We all know that participating in a company-sponsored 401(k) can be instrumental in saving for retirement. Not only that, but we're encouraged to max out our contributions year after year to take advantage of compounding and grow our nest eggs in time for retirement.

But while saving as much as possible in your 401(k) is certainly a responsible move in theory, your efforts could wind up being seriously hindered if you make the same mistake more than 90% of Americans make: failing to pay attention to fees.

According to a NerdWallet study, roughly 92% of Americans have no idea what they're paying in 401(k) fees. Many, in fact, underestimate their fees and wind up losing thousands of dollars in potential retirement income as a result. But while certain 401(k) fees are unavoidable, you do have the power to limit the amount you fork over each year.

How much money are you losing?

The average 401(k) plan's fees equal roughly 1% of assets under management. Now 1% may not seem like a lot, but you have to remember that you're not just forking over 1% (or more) of however much you're contributing; you're also giving up that same percentage of your assets as they grow.

In fact, the Center for American Progress estimates that the typical American worker who begins earning a median salary of $30,502 at age 25 and saves 5% of salary into a 401(k) is expected to lose a total of $138,336 in lifetime 401(k) fees.

Higher earners have it even worse. Those who begin earning $75,000 at age 25 will pay an estimated $340,147 in lifetime fees.

Calculator: How much are 401(k) fees costing you?

Let's stop and think about those numbers for a minute, and what they might mean as far as retirement goes. Forgoing $138,336 in income means having $576 less to spend per month over the course of a 20-year retirement. That's a major lifestyle difference, and one that should motivate you to take a closer look at your 401(k) going forward.

Furthermore, many retirees encounter high health care costs, with the average healthy 65-year-old couple currently expected to spend $377,000 on medical care over the course of retirement. That $138,336 (or, worse yet, $340,147) you're losing to fees could make a serious dent in your health care expenses and take one source of financial stress off your plate.

Look for lower-cost investments

All 401(k)s charge management and administrative fees, which are designed to cover the costs of maintaining the plan. (Those glossy booklets outlining your plan's key features? Your management and administrative fees are paying for those.) But while you can't avoid those basic fees, you can take steps to ease the burden elsewhere.

For one thing, you can review your plan's investment options and choose funds with the lowest expense ratios. This typically means opting for index funds, which are passively managed, as opposed to actively managed mutual funds. Because index funds simply seek to match the performance of existing indexes, their fees tend to be far lower than those charged by active management funds.

And if you're worried that putting your money into index funds will zap your returns, think again. A 2015 Morningstar study found that between 2004 and 2014, index funds outperformed actively managed funds pretty much across the board.

Another option to consider is moving some of your assets out of your 401(k) altogether and rolling them into an IRA. IRAs typically offer a wider range of investment choices, so if your options for low-cost investments are limited within your employer's plan, an IRA might actually save you money by minimizing the amount you lose to fees.

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Remember, your plan provider, by law, is required to disclose the fees associated with maintaining your 401(k), so consult your summary plan description and annual report to figure out exactly how much money you're dishing out.

Just as importantly, always review your individual investments and look for funds that offer the best returns at the lowest cost. The money you contribute to your 401(k) is money you work hard to earn, so don't make the mistake of sitting back and throwing it away.