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Interest Rate Calculator : Find Implied APR & Real Cost of Borrowing

Reverse-engineer any financing deal. If a lender gives you the loan amount, term, and monthly payment without clearly disclosing the interest rate, this calculator computes the exact implied APR using numerical root finding.

Interactive Calculator

100% Client-Side Engine
yrs
Implied Annual Interest Rate (APR)
8.354% per annum
Monthly Installment
$315
Total Interest Paid
$1,340
Total Repayment Amount
$11,340

Step-by-Step Examples

$10,000 Loan for 3 Years paying $315/month

Uncovering advertised loan interest.

Inputs: Principal: $10,000, Monthly Payment: $315, Term: 3 Years
Result: Implied Annual Rate: 8.448%, Total Interest: $1,340

Paying $315/mo on $10,000 over 36 months represents an 8.45% annual rate.

How to Use This Tool

  1. Enter principal loan amount borrowed.
  2. Enter exact monthly payment requested.
  3. Specify loan term in years.
  4. Calculate to find the exact annual percentage rate (APR).

Frequently Asked Questions

Because the interest rate variable r appears both linearly and exponentially in the amortization formula, it cannot be solved algebraically and requires iterative numerical methods like Newton-Raphson.

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