When investing in anything, it's crucial to understand the costs as well as the potential benefits, and this is true for your 401(k) as well. Only 27% of people know what their 401(k) fees are, according to a recent TD Ameritrade survey, and a surprising 37% of people don't believe they're paying any fees at all.

The truth is, while you may not be getting a bill in the mail, you are definitely paying for your 401(k). How much? That all depends on your portfolio. Here's a look at the three types of 401(k) fees and how much they could end up costing you over your lifetime.

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The three types of 401(k) fees

Investment fees make up the bulk of your 401(k) costs. These include expense ratios, a sort of annual fee all mutual funds charge to shareholders, sales loads that you pay upfront to buy or sell certain funds as well as any other costs directly related to managing your investments.

The manner in which those assets are managed will impact how much you pay in investment fees. Actively managed funds are typically more expensive than passively managed funds. The assets involved matter too. Some investment products, like index funds, typically have lower fees than other types of assets.

Another major expense associated with 401(k)s is the administrative fees. About 95% of 401(k) administrators charge these to cover things like accounting and record keeping, legal services and customer support. While some employers may pay these fees themselves, many companies pass these costs on to you.

Finally, there are individual service fees. You shouldn't have to pay these unless you require a special service, like a 401(k) loan or a 401(k) rollover to an IRA. If you plan to do one of these things, it's a good idea to check with your plan administrator to see what, if any, extra costs this will incur.

Some administrative fees may be a flat rate, but it's more common to see administrative and investment fees charged as a percentage of your assets. This is taken automatically from your 401(k) every year, which is why you never see a bill. Most experts recommend not paying more than 1% of your assets in 401(k) fees each year. This may not seem like that much, but it adds up quickly. The average worker earning the median salary will pay more than $138,000 in 401(k) fees in their lifetime, according to a study by the Center for American Progress. That's a few years' worth of living expenses in retirement.

Generally, larger companies are able to offer more affordable 401(k)s because they can distribute the administrative costs across more people. If you work for a larger corporation, your 401(k) fees may be less than 1% of your assets. On the other hand, small businesses may charge anywhere between 1.5% and 2% of assets. That means instead of losing $1,000 for every $100,000 you have, you'd lose $1,500 or $2,000.

How to find out what your 401(k) fees are

The easiest way to find out what 401(k) fees you're paying is to read your 401(k) plan's annual report. Ask your plan administrator if you don't know where to find this. It may not be spelled out clearly, so you'll have to do some digging. Look for things like "Expense Ratios," "Total Operating Expenses," or "Asset-Based Fees." Another option is to ask for the prospectus of the account funds. This document outlines the details of the fund, including any fees associated with it.

Your best option is always to ask someone who knows, like your plan administrator or the person in your company's HR department who deals with 401(k) plans. They should be able to explain the fees that you're paying and show you where you can find this information.

What to do if your 401(k) plan charges high fees

Your first step should be to evaluate if there are lower-cost options available to you. You may be able to trade in some of your more expensive assets for funds with lower investment fees. If this isn't possible, talk to your employer to see if they would be willing to add some more low-fee investment options for you to choose from.

If you're stuck with what you have, it may be best to not participate in the 401(k) plan and open an IRA instead. The exception to this is if your employer matches some of your contributions. In this case, if your employer match covers or exceeds the amount that you're paying in fees, you may be better off sticking with it, despite the high fees.

A 401(k) can be an excellent vehicle for retirement savings, but it can just as easily hurt you if its fees are taking too much of your assets. Do yourself a favor and figure out what you're paying for the privilege of your 401(k). Then decide if you're better off keeping things as they are or making some changes.