The basic formula for monthly interest

To find your monthly interest payment, multiply your current balance by your annual interest rate, then divide by 12. That is the simplest version.

Here is the formula written out: Monthly Interest = (Current Balance × Annual Interest Rate) ÷ 12. If you owe $5,000 on a loan with a 6% annual rate, your monthly interest is ($5,000 × 0.06) ÷ 12 = $25.

This calculation assumes your balance stays the same all month. In reality, your balance usually drops as you make payments, so the interest you owe next month will be slightly lower. But this formula gives you the amount due in any given month.

Key Takeaways

  • Monthly interest equals your current balance times your annual rate, divided by 12.
  • Your annual interest rate must be written as a decimal (6% becomes 0.06) before you multiply.
  • The interest portion of your payment changes each month because your balance changes.
  • Credit cards and variable-rate loans recalculate interest daily, so the exact amount depends on when in the month you check.

Converting your annual rate to a decimal

The most common mistake is forgetting to convert the percentage to a decimal. If your rate is 6%, you use 0.06 in the formula, not 6.

To convert: divide the percentage by 100. So 6% becomes 6 ÷ 100 = 0.06. A 12% rate becomes 0.12. A 0.5% rate becomes 0.005. Once you have the decimal, plug it into the formula.

Why your interest payment changes each month

When you make a payment, part of it goes toward interest and part goes toward the balance itself. The next month, your balance is lower, so the interest you owe is also lower.

For example, if you owe $5,000 at 6% annual interest, your first month's interest is $25. If you pay $200 that month, $25 goes to interest and $175 reduces your balance to $4,825. Next month, interest on $4,825 is only $24.13. Over time, more of each payment goes toward principal and less toward interest.

How credit cards calculate interest differently

Credit cards usually calculate interest daily rather than monthly. They divide your annual rate by 365 (or sometimes 360) to get a daily rate, then multiply that by your balance each day, then add up all those daily charges.

This matters because your balance changes every time you make a charge or payment. If you pay off your balance before the due date, you may owe no interest at all. If you carry a balance, the exact interest depends on which days you had which balance — information your statement will show you.

Most credit card statements show you the interest charged for that billing period, so you do not have to calculate it yourself. But if you want to estimate what you will owe, the monthly formula (balance × annual rate ÷ 12) gives you a reasonable approximation.

Interest on loans with fixed monthly payments

Mortgages, car loans, and personal loans usually have a fixed monthly payment. Part of that payment is interest, and part is principal. Early in the loan, most of the payment is interest. Late in the loan, most is principal.

You can calculate the interest portion for any month using the formula above: take your remaining balance at the start of that month, multiply by the annual rate, and divide by 12. The rest of your payment goes toward principal.

Your loan documents or monthly statement will show you exactly how much of each payment is interest and how much is principal. You do not have to calculate it yourself, but knowing the formula helps you understand why the split changes over time.

Comparing interest rates when shopping for loans

When you are comparing two loans, the monthly interest formula helps you see the real cost difference. A $10,000 loan at 5% costs $41.67 per month in interest. The same loan at 7% costs $58.33 per month. That $16.66 difference adds up over years.

Keep in mind that this shows only the interest in the first month. Over the life of the loan, the total interest you pay depends on how long you borrow and how much principal you pay down each month. But comparing the first month's interest gives you a quick sense of how much the rate difference matters.

Frequently Asked Questions

Do I need to calculate interest myself, or will my lender tell me?

Your lender will tell you the interest you owe each month on your statement. Calculating it yourself is useful for understanding how it works, checking your statement for errors, or estimating what you will owe before you take out a loan.

What if my interest rate changes during the year?

Use the rate that applies to that month. If your rate changes on the 15th, you may owe interest at two different rates that month — your statement will show the split. For future months, use the new rate in the formula.

Why do some lenders use 360 days instead of 365?

Some lenders divide the annual rate by 360 instead of 365 to simplify the math. This results in slightly higher daily interest. Your loan documents will tell you which method your lender uses. The difference is small but adds up over a year.

Can I use this formula for savings account interest?

Yes, the formula works the same way. If you have $2,000 in a savings account earning 0.5% annual interest, you earn ($2,000 × 0.005) ÷ 12 = about $0.83 per month. Most savings accounts compound interest daily or monthly, so your actual earnings will be slightly higher.