How Student Loan Payments Are Calculated
Fixed-rate student loans use the same standard amortization formula as mortgages and personal loans:
Where P is your loan balance, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments. Every payment first covers the interest that accrued since the last payment; the remainder reduces principal.
Federal Student Loan Interest Rates (2025–26)
| Loan Type | Borrower | 2025–26 Rate |
|---|---|---|
| Direct Subsidized | Undergraduate | 6.53% |
| Direct Unsubsidized | Undergraduate | 6.53% |
| Direct Unsubsidized | Graduate/Professional | 8.08% |
| Direct PLUS | Parents & Graduate Students | 9.08% |
Rates are fixed for the life of the loan disbursed during that academic year. Set annually by Congress based on the 10-year Treasury note.
Standard vs. Income-Driven Repayment
The standard 10-year repayment plan pays off your loan in the shortest time with the least total interest. Income-driven repayment (IDR) plans reduce your monthly payment based on your income and family size, which can free up cash flow — but results in more total interest paid over a longer repayment period.
| Plan | Payment | Term | Best For |
|---|---|---|---|
| Standard | Fixed (10 yr amortization) | 10 years | Paying off fastest |
| Graduated | Low start, rises every 2 yrs | 10 years | Expected income growth |
| SAVE / REPAYE | 5–10% discretionary income | 20–25 years | Low income, PSLF seekers |
| IBR | 10–15% discretionary income | 20–25 years | High loan-to-income ratio |
How Much Extra Payments Save on a $35,000 Loan
| Extra / Month | Interest Saved | Months Saved |
|---|---|---|
| $50 | ~$750 | ~8 months |
| $100 | ~$1,400 | ~15 months |
| $200 | ~$2,600 | ~26 months |
| $500 | ~$5,000 | ~50 months |
* Estimates for $35,000 at 6.53% on a 10-year term.
Worked Example: Applying the Formula Step by Step
Scenario: $42,000 in Direct Unsubsidized loans at 6.53% on the standard 10-year repayment plan. Here is exactly how the monthly payment is derived.
That first-month split — $229 to interest, $248 to principal — illustrates how amortization front-loads interest. By month 60 (the midpoint), the split has reversed: roughly $155 goes to interest and $322 to principal. Each extra dollar you pay in the early months eliminates future interest charges at full rate, which is why even a $100/month extra payment in years 1–3 saves disproportionately more than the same payment in years 8–10.
Questions You Might Ask
What are the current federal student loan interest rates?
For 2025–26: undergraduate Direct loans at 6.53%, graduate Direct Unsubsidized at 8.08%, and PLUS loans at 9.08%. Rates are set each July 1 based on the 10-year Treasury note and are fixed for the life of loans disbursed in that academic year.
What is income-driven repayment?
IDR plans cap payments at 5%–10% of your discretionary income (income above 150% of the federal poverty line). After 20–25 years of qualifying payments, the remaining balance is forgiven. The SAVE plan is currently the most borrower-friendly IDR option for most federal borrowers.
Should I pay off student loans early or invest?
At 6.53%, this is a genuinely close call. Max out any employer 401(k) match first (instant 50–100% return). Then, a Roth IRA and HYSA emergency fund typically take priority. After those are funded, extra loan payments at 6.53% vs. stock market returns (7–10% historically) is a judgment call based on your risk tolerance. Private loans at 9%+ almost always benefit from aggressive early payoff.
Can I deduct student loan interest?
Up to $2,500 per year, subject to income limits ($75,000 single / $155,000 MFJ MAGI for 2025, phasing out at $90,000 / $185,000). This is an above-the-line deduction — available without itemizing. Effectively reduces your after-tax interest rate by your marginal tax rate × the deductible amount.
Methodology
Student loan payments are calculated using the standard fixed-rate amortization formula consistent with U.S. Department of Education guidelines. Income-driven repayment estimates use 10% of discretionary income (defined as income above 150% of the 2025 federal poverty guideline for a single person, approximately $22,590), which approximates the SAVE and IBR plans for most borrowers. Actual IDR payments depend on your specific plan, family size, and annual recertification. Federal loan rates are sourced from the U.S. Department of Education's annual rate announcements. All calculations run client-side in your browser. Last updated: May 2026.