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Interest Rate Calculator

Calculate the true interest rate from your monthly payment amount.
Calculator
Enter your values
Analysis
Interpretation of the current calculator output

Enter values to see detailed analysis and insights.

How to Use

Step-by-step instructions
  1. 1Enter your total loan original principal.
  2. 2Input the loan term in years.
  3. 3Enter your exact monthly payment.
  4. 4The calculator will reverse-engineer the effective annual interest rate of your loan.

Iterative Rate Discovery

There is no algebraic way to solve for 'r' directly from the standard amortization formula. We use a binary search algorithm to test thousands of interest rates until we find the exact match for your monthly payment.
M = P[r(1+r)^n] / [(1+r)^n - 1]

Variables:

MMonthly Payment
PPrincipal Loan Amount
nTotal number of payments
rUnknown Interest Rate (Target)

Example

Finding the rate of a $25k Loan

Inputs:

Loan Amount:$25,000
Term:5 Years
Monthly Payment:$489.15

Steps:

  1. 1.Check total payment constraint: $489.15 * 60 = $29,349
  2. 2.Is total > principal? Yes. Interest is $4,349.
  3. 3.Algorithm executes binary search between 0% and 100%.
  4. 4.Locates correct rate match.
Result:
6.50% Annual Interest Rate

Frequently Asked Questions

Why might this rate differ slightly from my loan documents?

Many loans include originations fees, prepaid interest, or insurance costs rolled into the monthly payment. This calculator finds the raw geometric rate based purely on the numbers provided.

What if my result is 0%?

If your monthly payment multiplied by your term is less than or exactly equal to your principal, then no interest is being charged (0% APR).