Haley Garberg, a newly married 33-year-old physical education teacher, has been in various repayment plans for nearly a decade. Her first job after graduating in 2008 paid $22,000 annually — a salary that didn’t come close to covering her living expenses and a $700 monthly loan payment. With her parents’ help, she made those payments for a couple of years. But she eventually called her loan servicer and managed to get into a plan that saved her nearly $200 a month — enough wiggle room to afford internet service.

Still, Ms. Garberg was living close to the edge. She moved back with her parents in 2013 to build up her savings as she also dealt with a rare breathing condition that required three surgeries over the following year. A $3,000 insurance deductible meant she had to take out a personal loan to pay her share of the bills, and when she couldn’t afford her inhalers, at roughly $300 to $400 a month, she would do without them. She switched plans again in 2014, and pursued a master’s degree in hopes of boosting her earning power.

“Income-driven repayment doesn’t care that you have 18 bills to pay,” she said.

First instituted 25 years ago, income-dependent repayment was expanded during the administrations of George W. Bush and Barack Obama. It also grew more complicated. Borrowers must sort through an alphabet soup of income-driven repayment plans: I.C.R., I.B.R. (which comes in two flavors, new and classic), PAYE and REPAYE.

Monthly payments are often calculated as 10 to 15 percent of discretionary income, but one plan costs 20 percent . Discretionary income is defined as the amount earned above 150 percent of the poverty level, which is adjusted for household size. For a single person, the federal poverty level is typically $12,490, so single borrowers generally pay 10 percent of what they earn above $18,735. (After 20 years — sometimes 25 — any remaining debt is forgiven. So far, about 20 borrowers have remained enrolled long enough for that to happen, according to the Education Department . )

But the payment calculation is the same for all borrowers, and doesn’t account for local variations in cost of living. And, as Ms. Garberg discovered, it also doesn’t consider borrowers’ other costs.