30% of homeowners are making a mistake that could cost them thousands

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Nearly a third of American homeowners may have no idea what their mortgage rate is.

That's according to new data from Bankrate, which surveyed 2,194 adults, including 1,330 homeowners. When asked, 29 percent of respondents with a mortgage either didn't know their rate or wouldn't say.

Understanding your mortgage rate is crucial because even the smallest difference can add up to tens of thousands of dollars over time. It's an especially important number for homeowners with adjustable-rate mortgages, which rise and fall.

Let's use a $200,000 home, since that's approximately the median home value in the U.S., with 20 percent down and a 30-year mortgage as an example. A rate of 3 percent means you'd pay about $82,843 in interest over the length of the mortgage. A rate just 1 percent higher would cost $114,991 in interest — an increase of over $32,000.

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With mortgage rates rising to their highest level in years, it's becoming an increasingly important decision for homeowners with adjustable rates to consider switching to a fixed rate.

The Federal Reserve has already raised interest rates in 2018 and is expected to do so three more times. "If you have an adjustable-rate mortgage, you could be in for a doozy of a payment increase at the next reset," Greg McBride, chief financial analyst at, tells CNBC Make It. Although your bills might not have changed much right away, things could really start to add up by the end of the year and beyond.

In addition to the rate, the type of mortgage you get matters. Although more than 90 percent of buyers opt for a 30-year fixed rate mortgage, financial expert and former CNBC host Suze Orman says home-buyers are overlooking an easy way to save big. "I wish more people would take out a 15-year mortgage instead," she writes in a post on her blog.

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The reason Orman favors a shorter term loan is simple: It's cheaper. The shorter-term option locks in lower rates, which results in less interest paid out over time.

Again, even a change of just 1 percent can make a huge difference. On a $250,000 loan, paying 4.3 percent for 30 years amounts to $195,000 in interest, according to Orman, while 15 years at 3.5 percent comes out to only $72,000. That's more than $100,000 in savings.

However, a 15-year mortgage isn't the right choice for everyone. While the lower interest rate saves money in the long term, the monthly payments are much higher, which simply isn't possible for many owners.

No matter which type of mortgage is right for you, it's important to know your rate. It determines if you should opt for a different type of mortgage and if you can still afford your home overall.

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