The guardrail
Total undergraduate debt under the expected first-year salary keeps standard payments near 10% of gross — sustainable by most underwriting logic. A $45k-salary field supports ~$35–40k; a $75k field supports $60k+. The guardrail is a field-specific number, which is why "college costs too much" arguments that ignore major economics always feel half-right.
The safety net and its cost
Income-driven plans make any payment survivable — at the price of 20–25 years of payments and possible forgiveness tax consequences. They are excellent insurance against the guardrail failing, and a poor default for graduates who can pay standard. The loan calculator's term comparison shows exactly what the insurance costs in interest.
Graduate school is a different animal
Grad PLUS rates run higher, balances larger, and the salary payoff more variable (law, medicine, MBA outcomes spread enormously). The same guardrail applies but deserves stress-testing: run the payment at the 25th-percentile field salary, not the median — the borrowers who suffer are the ones who priced the average outcome and lived the tail.