handful of solid contenders that encompass much or all of the U.S. or world stock market: Vanguard S&P 500 ETF (NYSEMKT: VOO) Schwab U.S. Broad Market ETF (NYSEMKT: SCHB) iShares Core S&P Total U.S. Stock Market ETF (NYSEMKT: ITOT) Vanguard Total Stock Market ETF (NYSEMKT: VTI) Vanguard Total World Stock ETF (NYSEMKT: VT) The following ETFs focus on companies of certain sizes: Vanguard Large Cap ETF (NYSEMKT: VV) Schwab Large Cap ETF (NYSEMKT: SCHX) Vanguard Mid-Cap ETF (NYSEMKT: VO) Schwab Mid-Cap ETF (NYSEMKT: SCHM) iShares Core S&P Small Cap ETF (NYSEMKT: IJR) Vanguard Small-Cap ETF (NYSEMKT: VB) Finally, here are some ETFs that have particularly strong performances over the past 10 and 15 years. That's not necessarily guaranteed to continue, so be sure to read up on any of them before investing in them, to make sure you like what you see and that you're confident in their promise. Invesco QQQ Trust (NASDAQ: QQQ): This ETF tracks the Nasdaq-100 Index of the 100 largest non-financial companies that trade on the Nasdaq market. Vanguard Information Technology ETF (NYSEMKT: VGT): This ETF is heavily invested in technology stocks such as Apple, Microsoft, and NVIDIA. SPDR S&P Biotech ETF (NYSEMKT: XBI): This ETF is focused, obviously, on the biotech arena, with holdings such as Moderna and Curis. If you end up invested in one or more ETFs that grow significantly faster than the overall market, you may become a millionaire even faster than if you just stick with an S&P 500 or world market ETF. But even those very broad ETFs have very respectable records and are likely to be solid performers over the coming decades. 10 stocks we like better than WalmartWhen investing geniuses David and Tom Gardner have an investing 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.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Walmart 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 2/1/20Teresa Kersten, an employee of LinkedIn, a Microsoft subsidiary, is a member of The Motley Fool's board of directors. Charles Schwab is an advertising partner of The Ascent, a Motley Fool company. Selena Maranjian owns shares of Apple and Microsoft. The Motley Fool owns shares of and recommends Apple, Microsoft, NVIDIA, Vanguard Mid-Cap ETF, Vanguard S&P 500 ETF, and iShares S&P SmallCap 600 Index. The Motley Fool recommends Charles Schwab and Moderna Inc. and recommends the following options: long March 2023 $120 calls on Apple and short March 2023 $130 calls on Apple. The Motley Fool has a disclosure policy.Source