How to Calculate Monthly Interest
Calculating interest month-by-month is an essential skill. You’ll often see interest rates quoted as an annual percentage, but sometimes it’s more helpful to know exactly how much that adds up to in dollars and cents. We commonly think in terms of monthly costs.
For example, you have monthly utility bills, food costs, or a car payment. Interest is also a monthly (if not daily) event, and those recurring interest calculations add up to big numbers over the course of a year. Whether you’re paying interest on a loan or earning interest in a savings account, the process of converting from an annual rate to a monthly interest rate is the same.
Divide By 12
The first step is to calculate a monthly interest rate. To do so, divide the annual rate by 12 to account for the 12 months in every year (see Step 4 in the example below). You'll need to convert from percentage to decimal format to complete these steps. Divide by the number of time periods: You started with one annual time period, and you’re looking for 12 monthly periods. The same concept can be used with other time periods:
- For a daily interest rate, divide the annual rate by 360 (or 365, depending on your bank).
- For a quarterly rate, divide the annual rate by four.
- For a weekly rate, divide the annual rate by 52.
Example: assume you pay interest monthly at 10 percent per year. What is your monthly interest rate and how much will you pay (or earn) on $100?
- Convert the annual rate from percentage to decimal format (by dividing by 100)
- 10/100 = 0.1 monthly
- Divide the annual rate by 12
- 0.10/12 = .0083
- Calculate the monthly interest on $100
- 0.0083 x $100 = $0.83
- Convert the monthly rate in decimal format back to a percentage (by multiplying by 100)
- 0.0083 x 100 = 0.83 percent annually
Want a spreadsheet with this example filled in for you? See the free Monthly Interest Example spreadsheet, and make a copy of the sheet to use your own numbers. The example above is the most basic way to calculate monthly interest rates and costs for a single month.
But you might want a bigger-picture view of your finances. With many loans, your loan balance changes every month. With auto, home, and personal loans, you gradually pay down your balance over time, usually ending up with a lower balance each month.
That process is called amortization, and an amortization table helps you calculate (and shows you) exactly how much you pay in interest every month. Over time, you’ll notice that your monthly interest costs decrease — and the amount that goes towards your loan balance increases.
Home loans can be complicated. It is good to use an amortization schedule to understand your interest costs, but you may need to do extra work to figure out your actual rate. You might know the APR on your mortgage, but APR can contain additional costs besides interest charges (such as closing costs). Also, the rate on adjustable rate mortgages can change.
With credit cards, you can add new charges and pay off debt numerous times throughout the month. All that activity can make calculations cumbersome, but it’s still worth knowing how your monthly interest adds up. In many cases, you’ll use an average daily balance, which is the sum of each day’s balance divided by the number of days in each month (and the finance charge is calculated using the average daily balance). In other cases, interest is charged daily (so you calculate a daily interest rate — not a monthly rate).
With bank accounts, interest might be credited to your account monthly, daily, or quarterly. Use the same calculation shown above to convert to a monthly (or other) interest rate and multiply the rate by your account balance.
Be sure to use the interest rate in your calculations — not the annual percentage yield (APY).
APY accounts for compounding, which is interest you earn as your account grows due to interest payments. APY will be higher than your actual rate unless interest is compounded annually, so it will give an inaccurate result. That said, APY makes it easy to quickly find out how much you’ll earn annually on a savings account with no additions or withdrawals.
As you can see, interest can be calculated monthly, daily, annually, or over any other period. Whatever period is used, the rate you’ll use for calculations is called the periodic interest rate. You’ll most often see rates quoted in terms of an annual rate, so you’ll need to convert to whatever periodic rate matches your question or your financial product.