How this calculator works
This calculator finds the standard fixed monthly payment on a student loan using the same amortization formula that applies to any installment loan, then shows total interest over the full term. It works for federal and private student loans alike: federal loans generally set rates and terms through government programs that change over time, while private loans are priced individually by the lender based on credit and other factors, so always enter the actual rate and term from your loan documents or servicer rather than assuming a typical figure.
The extra monthly payment field simulates sending more than the required payment every month, with the entire extra amount applied straight to principal. That shrinks the balance faster, which means less interest accrues each following month — a compounding effect that grows the longer the loan would otherwise run.
The formula
Standard monthly payment: M = P × i ÷ (1 − (1 + i)^−n)With extra payments, the balance is simulated month by month: balance = balance × (1 + i) − (M + extra), until it reaches zero.P is the loan balance, i the monthly interest rate (annual rate ÷ 12), and n the number of scheduled monthly payments (years × 12). The simulation and the closed-form payment formula agree exactly when the extra payment is zero.
Worked example: $30,000 at 5.5% for 10 years, $100 extra a month
- Monthly rate i = 0.055 ÷ 12 = 0.004583; number of scheduled payments n = 120.
- Standard payment M = 30,000 × 0.004583 ÷ (1 − 1.004583^−120) ≈ $325.58, for total interest of about $9,069 over 10 years.
- Paying $425.58 a month instead (the standard payment plus $100) clears the balance in 86 months rather than 120 — 34 months, or nearly 3 years, sooner.
- Total interest under the accelerated schedule falls to about $6,316, a savings of roughly $2,753 compared with the standard 10-year schedule.
Frequently asked questions
Should I enter my federal or private loan rate?
Enter whichever loan you are analyzing. Federal student loan rates are typically fixed for the life of the loan and set through legislation that changes periodically, while private loan rates are set by the lender and may be fixed or variable — check your loan agreement or servicer account for the exact current rate rather than assuming a published average.
Does extra payment reduce my required minimum payment?
No. Extra payments reduce the balance faster and shorten how long the loan runs, but the required minimum payment stays the same unless the servicer recasts the loan. To see the benefit, keep paying at least the standard amount plus any extra you can afford.
What about income-driven repayment plans?
Income-driven plans base the payment on income rather than a fixed amortization schedule, and can extend the term well beyond what this calculator models. This tool is best suited to a standard fixed-payment loan; check your servicer for numbers specific to an income-driven plan.
Is refinancing or consolidating a student loan the same as paying extra?
No — refinancing replaces the loan with a new one, potentially at a different rate or term, while paying extra keeps the original loan but pays it down faster. Both can reduce total interest, but refinancing federal loans with a private lender also gives up federal borrower protections, which is worth weighing separately from the interest math.
Does making extra payments early matter more than making them later?
Yes. Because interest is charged on the outstanding balance, extra principal paid early avoids more months of interest than the same extra amount paid near the end of the loan, which is why starting extra payments as soon as possible maximizes the total interest saved.