It's 4 a.m. Do you know what you're getting charged in 401(k) fees?

It's not a question that keeps most people up at night -- but maybe it should. Or perhaps a better question is this: did you know that 401(k) plans have fees associated with them? If you didn't, you're in good company. In fact, seven in 10 people didn't realize that 401(k) plan providers charge fees to participants, according to a recent AARP study.

401(k) plans are still a powerhouse for your retirement funding, especially when some of your contributions are matched by your employer. But over decades, what seems like a small amount can add up big time...to the tune of thousands of dollars -- or more. Want to know how much?

New tools are available to help you find out about the potentially hidden costs you may not have noticed in your 401(k) plan's fine print. If you work for a large employer, CNN Money offers a free tool that shows the impact of the fees in your employer's plan. If your employer is not in CNN Money's database, you could use a tool like 401kfee.com just to calculate the impact that a couple of percentage points in fees makes over your lifetime.

Have your 401(k) statement for easy reference to use the calculator.

After you explore the fee scenarios, check out other tips to help make the most of your 401(k). Every dollar makes a difference, so make your dollars work for you.