Some refi rates are now around 2.5%. This is the credit score you'll need to get the best rates when refinancing your mortgage

Your credit score matters a lot when you refinance your mortgage.

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Lenders consider your credit score to be one of the most important factors they use when determining what interest rate you will qualify for, experts say. So even though you may see interest rates for a refinance at under 3%, that doesn't mean you'll qualify for those rates. (Some mortgage refi rates are around 2.5%; see the lowest rates you may qualify for below, and here).

In simple terms, the lower your credit score, the more interest you're likely to pay over the term of your mortgage. Even if you're only paying a quarter of a percentage point more than you would if your credit score were higher — the interest can add up to thousands of dollars. If you have a $500,000 mortgage at 4.5% and your monthly payment is $2,533, refinancing at 4.25% could mean reducing your monthly payment to $2,459, which saves $324 per month. ( Find the best mortgage refinance rates in your area here. )

So what's the credit score you need to get the lowest rates?

"Credit scores of 700 and above have an easier time qualifying, with the best terms available to those with credit scores of 740 and above. While it's possible to refinance with a credit score between 620 and 680, the terms aren't nearly as good," says Greg McBride, chief financial analyst at Bankrate. Adds Holden Lewis, home and mortgage expert at NerdWallet: "Life is easier for refinancers with credit scores of 740 or higher, because from the perspective of mortgage lenders, [they're the] least-risky borrowers.

However, as Jacob Channel, senior economic analyst at LendingTree, points out, you don't always have to have these scores: "For other loan types, like an FHA loan, a borrower could get approved with a credit score as low as 580."

What other factors impact my ability to get a refinance?

Of course, it's important to understand that getting approved at a good rate for a refi isn't just about your credit score. Lenders also look at your income and other aspects of your financial profile like your savings, debts and more.  "Lenders are typically unlikely to work with someone who has a credit score below 580, though there may be a few exceptions depending on the lender and the person applying for the refinance," says Channel. Adding a cosigner to the loan can make someone a more appealing candidate, as can having a lot of money saved up. "Keep in mind that even if the aspects of your overall financial profile apart from your credit score are great, you'll still face an uphill battle in getting approved," says Channel. (See the best mortgage refi rates you may qualify for here).

How can I boost my credit score?

If your credit score doesn't meet the minimum requirements, there are a few things you can do to get approved for a refinance. "First, you can look for a lender that has less stringent credit requirements. Just because one lender didn't approve your refinance doesn't mean every other lender will do the same," says Channel. To boost your credit score, doing things like making monthly payments on time and paying down debt to reduce your credit utilization ratio are among the steps Channel advises taking.

If your score is lower than you'd like, don't despair. "If you're making all your payments on time and paying down any revolving debts, time will heal the wounds. If you're close to the threshold, making a large payment against a revolving balance or utilizing something like Experian Boost could put you over the hump relatively quickly," says McBride.