Perhaps most critical, the total cost of attending a particular college or university, as estimated by the institution, sets the upper limit on how much a family can take out in both federal and private student loans, leading some students to borrow more than they need and some to borrow less and run out of money.

“This is how you get debt and no degree,” Dr. Goldrick-Rab said. “It’s not tuition that’s driving people out of school.”

Room, board and personal expenses make up about half of college costs. Most students live off-campus — 87 percent — but even prospective students who plan to live in dorms can be affected by shaky statistics. That’s because some college planning websites, including the College Scorecard, average schools’ on- and off-campus living expenses to reach their net price — the cost of attending less the average amount of grants and scholarships awarded.

Miscalculations can be traced to the thicket of regulations that govern college pricing. The Department of Education requires colleges to report the cost of attendance. It tells them what kinds of costs to include — tuition, room and board, books and supplies, and personal expenses — but gives lots of wiggle room in how they come up with the numbers. With no federal rules and few recommendations to guide them, colleges use a hodgepodge of methods to predict costs.

That might explain why room and board estimates for colleges within the same county can vary so much — on average, by $6,448, according to research by the New America foundation published in May.

Some estimates are so low that students could not live on them even with a crowd of roommates, according to the real estate website Trulia, which came out with its own study last September. For example, the University of California, Santa Barbara, posted its 2015-16 housing budget as $6,345, but Trulia calculated that students would need to spend $13,478 each to share a two-bedroom apartment within the university’s ZIP code. Even if five students shared a four-bedroom apartment, they would each have to pay $8,460. (Of course, students often move outside of pricey ZIP codes, trading higher rents for longer commutes from less desirable areas.)

Some colleges keep estimates low, they say, to prevent their students from going into too much debt. If students get more than they need, it’s easy to forget it’s a loan and spend it.