// the true cost of borrowing
Your loan has a second price tag.
This is the interest.
Banks show you the monthly payment. We show you the total interest — the number that decides whether your loan costs thousands more, or thousands less.
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Why total interest is the number that matters
Every loan quote leads with the monthly payment, because the monthly payment always looks
manageable. The total interest — what the loan actually costs — is usually buried in
fine print or row eleven of a results table. On a typical $400,000 mortgage at 6.5% over 30
years, that buried number is over $510,000. You pay for the house twice.
This site exists to put that number first. Enter any loan above and the big red figure is
your total interest. Then drag the green slider: even modest extra payments redirect money
from the red side of the bar to your side.
How the math works
Loans amortize: each payment covers that month's interest first, and whatever remains
reduces your balance. Early on, the balance is large, so interest devours most of the payment.
The chart above marks the crossover point — the month your payment finally puts more
toward principal than interest. On a 30-year loan at today's rates, that can take well over a
decade. Extra principal payments drag the crossover earlier, which is why they are so
disproportionately powerful.
Frequently asked questions
What is total interest on a loan?
Total interest is the sum of every interest charge over the life of a loan — the difference between what you borrow and what you actually repay. On a 30-year mortgage it often equals or exceeds the amount borrowed.
How is total interest calculated?
Each month, interest equals your remaining balance times the monthly rate. The rest of your payment reduces the balance. Summing the interest portion of every payment gives total interest. See our methodology for the exact formula.
How can I reduce the total interest I pay?
Three proven levers: pay extra toward principal each month, choose a shorter term, or refinance to a lower rate. Use the slider in the calculator to see exactly what each extra dollar saves on your loan.
Is this calculator accurate?
It uses the standard amortization formula every lender uses, computed to the cent with documented rounding rules and verified by an automated test suite. Lender figures may differ slightly due to start dates and fee structures.