Monthly payments, total interest, and the extra-payment math — federal and private
2025-26 federal: undergrad 6.39%, grad 7.94%, PLUS 8.94%
| Minimum only | With +$100/mo |
|---|
Student loan math is mortgage math with worse defaults: the standard 10-year federal plan front-loads interest, servicers apply extra payments uselessly unless instructed otherwise, and the borrowing decision itself happens at 17 without a salary attached. This calculator runs the honest numbers — payment, total interest, the extra-payment acceleration — plus the guardrail that should govern the original borrowing: total debt under 1.0× expected first-year salary.
| Loan | Rate | Limits |
|---|---|---|
| Direct undergraduate | 6.39% | $5,500–7,500/yr; $31k aggregate (dependent) |
| Direct graduate | 7.94% | Higher limits — where balances explode |
| PLUS (parent/grad) | 8.94% | Up to cost of attendance — the dangerous one |
| Private | 4–16% by credit | Co-signer usually required; fewest protections |
The structural advice hiding in that table: max the undergraduate Direct loans first (lowest rate, best protections), treat PLUS loans as a flag that the school costs too much, and let private loans be the last resort they're priced as.
The 1.0× rule (total debt ≤ realistic first-year salary in the intended field) is blunt and effective: it keeps payments near 10% of gross on the 10-year plan, and it converts "dream school" conversations into arithmetic. Salary data by major is public (BLS, College Scorecard); the 17-year-old deserves to see it before the parents co-sign anything.
By rate: above ~7% (PLUS, grad, private), paying down is a guaranteed return beating expected market returns risk-adjusted; below ~5%, investing (especially 401k matches — always take the match first) usually wins; between, temperament decides. Never skip the employer match to prepay a 6% loan.
Avalanche (highest rate first) wins mathematically; snowball (smallest balance first) wins behaviorally for people who need visible progress. With federal+private mixes, the private loans' higher rates usually make the avalanche pick them anyway.
One written instruction to the servicer: apply overpayments to PRINCIPAL on [highest-rate loan], do NOT advance the due date. Then verify on the next statement — servicers default to the option that helps them. Biweekly half-payments are a painless stealth version (13 full payments/year).
Only for high-earning borrowers certain they'll never need income-driven plans, forgiveness, or hardship protections — a Big-Tech engineer with 8.9% PLUS loans, maybe. For everyone else the surrendered options are worth more than 1-2% of rate. Private loans: refinance freely and repeatedly.
Federal: income-driven plans can take payments to $0, deferment/forbearance bridge gaps, and default takes 270 days with rehabilitation paths after. Private: fewer options — call before missing (hardship programs exist unadvertised), because private default hits credit fast and co-signers hard. The worst move in either system is silence.
Federal: discharged at death, and parent PLUS discharges when either parent or student dies. Private: varies — many discharge, some pursue estates and co-signers (co-signer release clauses matter at signing). Morbid but worth knowing before a co-signature.
Yes — every figure computes locally in your browser.
Borrow under the 1.0× line, send the principal-application email, and aim the extra $100 at the ugliest rate. Student debt is beatable arithmetic — it just ships with defaults designed for the lender.