Interest Rate Calculator
Calculate the exact interest rate on any loan given your payment amount — or find the return rate needed to reach your investment goal.
How to Use the Calculator
- 1Use the Loan Interest Rate tab to find the annual interest rate on any loan. Enter the original loan amount your fixed monthly payment and the loan term in months. This is useful for checking if the rate you were offered matches what you are actually paying.
- 2Use the Investment Return Rate tab to find the annual return needed to grow an investment from a starting value to a target value over a given number of years. This uses the compound annual growth rate formula.
- 3Use the Simple Interest Rate tab to calculate the interest rate on simple interest loans or deposits where interest does not compound. Enter the principal total interest paid and time period.
- 4Download the free PDF for a complete breakdown of your interest rate calculation to share with your lender or financial advisor.
Example Calculation
Tab 1: Loan $10,000 | Payment $222.44/mo | 48 months
- Monthly Rate: 0.271%
- Annual Rate: 3.25%
- APR (Effective): 3.30%
- Total Paid: $10,677.12
- Total Interest: $677.12
Tab 2: Investment PV $5,000 | FV $8,000 | 5 years
- Required Annual Return: 9.86%
- Total Growth: $3,000 (60%)
Tab 3: Simple Interest — Principal $5,000 | Interest $1,500 | 3 years
- Simple Interest Rate: 10.00%
Frequently Asked Questions
What is the difference between interest rate and APR?
The interest rate is the basic cost of borrowing expressed as an annual percentage. APR (Annual Percentage Rate) is the effective annual rate that accounts for compounding. For a loan with monthly compounding a 3.25% annual rate has an APR of approximately 3.30% because interest compounds each month. The APR more accurately represents the true cost of borrowing and is the rate lenders are required to disclose under US law.
What is the difference between nominal and effective interest rates?
The nominal rate is the stated annual interest rate without accounting for compounding frequency. The effective rate (APR) reflects the actual return or cost after compounding is applied. For example a 12% nominal rate compounded monthly has an effective annual rate of 12.68%. The more frequently interest compounds the larger the gap between nominal and effective rates. For most consumer loans with monthly payments the nominal rate and APR are very close.
How do I use this calculator to check if my lender is charging the right rate?
Enter your original loan amount your fixed monthly payment and the number of months remaining. The calculator uses Newton-Raphson iteration to find the exact interest rate implied by your payment. Compare this to the rate in your loan agreement. If they match your lender is charging correctly. If the calculated rate is higher your actual cost of borrowing is higher than the stated rate.
What is CAGR and how is it used for investments?
CAGR stands for Compound Annual Growth Rate. It represents the steady annual rate at which an investment would need to grow to reach a target value from a starting value. For example growing $5,000 to $8,000 in 5 years requires a 9.86% CAGR. CAGR is used to compare investment performance evaluate business growth and set realistic return expectations for financial planning.
What is simple interest and when is it used?
Simple interest is calculated only on the original principal without compounding. The formula is Interest equals Principal times Rate times Time. Simple interest is used for some short-term personal loans certain savings accounts and as an approximation for longer term calculations. Most consumer loans like mortgages and auto loans use compound interest which costs more over time because interest is charged on previously accumulated interest.