Introduction
This monthly interest calculator helps you find out how much interest you owe on a late payment. Just enter the amount you owe, the annual interest rate, the number of full months late, and any extra days beyond those months. The calculator shows the total interest, the full amount due, and a step-by-step breakdown of how it all adds up. It uses monthly compounding for full months and simple daily interest for any remaining days, based on a 360-day year. That fits a loan, bill or other debt that charges interest this way. Some credit card issuers instead add interest to the balance every day, so the interest compounds daily.4 Check your agreement to see how your interest is worked out.
How to Use Our Monthly Interest Calculator
Enter a few details about your loan or debt below. The calculator will show you how much interest you owe, your total payment due, and a full step-by-step breakdown of the math.
Principal Amount ($): Type in the original amount of money owed. This is the starting balance before any interest is added. You can enter up to two decimal places (for example, 5000.00).
Annual Interest Rate (%): Enter the yearly interest rate on your loan or debt. Type just the number, not the percent sign. For example, if your rate is 4.125%, type 4.125. You can enter up to three decimal places.
Number of Full Months Late: Enter how many complete months the payment is past due. Use whole numbers from 0 to 1,200, such as 0, 1, 2, or 3. Do not count partial months here.
Remaining Days (Beyond Full Months): Enter any extra days left over after counting full months. For example, if a payment is 3 months and 15 days late, put 3 in the months field and 15 here. This number must be between 0 and 31.
Click the Calculate button to see your results. The calculator will display the interest owed, the total payment due, the effective daily interest rate, and the interest as a percentage of your principal. It also shows a step-by-step solution and two charts so you can see exactly how your balance grows over time.
What Is Monthly Interest?
Monthly interest is the cost you pay for borrowing money, calculated each month on the amount you owe.2 When you take out a loan or carry a balance, the lender charges you a percentage of what you still owe.2 This charge is based on your annual interest rate, which is the yearly cost of the loan split into monthly pieces.2
How Monthly Interest Works
To get a monthly rate, divide the annual interest rate by 12.2 Each month, that rate is applied to your remaining balance. This is called compound interest, meaning interest builds on top of interest already added.3 The longer you wait to pay, the more your balance grows.
If your late period includes extra days beyond full months, interest for those days can be worked out as simple daily interest: the balance times the annual rate divided by the number of days in a year, times the number of days.2 Some banks use 360 as the number of days in a year, so they divide the annual rate by 360 instead of 365.2 Interest that federal agencies owe on late payments is also based on a 360-day year, but unpaid interest is added to the principal at the end of each 30-day period, and it stops building after one year.1 So for a federal late payment, a result for more than 12 months is too high.
Key Terms to Know
- Principal: The original amount of money you owe before any interest is added.2
- Annual Interest Rate: The percentage a lender charges you per year for borrowing money.5 This is different from the APR, which can include fees and other costs beyond the base rate.5
- Compound Interest: Interest that is calculated on both the original amount and any interest already added.3 It makes your balance grow faster over time.
- Simple Interest: Interest worked out only on the amount at the start, with no interest added to the balance along the way.2 Here, the extra days beyond the full months are charged simple interest on the balance after those months.
- Effective Daily Rate: The annual rate divided by 360, the number of days in a year used here, showing how much interest accrues in a single day.
Why This Matters
Even a small interest rate can add up quickly when payments are late. Knowing exactly how much interest you owe helps you plan your budget, avoid surprises, and pay off debt faster. Just enter your principal, rate, and how long the payment is overdue, and this monthly interest calculator shows your total amount due.