Interest Rate Calculator - Calculator See


Interest Rate Calculator

Calculate the exact implied annual interest rate charged on loans or earned on investments given payments, term, and balances.

Calculator Inputs

$
Starting principal balance.
$
Regular monthly installment.
mos
Number of monthly installments (60 mos = 5 yrs).

Calculated Results

Calculated Annual Interest Rate
6.50% APR
Effective Annual Rate (EAR) 6.70%
Total Lifetime Interest Paid $5,989.44
Monthly Periodic Interest 0.542% / month
Values calculate live as you adjust inputs.

About Interest Rate Calculator

Uses numerical root-finding (Newton-Raphson method) to solve for the true implicit interest rate embedded in fixed loan financing agreements.

What is Interest Rate Calculator?

Uses numerical root-finding (Newton-Raphson method) to solve for the true implicit interest rate embedded in fixed loan financing agreements.

How to Use This Calculator

  1. Enter loan principal borrowed.
  2. Enter monthly payment.
  3. Enter total loan duration in months.
  4. View the implied APR and effective interest rate.

The Mathematical Formula & Variables

Iteratively solve for monthly periodic rate r matching present value annuity equation

Step-by-Step Worked Example

Scenario: Borrow $25,000 with $516.49 monthly payment for 60 months.

  1. Total paid = 516.49 * 60 = $30,989.40.
  2. Interest paid = $5,989.40.
  3. Implicit monthly rate r = 0.005417 -> Annualized APR = 6.50%.

Result: 6.50% APR

Frequently Asked Questions

Why can't interest rate be calculated with simple division?

Because monthly payments pay down principal, causing interest charges to compound and decline over time rather than remaining flat.