The basic formula: your balance times the rate times the time

Banks calculate savings account interest by multiplying three things: the money you have in the account, the interest rate the bank is paying, and how long your money sits there. The result is the interest you earn. Most banks use a method called daily compounding, which means they calculate interest on your balance every single day, then add that interest back into your account so the next day's calculation includes it.

Here is the simplest version: if you have $1,000 in an account earning 4% annual interest, and the bank calculates interest once per year, you would earn $40 that year. But because most banks compound daily, you actually earn slightly more — the interest gets added to your balance partway through the year, and then you earn interest on that interest too.

Key Takeaways

  • Banks multiply your account balance by the annual interest rate and divide by 365 to find what you earn each day.
  • Daily compounding means interest gets added to your account every day, so tomorrow's interest calculation includes today's interest.
  • The Annual Percentage Yield (APY) shown on your account paperwork already includes the effect of compounding, so it is the number to compare between banks.
  • Your actual interest earned depends on your lowest balance during the period, not your average or highest balance — most banks use the "daily balance method."
  • Interest rates change over time, so the rate you see today may not be the rate you earn next month.

Why banks use daily compounding instead of yearly

If a bank calculated interest only once per year, you would wait 12 months to see your money grow. Daily compounding means your interest gets added to your account every day, and starting the next day, you earn interest on that interest too. This is called the compounding effect, and it is why the total interest you earn is slightly higher than a straightforward yearly calculation would suggest.

For example: with $1,000 at 4% APY compounded daily, you do not earn exactly $40. You earn about $40.80 over the year because of compounding. The difference grows larger as your balance grows larger and as you leave the money untouched for longer.

The difference between APR and APY

APR stands for Annual Percentage Rate — it is the interest rate before compounding is factored in. APY stands for Annual Percentage Yield — it is the rate after compounding is included. Banks are required to show you the APY on savings accounts, and that is the number you should use when comparing accounts between banks.

If one bank advertises 4% APY and another advertises 4% APR, the APY account will earn you more money, even though the APR sounds lower. Always look for APY when you are deciding where to open an account.

How the daily balance method works

Most banks use the daily balance method to decide which balance they use for calculating interest. This means they look at your lowest balance on any single day during the month (or the period they are calculating for), and they use that balance to calculate your interest for the entire period.

This matters because it means withdrawing money early in the month can reduce the interest you earn for the whole month, even if you deposit it back later. For example: if you start the month with $5,000, withdraw $2,000 on day 5, and deposit it back on day 25, the bank calculates interest for the entire month using $3,000 as your balance, not the average of $4,500.

Some banks use different methods — the average daily balance method adds up your balance for each day and divides by the number of days, which can be slightly better for you if you make withdrawals. Ask your bank which method they use when you open an account.

Why your interest rate changes over time

Banks set savings account interest rates based on what the Federal Reserve does with something called the federal funds rate. When the Federal Reserve raises or lowers this rate, banks usually raise or lower the rates they pay on savings accounts within days or weeks. This means the 4% rate you see today might become 3.5% next month, or it might rise to 4.5%.

Banks are not required to notify you before they lower your rate, though they usually do. You can check your account statement or log into your online banking to see your current rate. If your rate drops and you find a better rate elsewhere, you can move your money to a different bank — there is no penalty for closing a savings account and opening one somewhere else.

How to find the interest you earned on your statement

Your bank statement shows the interest you earned during that period, usually listed as "Interest Paid" or "Interest Earned" near the bottom. This is the actual money the bank added to your account. The statement also shows the rate that was in effect during that period, though sometimes you have to look in the account details section rather than on the main statement page.

If you want to verify the calculation yourself, you can ask your bank for the daily balance method they used and the rate for each day. Most banks will provide this information if you call or visit in person, though it may take a few business days. Online banks often show this information in your account dashboard.

What happens to interest if you close your account early

If you close your savings account before the end of the month or statement period, the bank calculates interest only for the days you had the account open. You do not lose any interest you already earned — the bank pays you the full amount you are owed up to the day you close it. The interest is added to your final balance before the account closes.

Some savings accounts have minimum balance requirements, meaning if your balance drops below a certain amount (often $25 or $100), the bank may charge you a monthly fee or stop paying interest. Check your account agreement to see if yours has this rule, and if it does, keep your balance above the minimum to avoid losing money to fees.

Frequently Asked Questions

If I deposit money halfway through the month, do I earn interest on it right away?

No. Most banks calculate interest based on your lowest balance during the statement period, so a deposit halfway through does not help you earn more interest that month. The deposit starts earning interest the next day, and it counts toward your balance for the next statement period. If you want to maximize interest, deposit money early in the month.

Why does my interest sometimes look different from what I calculated?

The most common reason is that your balance changed during the month. Banks use your lowest balance for the entire period, not your average or ending balance. Also, if the interest rate changed during the month, the bank may have used different rates for different days. Check your statement to see the rate and balance they used.

Can I lose money if interest rates drop?

No. Interest rates dropping means you will earn less interest going forward, but you will not lose the money you already have. The interest you already earned stays in your account. You only earn less on new deposits or on future months if the rate stays low.

Is the interest I earn on a savings account taxed?

Yes. Interest income is taxable as ordinary income on your federal tax return. Banks send you a form called a 1099-INT if you earned $10 or more in interest during the year. Keep your statements so you have the numbers when you file taxes. Some states also tax interest income.

What is the difference between a savings account and a money market account for interest?

Money market accounts often pay higher interest rates than regular savings accounts, but they usually require a larger minimum balance to open and maintain. Both use daily compounding and the daily balance method. The trade-off is higher interest for less access to your money — money market accounts often limit how many withdrawals you can make per month.