Everyone makes mistakes, especially with finances. It's nothing to be ashamed of, as long as you recognize it and correct them before they become problematic. If you wait too long (or if you don't realize you're making a mistake at all), you could be putting your finances in jeopardy.

So what do Americans think is the No. 1 retirement mistake you can make with your money? It involves your 401(k).

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Why a 401(k) is a vital retirement tool

The biggest financial mistake you can make is not taking advantage of your 401(k), according to a survey from TD Ameritrade. A whopping 72% of the survey participants said not investing in a 401(k) is a mistake, with roughly 52% of those people admitting it's a major one.

Why is not putting money into a 401(k) considered a big blunder? Because it's one of the best retirement saving tools there is.

There are several reasons a 401(k) is one of the best places to stash your savings. For one, your employer may match your contributions up to a certain percentage of your salary. These matching contributions are essentially free money, so it's smart to take full advantage of them.

Second, 401(k) plans have much higher annual contribution limits than other types of accounts. As of 2019, you can contribute up to $19,000 per year in your 401(k), plus an additional $6,000 if you're 50 or older. IRAs, on the other hand, have a yearly contribution limit of just $6,000, plus an extra $1,000 for those 50 and up. Although most people can't afford to max out a 401(k), it's nice to have a higher limit in case you want to supercharge your savings.

Another perk of investing in a 401(k) is that it's relatively easy to get started and save consistently. Oftentimes, employers allow you to automatically transfer a portion of each paycheck straight into your retirement account. It makes it easy to save and can help avoid the temptation of spending that money.

How to maximize your 401(k)

The first step to making the most of your 401(k) is to contribute at least enough to earn the full employer match. Not doing so means you're missing out on free money and the opportunity to significantly increase your savings.

Next, make sure you know what you're paying in fees. All retirement accounts -- whether it's a 401(k), traditional IRA, Roth IRA, etc. -- charge fees, but they aren't always obvious. You typically won't see a bill for the fees you owe, because the money is simply taken out your account balance. And you also can't expect to receive a statement telling you exactly how much you're paying in fees, so it's tough to tell how much of your contributions are going toward your savings and how much is being spent on fees.

The average 401(k) charges fees of around 1% of total assets under management, according to a report from the Center for American Progress. So if you have $100,000 in your account, around $1,000 per year is going toward fees. It's not unrealistic to end up paying tens or even hundreds of thousands of dollars in fees over a lifetime. In fact, the Center for American Progress also found that the average worker paying 1% in fees loses around $138,000 to fees over a lifetime. Raise that to 1.3%, and the loss jumps to around $166,000 over a lifetime.

To check what you're paying in fees, you can either talk to your plan administrator or dig through the fine print in your statements. The key figure to look for is the expense ratio, which tells you how much of your money is going to fees. If you find you're paying higher-than-average fees, it might be worth it to consider investing elsewhere. While you should still contribute enough to your 401(k) to earn the full match, it might be wise to invest the rest of your cash in an IRA that charges lower fees.

Investing in a 401(k) is one of the best ways to build a healthy retirement fund, so if you're not taking advantage of it, you could be putting your financial future at risk. And the more you know about how to maximize it, the better the chance you'll have of reaching your long-term financial goals.