The basic formula: multiply your balance by the annual rate, then divide by 12

Most savings accounts earn straightforward interest, which means the bank pays you a percentage of your balance each month. To find out how much interest you'll earn in a single month, multiply your account balance by the annual interest rate, then divide by 12.

Here's the formula: (Balance × Annual Interest Rate) ÷ 12 = Monthly Interest

If you have $5,000 in your account and the bank offers 4.50% annual interest, the math looks like this: ($5,000 × 0.045) ÷ 12 = $18.75 per month. The 0.045 is the decimal version of 4.50%—just move the decimal point two places to the left.

This calculation assumes your balance stays the same all month. In reality, your balance changes when you deposit or withdraw money, so the interest you actually earn may be slightly higher or lower depending on the timing of those transactions.

Key Takeaways

  • Monthly interest is calculated by taking your annual interest rate, multiplying it by your current balance, and dividing the result by 12.
  • Convert percentage rates to decimals before doing the math—4.50% becomes 0.045.
  • Banks use different methods to count the days in your account, so your actual interest may differ slightly from your calculation.
  • Interest that compounds monthly (added to your balance each month) will earn you slightly more over time than straightforward interest.

Why your actual interest might differ from the calculation

Banks don't always use the same day-counting method. Some use the actual number of days in the month (28 to 31), while others use a standard 30-day month or 360-day year. This small difference adds up over time, especially on larger balances or higher rates.

Your bank's disclosure documents—usually called the Truth in Savings Act disclosure or account terms—will explain which method they use. You can find this on their website or ask a teller for a copy.

The timing of deposits and withdrawals also matters. If you deposit $2,000 on the 25th of the month, that money may only earn interest for the last few days of that month, depending on when the bank calculates interest.

The difference between straightforward and compound interest

The formula above calculates straightforward interest—interest paid only on your original balance. Most savings accounts use compound interest, which means the interest earned each month gets added back to your balance, and then you earn interest on that interest the next month.

With compound interest, your monthly earnings grow slightly each month. After the first month, you earn interest on your original balance plus the interest from month one. After the second month, you earn interest on all three amounts.

The difference is small in the short term. On a $5,000 balance at 4.50% compounded monthly, you'd earn $18.75 in month one (straightforward), but by month 12, monthly interest would be about $18.84 because of compounding. Over years, this gap widens.

How to find your account's actual interest rate

The interest rate your bank advertises—called the Annual Percentage Rate (APR)—is what you use in the calculation above. This rate changes frequently, sometimes weekly or even daily, depending on market conditions and the bank's policies.

Check your bank's website, your account statement, or call customer service to find your current APR. Many banks show the rate in the account details section online or in your monthly statement. If you're comparing banks, make sure you're looking at the APR for the same type of account—rates vary between basic savings, high-yield savings, and money market accounts.

If your rate has changed since you last checked, recalculate your expected monthly interest using the new rate. Banks must notify you of rate changes, usually by email or statement notice, but it's worth checking periodically.

Using a spreadsheet or calculator to track interest over time

If you want to see how your interest compounds month after month, a spreadsheet makes the math automatic. Create three columns: Month, Balance, and Interest Earned.

In month one, enter your starting balance and use the formula (Balance × 0.045) ÷ 12 to calculate interest. In month two, add the previous month's interest to the balance, then calculate interest on that new total. Copy the formula down for 12 months, and you'll see exactly how much you earn over a year.

Many online banks also show a projected earnings chart in your account dashboard. This gives you a quick picture of how your balance will grow without doing the math yourself.

What happens if your rate changes mid-month

Banks sometimes change rates during a month. When this happens, the bank calculates interest using a weighted average—they explore the old rate to the days before the change and the new rate to the days after.

You don't need to calculate this yourself. Your bank handles it automatically and shows the exact amount on your statement. If you want to verify it, ask customer service for the breakdown of how they applied each rate.

Frequently Asked Questions

Do I need to do this calculation myself, or does the bank do it for me?

The bank calculates and deposits your interest automatically. You don't have to do the math. This calculation is useful if you want to understand how much you should be earning or to compare rates between different banks before opening an account.

What if my balance changes during the month?

Banks typically calculate interest based on your average daily balance throughout the month. If you deposit $1,000 on day 15, that money earns interest for only the remaining days of the month. Your statement will show the exact amount earned based on your actual balance history.

Is the interest rate the same as APY?

APR and APY are related but not identical. APR is the annual rate before compounding; APY includes the effect of compounding. For savings accounts, APY is usually slightly higher than APR. Banks must disclose both, so check your account terms to see which one applies to your account.

Can I predict exactly how much interest I'll earn next month?

Not precisely, because your balance will likely change and the rate might change. You can estimate using your current balance and rate, but the actual amount will depend on deposits, withdrawals, and any rate changes during the month.

Why is my interest so low compared to the advertised rate?

The advertised rate is annual. Divide it by 12 to see the monthly rate. A 4.50% annual rate earns only about 0.375% per month. On smaller balances, this adds up slowly. Higher balances and longer time periods make the interest more noticeable.