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Loan Interest Calculator

A loan interest calculator totals the interest you will pay over the life of a fixed-rate loan. Enter the amount, rate, and term to see the monthly payment, total interest, and total repaid.

By the Personalloaned Editorial Team · Last updated 2026-09-16

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Method

How this calculator works

This calculator totals the interest you would pay over the life of a fixed-rate loan, assuming you make every payment on schedule.

M = P * r * (1 + r)^n / ((1 + r)^n - 1)

Total interest = (M * n) - P.

  • P = amount borrowed
  • r = monthly rate = annual rate / 12 / 100
  • n = number of monthly payments

Interest grows with the rate and the term. Paying extra toward principal, or choosing a shorter term, reduces the total.

Rates are user inputs here because they depend on the lender and your credit profile. Enter the rate you want to test.

Common questions

Frequently asked questions

Is total interest the same as the APR?
No. Total interest is the dollar amount paid over the life of the loan. The APR is an annualized percentage that also reflects upfront fees.
How can I reduce the total interest?
Pay extra toward principal, choose a shorter term, or find a lower rate. Each of those lowers either the balance, the number of months, or the cost of each month.
Does this assume a fixed rate?
Yes. The calculation assumes the rate stays the same for the whole term. A variable-rate loan can cost more or less as the rate changes.
Why is the total interest higher on a longer loan?
A longer term means interest is charged for more months, even though each payment is smaller. The total of those smaller payments is usually larger.

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