Student Loan Calculator
A $35,000 student loan at 6.5% over 10 years costs $397 a month and $12,690 in total interest. Adding $100 a month pays it off in 7.4 years instead of 10 and cuts the interest to $9,186, saving $3,504. Extending to 20 years drops the payment to $261 but more than doubles the interest.
The term is the biggest lever on a student loan — and extra principal is the cheapest one to pull.
The term sets the interest more than the rate does. The same $35,000 at 6.5% costs $12,690 of interest over 10 years, $19,880 over 15, and $27,628 over 20. Cutting the payment from $397 to $261 by stretching to 20 years costs you $14,938 — more than the rate difference between 5.5% and 7.5% over ten years.
Calculator
Enter your loan and any extra payment. Results update instantly.
How This Calculator Works
The scheduled payment comes from the standard amortization formula: balance, rate, and term. If you add an extra monthly amount, the calculator re-solves the payoff time using the higher payment, since student loans apply any surplus to principal.
Extra principal saves interest twice: it removes balance that would have accrued interest, and it shortens the period over which interest accrues at all.
M = P × r(1+r)n / ((1+r)n − 1) — npayoff = −ln(1 − rP/M) / ln(1+r) with M = payment + extra
Worked example
Using the defaults — $35,000 at 6.5% over 10 years, plus $100 extra a month:
The standard formula gives $397.42 a month. 120 payments total $47,690, so interest is $12,690 — 36% of the amount borrowed.
Paying $497 instead of $397 clears the balance in about 89 months (7.4 years) instead of 120 — two years and seven months early.
Total interest falls to $9,186. That is $3,504 saved for $100 a month — effectively a guaranteed 6.5% return on every extra dollar, tax-free.
The calculator above returns these figures exactly. Federal loans apply extra payments to accrued interest first, then principal, unless you instruct the servicer otherwise in writing.
The same $35,000 across terms
Federal student loans are usually sold on a 10-year schedule, but income-driven plans can stretch to 20 or 25 years. The interest cost follows.
| Term | Monthly payment | Total interest | Total paid |
|---|---|---|---|
| 10 years (standard) | $397 | $12,690 | $47,690 |
| 15 years | $305 | $19,880 | $54,880 |
| 20 years | $261 | $27,628 | $62,628 |
| 25 years | $236 | $35,897 | $70,897 |
A 25-year term cuts the payment by $161 a month but adds $23,207 of interest — nearly two thirds of the loan again. Use the shortest term you can carry, then attack it with extra principal.
Common mistakes with this calculation
- Choosing the lowest payment, not the lowest cost. Income-driven plans can be lifelines, but as a default choice they stretch the term to 20-25 years and double the interest. Take the standard term when you can afford it and prepay instead.
- Paying extra without instructing the servicer. Many servicers apply surplus to next month's bill by default, which advances the due date but saves almost nothing. Write to direct the extra specifically to principal, and verify on the statement.
- Refinancing federal loans to private too early. A lower private rate can beat 6.5%, but refinancing surrenders federal protections: income-driven plans, deferment, and forgiveness programmes. Price that insurance before you trade it for half a point.
- Ignoring the loan in credit decisions. A $397 payment is a $67,000 mortgage-cap reduction at 6.5% over 30 years. Plan the payoff order around what each debt costs you in future borrowing.
When this calculator does not apply
- The calculator models a single fixed-rate loan. Borrowers usually hold several loans with different rates and balances; run each separately.
- It does not model income-driven repayment, where the payment is set by income and the balance can grow if the payment is below accrued interest.
- Federal forgiveness programmes change the economics entirely and are outside the projection.
- Capitalisation events — unpaid interest added to principal after grace or deferment — are not modelled and raise the effective cost.
- Fees and the tax deductibility of student-loan interest (up to $2,500 a year in the US, income-limited) are excluded.
Frequently Asked Questions
How much interest will I pay on a $35,000 student loan?
At 6.5% over the standard 10-year term, $12,690 — about 36% of what you borrowed. Over 20 years the interest rises to $27,628, more than three quarters of the principal.
Is it worth paying extra on student loans?
It is one of the safest returns available: every extra dollar earns the loan rate guaranteed and tax-free. $100 a month extra on the default example saves $3,504 and retires the loan almost two and a half years early.
Should I refinance my federal student loan?
Only if you have no need for federal protections. Refinancing to a lower private rate can save real money, but you permanently give up income-driven plans, deferment options, and any forgiveness eligibility.
Should I pay student loans or invest instead?
Compare the loan rate with your expected after-tax investment return. Paying off a 6.5% loan is a guaranteed 6.5% return; a reasonable baseline is to capture any employer retirement match first, then split between the loan and investing.
Sources & Methodology
This calculator uses standard financial formulas. See our methodology page for the full derivation.
- Repaying Student Debt Consumer Financial Protection Bureau
- Federal Student Aid: Repayment Plans U.S. Department of Education
- Interest Rates and Fees U.S. Department of Education
Last reviewed .
Key takeaways
- $35,000 at 6.5% over 10 years: $397 a month, $12,690 total interest.
- $100 extra a month saves $3,504 and finishes the loan 2.6 years early.
- Stretching 10 to 20 years more than doubles the interest.
- Direct extra payments to principal in writing, or the servicer may just advance your due date.
- Refinancing federal loans trades protection for rate — price the protection first.