How the Student Loan Repayment Calculator works
This tool answers the repayment-phase question every borrower eventually asks: given a balance, a rate, and a term, what is the fixed monthly payment that pays the loan off exactly on schedule — and how much of that payment is interest versus principal? It uses the standard loan amortization formula, the same math used for mortgages, auto loans, and federal or private student loans with a fixed rate.
The formula
For a loan balance P, a monthly interest rate r (the annual rate divided by 12), and n total monthly payments (repayment term in years × 12), the fixed monthly payment is:
M = P × r(1 + r)n ÷ [(1 + r)n − 1]
If the interest rate is 0%, the formula reduces to M = P ÷ n — the balance split into equal installments with no interest added. Each month of a normal repayment, interest is charged on the remaining balance (balance × r), and whatever is left of the payment reduces principal. Because the balance shrinks every month, the interest portion shrinks and the principal portion grows over the life of the loan, even though the total payment stays the same.
Worked example
Take a $30,000 balance at 5.5% annual interest, repaid over 10 years. The monthly rate is 0.055 / 12 ≈ 0.004583 and n = 120 payments. The formula gives a payment of about $325.58 per month. Over the full 10 years that totals roughly $39,069 — the original $30,000 of principal plus about $9,069 of interest.
COVID-19 forbearance: how it fits the math
From March 2020 through September 2023, the U.S. Department of Education suspended required payments on federally held student loans and set the interest rate to 0% for that period — a stretch of roughly 42 months. Because the rate was 0%, balances did not grow while payments were paused; the loan simply sat unchanged until repayment resumed. This calculator models that by letting you add a number of forbearance months: the loan balance carries forward unchanged for that many months, and the standard amortization schedule then runs on top of it, so the total time from today to a zero balance is the forbearance months plus the months of active repayment. Forbearance does not add interest in this model — it only delays when the amortization clock starts.
Extra payments and payoff speed
Any extra amount you add to the required monthly payment goes straight to principal, since the required amount already covers that month's interest in full. The calculator re-amortizes the loan month by month with your required payment plus the extra amount, tracking the balance until it reaches zero — that is how it finds the shorter payoff time and the reduced total interest shown in the results. Extra payments matter most early in the loan, because a lower balance earlier means less interest accrues in every month that follows.
What moves the payment and total interest most
- Interest rate: a higher rate raises both the monthly payment and the total interest, since more of each early payment goes toward interest rather than principal.
- Repayment term: a longer term lowers the monthly payment but increases total interest paid, because the balance stays outstanding — and accruing interest — for more months.
- Extra payments: even a modest recurring extra payment can meaningfully shorten the payoff time and cut total interest, because it compounds against a smaller balance every month it's applied.
- Forbearance: a 0% forbearance period does not change the balance or the eventual monthly payment, but it does push out the calendar date of the final payment by the number of forbearance months.