Most parents and students have no idea how much college really costs 

  • Only around 10 percent of ninth graders correctly estimated the cost of one year at a public four-year college in their state, according to new data.
  • Students' forecasts were off by an average of $10,500, and parents by $8,800.
  • Here's how to find out what you will actually pay.
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Most parents and students are in the dark about how much college costs.

That's the main takeaway from the latest data in an ongoing study by the National Center for Education Statistics, part of the Education Department's research division.

Only around 10 percent of ninth graders correctly estimated the tuition and fees for one year at a public four-year college in their state. Around 57 percent overestimated the costs, and 32 percent underestimated them.

Across the board, students' forecasts were off by an average of $10,500, and parents by $8,800.

Some 26,000 students who were in ninth grade in 2009 were followed and interviewed throughout high school, college, their work-life and beyond. Their parents were also questioned.

Students and parents tend to focus on a school's sticker price rather than its net price — what they'll actually pay after accounting for scholarships and grants, said Mark Kantrowitz, publisher of SavingForCollege.com.

The net price is more personal and therefore accurate.

Most colleges include a net price calculator on their website, Kantrowitz said, and parents and students should plug in their information to evaluate whether the college is "inside or outside the ballpark of affordability."

With college costs climbing, it can be hard to tell what's affordable. Kantrowitz recommends dividing the net price for one year at the college by your family's total income, which is the sum of your adjusted gross income and untaxed income. "If this ratio is less than 25 percent, the college is affordable," Kantrowitz said.

The Institute for College Access and Success has a list of tips for navigating the financial aid system, and the Consumer Financial Protection Bureau allows you to compare different colleges' financial aid packages.

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But you won't get access to any aid unless you fill out the Free Application for Federal Student Aid (FAFSA).

Over a third of this year's high school graduates didn't complete the form, personal finance website NerdWallet recently found, leaving behind $2.6 billion in free college money as a result.

The average person who filed the FAFSA in the 2015-2016 academic year received around $8,500 in federal aid.

"No matter how much your family earns, it's crucial to fill out the FAFSA," said Brianna McGurran, NerdWallet's student finance expert.