Payment Calculator
Solve any loan for either the monthly payment given a term, or the number of months needed to pay it off given a fixed monthly payment.
How to use the Payment Calculator
- Enter your inputs into the Payment Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The payment-and-term formulas
M = P · r(1 + r)^n / ((1 + r)^n − 1) · · · n = −log(1 − P·r/M) / log(1 + r)
The first formula solves for payment given principal P, periodic rate r and term n. The second solves for the number of payments given a fixed payment M.
Worked example
For a $15,000 loan at 6% APR, a 36-month term gives M ≈ $456.33/month. If you can only afford $300/month at the same rate, the payoff stretches to about 60 months (5 years) and total interest doubles.
Frequently asked questions
Can I solve for payoff time instead of payment?
Yes — this calculator works both directions. Enter loan amount, rate and either the payment or the term, and it will solve for the missing value.
What happens if my payment is below the interest accrual?
If the monthly payment is smaller than one month of interest, the loan balance grows instead of shrinking and never pays off. The calculator will flag this case.
Does this work for credit cards?
Credit card payoff math uses the same formula, but the minimum payment is usually a percentage of the balance — so payment falls as balance falls, which extends payoff time enormously.