Good news: The average 401(k) contribution could fund a millionaire retirement. That average contribution, according to a 2021 retirement report from Fidelity, is $640 per month.

The thing is, making the right contribution is only part of the retirement funding challenge. You also must stick with those contributions and invest them appropriately over time.

Read on for a quick look at two easy retirement investment plans -- both can help you turn an average contribution into an above-average retirement.

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1. The index fund portfolio

The long-term average growth rate of the S&P 500 is about 7% after inflation. At this rate, your $640 monthly contribution would grow to $1 million in 35 years.

Add employer matching contributions to that $640 monthly, and you reach seven figures even faster. Specifically, an average matching contribution of $340 monthly gets you to millionaire status in less than 30 years.

  • Achieving market-level returns: What's exciting here is that you can invest directly in the S&P 500 to earn those market-level returns. An S&P 500 index fund mimics the S&P 500's performance, with only a slight drag to cover fund expenses. Choose a fund with an ultra-low expense ratio, and the returns should be just a notch below the index itself.
  • Limiting volatility in your portfolio: Although the S&P 500 averages 7% annual growth over long periods of time, the short-term performance can be volatile. If you invest 100% of your contributions in an S&P 500 fund, your portfolio balance will reflect the full strength of the market's ups and downs. That can be stressful to watch.

To limit that volatility, you can pair your S&P 500 fund with a more stable security, like a U.S. Treasury bond fund. You might hold 10% of your contributions in the bond fund when you're young, gradually increasing that percentage as you near retirement.

Adding a bond fund to your portfolio will moderate your returns. But many investors accept this in exchange for a smoother growth path.

2. The target date fund

If you want something easier than the index fund portfolio, try a target date fund (TDF). TDFs combine stocks and bonds into one fund.

Even better, the composition of stocks and bonds in a TDF is tailored to your retirement timeline. That means you don't have to shift your portfolio to be more conservative as you get older. The fund does this for you automatically.

Your 401(k) probably offers you one family of TDFs with multiple vintages. The vintage -- the date in the name of the fund -- should align with your planned retirement year.

It's smart to take the extra step of reviewing the fund documentation. Specifically, look for how the fund transitions from aggressive to conservative over time. If that transition feels too conservative or aggressive, you can choose a different vintage. A fund with a later target year would be more aggressive. You'd select an earlier vintage to be more conservative.

Building an above-average retirement

Building wealth for retirement takes time, but it doesn't have to be hard or complicated. A simple portfolio of two index funds or a single TDF can get the job done, even with an average retirement contribution.

If you like the idea of retiring as a millionaire, check your 401(k) contributions and investment selections today. From there, plan on reviewing your account's performance periodically. Momentum will be slow at first, but stick with your plan. Eventually, you'll look forward to seeing how much closer you are to becoming a millionaire retiree.