Amassing $1 million in retirement savings takes some effort, but it's within reach for most people to hit this milestone. That's especially true if you already have $100,000 available to further your efforts.

Not sure how to do that? Here are three different ways to grow $100,000 into a six-figure account balance. 

Investor looking at computer charts.

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1. Invest in an S&P fund and wait 25 years

If you have $100,000 and want to end up with $1 million, there's a way to do this in one simple step without much effort. You can put the money into an S&P 500 index fund and leave it alone for 25 years. 

The S&P 500 is a financial index that tracks the performance of approximately 500 large U.S. companies. If you buy an S&P 500 exchange-traded fund (ETF), you'll acquire a small ownership stake in some of the most established and successful companies in the country. You'll also stand a solid chance of earning a 10% average annual return, since that's the return on investment (ROI) the S&P 500 has historically provided. 

If you can earn an average annual 10% return on your $100,000 investment and you wait 25 years, you would end up with around $1,083,470 (not accounting for investment fees). Since S&P 500 ETFs generally have very low investment expenses, you should be able to hit your $1 million target, even after accounting for the costs of investing. 

2. Add to your investment

What if you don't have 25 years to wait? You can also end up a millionaire retiree by investing your $100,000 in an S&P 500 index fund and adding some money to your account over time. The exact amount you would need to contribute each month will vary depending on your timeline, but the calculators on can help you figure it out.

For example, let's say you're planning on retiring in 10 years. You have $100,000 now and want to end up with $1 million. Therefore, you'd need to add around $3,872 per month to your retirement account in order to hit your target.

If you contribute to a tax-advantaged account, such as a 401(k), or if you earn an employer match, you'll get some help from Uncle Sam or the company you work for. So not all that money has to come directly out of your pocket. But if you find you can't add enough to your initial $100,000 to end up with $1 million on your desired timeline, you may have to either change your budget to invest more or adjust your retirement date to give yourself more time. 

3. Make very smart investments

Finally, your last option is to make really smart investments to try to beat the market so you can grow your $100,000 into $1 million quickly. 

The S&P 500 is widely viewed as a measurement of how the overall "market" performs. But you might be able do better than the ROI that an S&P fund would provide if you pick the right stocks that see their share prices rise quickly. If you can consistently earn a 30% average annual ROI on your $100,000, for example, you'd be able to end up a millionaire in around nine years.

It's not easy to beat the market -- especially by a huge amount and consistently over time. But if you research investments carefully and take calculated risks that are appropriate for your age and investing timeline, you may be able to do pretty well and grow your $100,000 much faster than you could if you invested in an S&P fund alone.

Ultimately, the right path for you is going to depend on your comfort level with selecting stocks, the money you have available to you, and the length of time you have to invest. But if you already have $100,000 to get started, you're in a good position to find yourself with seven figures in your senior years.