Repayment Calculator
$
6.00%
$
Payoff Time4 yrs 10 mos
Total Interest$2,400.00
Total Paid$17,400.00
total paid$17.4K
Principal$15,000.00
Interest$2,400.00
This repayment calculator works backward from a fixed monthly payment to tell you how long a debt will take to clear and how much interest you will pay getting there. Instead of solving for a payment, it solves for time: enter the outstanding balance, the annual interest rate, and the amount you can put toward it each month, and it returns the payoff in years and months plus the total interest and total paid. It also warns you when a payment is too small to ever beat the interest.
Formula
n = −log(1 − rB ÷ P) ÷ log(1 + r), rounded up
- n
- Number of monthly payments until the balance hits zero
- B
- Current outstanding balance
- r
- Monthly interest rate = annual rate ÷ 12 ÷ 100
- P
- Fixed monthly payment; must exceed B × r or the debt never clears
How it works
- Enter the Loan Balance you still owe and set the Interest Rate slider (0%-30% in 0.125% steps).
- Enter the fixed Monthly Payment you intend to make.
- Using the loan payoff formula, the tool reports Payoff Time split into years and months, the Total Interest, and the Total Paid. If the payment does not even cover the first month's interest, it flags the payment as too low because the balance would never fall.
Worked example
Paying off a $20,000 balance at 18% APR with $500 per month.
- Monthly rate r = 18 ÷ 12 ÷ 100 = 0.015; first month interest = 20,000 × 0.015 = $300, below the $500 payment, so it pays down.
- n = −log(1 − 0.015 × 20,000 ÷ 500) ÷ log(1.015) ≈ 61.02, rounded up to 62 months.
- 62 months = 5 years and 2 months; total paid = 500 × 62 = $31,000; total interest = 31,000 − 20,000 = $11,000.
The debt clears in 5 years and 2 months, with $11,000 of interest and $31,000 paid in total.
Frequently asked questions
- What does "payment too low" mean?
- If your monthly payment is less than or equal to the first month's interest, every payment is swallowed by interest and the balance never shrinks. The tool flags this so you know the debt would, in theory, never be repaid at that payment.
- Why is the payoff rounded up to a whole month?
- Loans are repaid in whole monthly installments, so a computed 61.02 months becomes 62. The final payment is usually smaller than the rest, but the schedule still spans the rounded-up number of months.
- How does the interest rate change the payoff time?
- A higher rate sends more of each payment to interest, leaving less to reduce principal, so the balance falls more slowly and the payoff stretches out. Lowering the rate or raising the payment both shorten the timeline.
- How is this different from a loan payment calculator?
- A standard loan calculator fixes the term and solves for the monthly payment. This tool does the reverse: you fix the payment and it solves for how many months you need, which is ideal for "what if I pay $X per month" planning.