The prospect of having more than one child attend college at the same time — whether it’s twins, triplets or closely spaced siblings — leaves families worried about how they’re going to foot multiple tuition bills at once.

The average cost at a four-year, public, in-state university was $20,090 in 2016-2017, according to the College Board, and $45,370 at a private university. That includes tuition, fees and room and board. Multiply that by two, three or more children — and most families feel the financial stress.

But the good news is that “having more than one child in college at the same time really has a dramatic effect on financial aid,” says Shannon Vasconcelos, college finance consultant at College Coach and a former senior financial aid officer at Boston University and Tufts University.

Some financial aid experts call this “the sibling factor.”

“It is hard to pay for two colleges ; there’s no getting around that,” she says. “The only thing that sometimes eases famil ie s ‘ minds is the knowledge that, in fact, it will be taken into consideration in financial aid packages.”

[Here are seven questions families should discuss when choosing colleges.]

Financial Aid Formula and Siblings

Here’s how it works : When families fill out the Free Application for Federal Student Aid, or FAFSA, they receive an Expected Family Contribution, or the amount that the federal government expects a family to be able to contribute to a child’s college education.

That contribution, known as the EFC, gets split in half if two kids are in college and in thirds if three children are in college. That’s based on a student attending at least half time at a program that leads to a college degree or certificate.

So, for instance, if your EFC is $20,000 for one child in college, it would be $10,000 each for two children in college under the federal formula.

How That Affects Financial Aid

Joe Messinger, co-founder and director of college planning at Capstone Wealth Partners in Dublin, Ohio, says his office sees parents of multiples on a regular basis who are “really concerned” about paying for college.

“I think we’re able to kind of put them at ease, because we walk them through how that can actually work to their advantage when they have multiples,” he says.

A family may not qualify for need-based aid — or financial assistance based on a family’s demonstrated financial need — when they have only one child in college, but that may change once more than one child is in college, Messinger says.

Take the example where a family’s expected contribution is $20,000 for one child. If a college costs $30,000, the family only has $10,000 in financial aid eligibility with one child in college. But if two children are in college, the student becomes eligible for $20,000 in financial aid.

[Learn the definitions of 12 college financial aid terms.]

Not a Guarantee

It’s important for families to understand how a college grants financial aid, Messinger says. Some schools say they will meet 100 percent of a student’s need. That means if a sibling comes on board, that school will likely provide more financial aid to fill the gap, he says.

However, if a school makes no such promises, a student’s financial aid package may not increase with the added pressure of a sibling’s college costs.

“Most schools don’t guarantee they’re going to meet every student’s full eligibility,” Vasconcelos says. “The oldest child will have more eligibility next year when the sibling is going to school, but is the college actually going to give them more financial aid? They may , or they may not.”

Vasconcelos recommends checking with a school’s financial aid office to find out how a sibling enrolling in college will affect the student’s financial aid package.

Further, she cautions that if a younger child receives a generous financial aid package because an older child is already in college, families should be prepared to see that aid decrease when the older child graduates.

[Read the seven questions college financial aid officers wish families would ask.]

CSS/Financial Aid Profile

A lmost 400 organizations — many of which are elite, private colleges — use a form created by the College Board called the CSS/Financial Aid P rofile to award grants and scholarships.

That calculation for siblings is “not as generous” as the calculation used on the FAFSA, Vasconcelos says, and assumes that a family will be able to contribute 60 percent of the EFC if two children are in college.

“If you have two kids that both end up attending schools that use the P rofile , you end up actually having to pay 120 percent of EFC because both schools assume you can contribute 60 percent to them,” she says.

That number changes to 45 percent for three children in college, meaning a family with three children attending schools that use the P rofile could end up paying 135 percent of their Estimated Family Contribution.

Trying to fund your education? Get tips and more in the U.S. News Paying for College center.

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