The basic formula for monthly savings account interest

To find out how much interest your account earns in a month, you need three numbers: your account balance, the annual interest rate (called APY), and the number of days in that month. The formula is: Balance × APY ÷ 365 × Number of Days in Month = Monthly Interest.

Here's a concrete example. Say you have $5,000 in your account and your bank pays 4.50% APY. In January (31 days), you would earn: $5,000 × 0.045 ÷ 365 × 31 = $19.11. In February (28 days in a non-leap year), the same balance earns: $5,000 × 0.045 ÷ 365 × 28 = $17.26.

The reason you divide by 365 is that APY is an annual rate—it tells you what you'd earn in a full year. Dividing by 365 converts that yearly rate to a daily rate, then multiplying by the number of days in the month gives you that month's earnings.

Key Takeaways

  • Monthly interest = (Balance × APY ÷ 365) × number of days in the month, and the result changes if your balance changes during the month.
  • Banks use the average daily balance method, which means they add up your balance for each day of the month, then divide by the number of days to find the average they'll use for interest calculation.
  • Your actual monthly interest may be lower than the formula shows if your balance fluctuates, because interest is calculated on the average balance, not the ending balance.
  • The APY your bank advertises is the rate you'll see on your account details; if it changes, your monthly interest changes too, even if your balance stays the same.

Why your balance matters more than the rate

The interest you earn is directly tied to how much money sits in your account. If you deposit $1,000 mid-month, your interest for that month is lower than if you'd had the full amount there from day one. Banks calculate this using the average daily balance method.

Here's how it works in practice. Suppose you start January with $10,000. On January 15, you withdraw $3,000. The bank adds your balance for each day: $10,000 for days 1–14 (14 days), then $7,000 for days 15–31 (17 days). The average is ($10,000 × 14 + $7,000 × 17) ÷ 31 = $8,419.35. That's the balance the bank uses to calculate your interest, not the $7,000 you ended with.

This is why timing matters. Deposits made early in the month earn interest for more days than deposits made late in the month. Withdrawals late in the month reduce your average less than withdrawals early in the month.

How to find your APY and check your bank's calculation

Your APY is listed on your account statement, in your online banking portal, or in the account agreement your bank gave you when you opened the account. It's different from the interest rate (APR) because APY includes the effect of compounding—interest earned on interest. For savings accounts, the difference is usually small, but it matters.

To check whether your bank calculated your interest correctly, gather your statement and your APY. Find the average daily balance (many banks show this on the statement itself). Then use the formula: Average Daily Balance × APY ÷ 365 × Number of Days in Month. Compare your result to the interest shown on the statement. They should match or be within a few cents.

If your bank doesn't show the average daily balance on the statement, you can ask customer service for it, or you can request a detailed transaction history and calculate it yourself by adding up each day's balance and dividing by the number of days.

What happens when your APY changes

Banks change their APY regularly, especially when the Federal Reserve changes interest rates. When your rate goes up, your monthly interest goes up immediately. When it goes down, your monthly interest goes down. The change applies to the next interest period, not retroactively.

If your bank raises the APY from 4.50% to 5.00%, and you have a $5,000 average daily balance, your monthly interest in a 31-day month rises from $19.11 to $21.23. The difference is small in one month, but over a year it adds up. This is why it's worth comparing rates between banks—a difference of 0.50% on $5,000 means about $25 more per year.

Your bank will notify you of rate changes, usually by email or through your online account. Read these notices carefully, because some banks lower rates without much warning.

The difference between simple and compound interest

Most savings accounts use daily compounding, which means the bank calculates interest on your balance plus any interest you've already earned. This is better for you than simple interest, where interest is calculated only on your original deposit.

Here's the difference over a year. With $5,000 at 4.50% APY and simple interest, you'd earn $225 total ($5,000 × 0.045). With daily compounding, you earn slightly more because each day's interest gets added to your balance, and the next day's interest is calculated on that larger amount. Over a year, the difference is usually $1 to $3 on a small balance, but it grows as your balance grows.

When you see "APY" on your account, that rate already includes the effect of compounding. The bank has done the math for you. If you see "APR" instead, that's a simple interest rate, and the actual return will be lower.

Using a calculator to save time

If you don't want to do the math by hand, you can use a savings account interest calculator. Search "savings account interest calculator" and you'll find free tools from financial websites. Plug in your balance, APY, and the number of days in the month, and the calculator gives you the interest earned.

These calculators use the same formula described above. They're accurate as long as your balance doesn't change during the month. If it does, you'll need to either calculate the average daily balance yourself or use a more detailed calculator that lets you enter multiple transactions.

Your bank's online portal may also have a tool that shows you projected interest based on your current balance and rate. This is a quick way to see what you'll earn without doing any calculation.

Common mistakes when calculating interest

The most common mistake is using your ending balance instead of your average daily balance. If you had $10,000 for most of the month and withdrew $5,000 on the last day, using $5,000 to calculate interest will underestimate what you actually earned. Always use the average daily balance if your balance changed during the month.

Another mistake is forgetting to divide the APY by 365. If you multiply your balance by the APY directly, you'll get the annual interest, not the monthly interest. For example, $5,000 × 0.045 = $225 per year, not per month. Dividing by 365 and multiplying by the number of days in the month gives you the correct monthly amount.

A third mistake is using the wrong number of days. February has 28 days in most years and 29 in leap years. Don't assume every month has 30 days. Your bank counts the actual number of days in the calendar month.

Frequently Asked Questions

Does interest post to my account every month?

Most banks post interest monthly, but some post it quarterly or even annually. Check your account agreement or ask your bank. When interest posts, it's added to your balance and becomes part of the amount that earns interest the next period.

What if I have multiple savings accounts at the same bank?

Each account earns interest separately based on its own balance and APY. The bank calculates interest for each account independently. If one account has a higher APY than another, that's usually because it has different terms or a higher minimum balance requirement.

Can I calculate interest if my APY changes mid-month?

Yes, but you need to split the calculation. Calculate interest for the days under the old rate, then calculate interest for the days under the new rate, and add them together. Most banks handle this automatically and show the breakdown on your statement.

Is the interest I earn taxable?

Yes. Interest earned in a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. This is separate from calculating how much interest you earned—it's about taxes owed on that interest.

Why does my calculated interest not match what the bank shows?

The most likely reason is that you used your ending balance instead of your average daily balance, or you used the wrong number of days. Ask your bank for the average daily balance they used, then recalculate. If it still doesn't match, contact customer service—errors are rare but do happen.