Debt Repayment Calculator
Calculate your monthly loan or debt repayment amount, total interest paid, and payoff timeline from principal, interest rate and term using standard amortization — and see how an extra monthly payment shortens the term and cuts interest.
Input
Optional. Paid on top of the required monthly payment to pay off the loan faster and save on interest.
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
What this calculator does
Enter a loan or debt's principal, annual interest rate and term, and this calculator works out your required monthly payment, how much interest you'll pay in total over the life of the loan, and the total amount paid (principal plus interest). It also lets you add an optional extra monthly payment to see exactly how much faster the debt is paid off — and how much interest that saves — by simulating the amortization month by month rather than just scaling a formula.
This works for mortgages, auto loans, personal loans, student loans or any other fixed-rate, fixed-term debt that's paid down with equal monthly installments.
How it's calculated
The standard loan amortization formula gives the fixed monthly payment that fully repays a loan over its term:
M = P x [r(1 + r)^n] / [(1 + r)^n - 1]
Where P is the principal (loan amount), r is the monthly interest rate (annual rate
divided by 12), and n is the total number of monthly payments (term in years times 12).
Total interest paid is simply M x n - P.
If the interest rate is 0%, the formula above divides by zero, so a separate rule
applies: the payment is just the principal spread evenly across the term, M = P / n,
with no interest.
When an extra monthly payment is added, the calculator no longer uses the formula alone — it simulates the loan payment by payment: each month, interest is charged on the remaining balance, the rest of the payment (base payment plus extra) reduces the balance, and the loop continues until the balance reaches zero. This is the only accurate way to model extra payments, since paying more than required changes how quickly the balance — and therefore future interest — shrinks. The simulation is capped at 1,200 months (100 years) so a payment too small to even cover the interest can't cause it to run indefinitely; the calculator reports a clear message instead.
How to use this tool
- Enter the Loan Amount — the principal you're borrowing or currently owe.
- Enter the Annual Interest Rate as a percentage (e.g.
6for 6%). - Enter the Loan Term in years.
- Optionally, enter an Extra Monthly Payment — an amount paid on top of the required payment every month. Leave it at 0 to see the standard repayment schedule.
The summary table updates instantly and, when an extra payment is set, shows both the original and accelerated payoff time plus the interest saved by paying more.
FAQ
Does extra payment always reduce the term? Yes, as long as it's greater than $0 — any amount paid above the required minimum goes straight toward principal, so the balance clears sooner and less interest accrues along the way.
Why is my last payment a different amount? Real amortization schedules true-up the final payment to exactly clear the remaining balance, which is usually a few cents different from a regular payment. The totals shown here already account for that.
Can I use this for a fixed extra one-time payment instead of a recurring one? No — the extra payment field applies every month for the life of the loan. For a one-time lump-sum payoff scenario, treat the remaining balance after the lump sum as a new principal and recalculate.
Privacy
All calculations run entirely in your browser. Nothing you enter — loan amounts, rates or terms — is sent to a server or stored anywhere.