About the Payment calculator
This payment calculator answers two everyday loan questions. If you know the term, it finds the monthly payment. If you know what you can pay each month, it finds how long the loan will take to pay off.
Either way you get total interest and total paid. In payoff mode it also tells you the size of the smaller final payment. The working shows the formula used for your choice, filled in with your numbers, and warns you if a payment is too small to ever cover the interest.
How to use it
- Under I know, choose The loan term or My monthly payment.
- Enter the Loan amount and annual Interest rate.
- If you chose the term, enter Term (years) to get the monthly payment.
- If you chose the payment, enter your Monthly payment to get the payoff time.
- Compare total interest across a few payments or terms.
The formula
Payment from term:
M = P × r(1 + r)^n ÷ ((1 + r)^n − 1)
Months from payment:
n = −ln(1 − P × r ÷ M) ÷ ln(1 + r)
- M — monthly payment
- P — loan amount
- r — monthly rate = annual rate ÷ 12 ÷ 100
- n — number of monthly payments
- ln — natural logarithm
The month count is rounded up, because a partial month still needs a (smaller) payment. The payment must be larger than P × r, the first month’s interest, or the balance never falls.
Why the payment has to beat the interest
Each month the lender adds interest of balance × r. If your payment is equal to or below that, the balance stays flat or grows, and no number of months will pay it off. That is why the calculator stops with a warning instead of giving a result.
Just above that threshold, payoff times get very long. Small increases in the payment make a big difference there. On a $30,000 loan at 8%, the first month’s interest is $200. A $250 payment takes many years, while $700 clears the loan in 51 payments with about $5,450 of interest.
If you are deciding how much to pay, try several amounts in payment mode and watch how quickly total interest falls. The first few extra dollars above the interest usually save the most.
Worked example both ways
Say you borrow $30,000 at 8%. In term mode with 5 years, r = 0.08 ÷ 12 ≈ 0.0066667 and n = 60, giving a payment of about $608.29 and total interest of about $6,498.
Now switch to payment mode and enter $700. The formula gives n = −ln(1 − 30,000 × 0.0066667 ÷ 700) ÷ ln(1.0066667), a little over 50 months. Rounded up, that is 51 payments, or 4 years 3 months. The last payment is about $447.76, and total interest drops to roughly $5,448.
Paying about $92 more a month saves over $1,000 and finishes the loan nine months early.
Frequently asked questions
How long will it take to pay off my loan?
Choose My monthly payment, enter the balance, rate and payment, and the calculator returns the number of payments and the time in years and months.
What if my payment is too low?
If the payment does not exceed the monthly interest (balance × rate ÷ 12), the loan never gets paid off, and the calculator tells you so.
Why is the last payment different?
The final month only needs the remaining balance plus its interest, so it is usually smaller than the regular payment.
Does this include fees or insurance?
No. It covers principal and interest on a fixed-rate loan. Add any monthly fees separately.
Last updated October 24, 2026. Results are estimates for education and planning — see our disclaimer.