One perk of marrying into royalty? Kate Middleton isn’t up at night worrying about Prince William’s credit score.

For everyone else, a spouse’s financial track rec­ord can be cause for concern. Marriage itself doesn’t automatically trigger changes in scores, but the history each spouse brings into the union can significantly affect shared and individual fortunes.

The exact formula varies, but credit scores are based on complex algorithms that include factors such as the length of your credit history, the amount of available credit, outstanding debt and any negative marks such as bankruptcies. Several different scores are marketed to consumers, but the FICO score is still the most widely used by lenders and ranges from 300 to 850.

Here’s a look at how credit scores factor into married life:

MORTGAGES

A common milestone after marriage is buying a home. This is perhaps when a spouse’s credit score is most critical.

If the mortgage is going to be under both names, the bank will check each spouse’s credit score in determining eligibility and the terms of the loan. But how the scores — and other financial information — are weighed will vary depending on the lender.

The lender might take an average of the two scores or, in many cases, simply use the lower score to err on the side of being conservative, said Careen Foster, director of scores product management at FICO Inc.

So even if your score is an impressive 780, the bank might base the loan’s interest rate on your spouse’s score of 620.

That would translate into a big financial hit; on a $200,000 mortgage the payment would be about $1,000 a month for the stronger score, versus about $1,200 a month for the lower score, according to FICO. That’s for a 30-year, fixed-rate mortgage, assuming 4.44 percent and 6.03 percent interest rates for the respective scores.

There are a few ways to mitigate the impact of a bad score. The ideal situation is to take some time to rehabilitate the credit score before applying for a mortgage.

Another option is for the spouse with the stronger score to take out the mortgage. But this could significantly reduce the amount the couple can borrow because only one person’s income will be considered for the loan. It also means only that spouse will be liable for the debt.

Keep in mind that credit scores aren’t the only factors used to determine whether you qualify for a mortgage. Assets such as savings and investment accounts can help offset bad scores.

MERGING MONEY

It’s the mingling of finances that often comes with marriage that leaves scores vulnerable. With joint credit card accounts, for example, the account is factored into each spouse’s credit score just as if it were an individual account. So if one spouse has limited available credit, this could be a way to help lift his or her score. Scores benefit when borrowers have a higher ratio of available credit to outstanding debt.

On the flip side, if your spouse racks up charges you can’t cover, both your scores could take hits. Keep in mind that banks might also require credit checks before a spouse can be added as a joint owner of an account.

If you simply add your spouse as an authorized user on a credit card, however, the impact might not be as significant.Authorized users aren’t liable for the debt and therefore aren’t weighted as heavily in the newest version of the FICO scoring model. That’s a change from older versions of the score, when there was no differentiation between authorized user accounts, individual accounts or joint accounts. FICO says 3,500 lenders are already using the new score and that the company is in the proc­ess of moving major banks over.

Exactly how much an authorized user account will affect a score depends on other factors. Its impact will be minimized for those with a longer and fuller credit history.

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Candace Choi is an personal finance writer.