Banks multiply your balance by an interest rate to calculate what they owe you

Your bank takes the money in your account, lends it out or invests it, and pays you a portion of what they earn. The amount they pay you depends on three things: how much money you have in the account, what interest rate the bank offers, and how often the bank compounds the interest—meaning how often it adds earned interest back into your balance so you earn interest on that interest too.

Most savings accounts use daily compounding, which means the bank calculates interest every single day and adds it to your balance. Some accounts compound monthly or quarterly instead. The more often interest compounds, the more you earn, because each time the bank adds interest to your balance, your next interest calculation is based on a slightly larger amount.

The actual math is straightforward: the bank divides the annual interest rate by the number of days in a year (365 or 366), multiplies that daily rate by your account balance, and adds the result to your account. Then tomorrow it does the same thing with the new, slightly higher balance. Over a year, these tiny daily additions add up.

Key Takeaways

  • Interest is calculated by multiplying your account balance by the interest rate, usually on a daily basis.
  • Compounding means the bank adds earned interest back into your balance, so you earn interest on interest.
  • Daily compounding produces more earnings than monthly or quarterly compounding because interest is calculated more often.
  • The annual percentage yield (APY) already accounts for compounding, so it shows the real amount you will earn in a year.
  • Your actual earnings depend on your balance staying the same—withdrawals reduce what the bank calculates interest on.

The difference between APR and APY

Banks advertise two different rates: APR (annual percentage rate) and APY (annual percentage yield). APR is the basic interest rate without compounding factored in. APY is the real rate you earn after compounding is included. For savings accounts, APY is the number that matters because it shows what you actually take home.

For example, a bank might offer 4.50% APR on a savings account. If that account compounds daily, the APY might be 4.60%. The difference is small but real—over a year on a $10,000 balance, daily compounding adds roughly $100 more than simple interest would. The higher the APR and the more often it compounds, the bigger the gap between APR and APY grows.

Your bank statement or account details page will show the APY. That is the number to use when comparing accounts at different banks, because it already includes the effect of compounding.

How your balance affects what you earn

Interest calculations are based on your account balance on the day the calculation happens. If you have $5,000 in the account on Monday and withdraw $2,000 on Tuesday, the bank calculates interest on $5,000 for Monday and $3,000 for Tuesday onward. Deposits work the same way—money you add starts earning interest the day it lands in the account.

Some banks use the average daily balance method instead of calculating daily. With this method, the bank adds up your balance at the end of each day during the month, divides by the number of days, and calculates interest on that average. This method smooths out the effect of deposits and withdrawals, so a large withdrawal late in the month hurts your earnings less than it would under daily calculation.

Most savings accounts today use daily compounding, but it is worth checking your account details to see which method your bank uses. The difference is usually small unless your balance changes a lot from day to day.

Why interest rates change

Banks set their own interest rates based on what the Federal Reserve does. When the Fed raises its benchmark rate, banks typically raise the rates they offer on savings accounts. When the Fed cuts rates, banks cut theirs too—usually faster than they raise them. This means the APY on your account can go up or down without you doing anything.

Some accounts offer a fixed rate that does not change for a set period, usually three months to a year. Others offer a variable rate that can change at any time. High-yield savings accounts almost always have variable rates, which is why their APY can jump from 4.50% to 5.25% in a few weeks when the Fed moves. Traditional bank savings accounts often have lower rates that change less frequently.

If you are comparing accounts, check whether the rate is fixed or variable and for how long. A high rate that can drop next month is less valuable than a slightly lower rate that is locked in.

How to calculate your own interest earnings

You can estimate what you will earn using a simple formula: multiply your balance by the APY, then divide by 12 for a monthly estimate. For example, $10,000 at 4.50% APY earns roughly $37.50 per month ($10,000 × 0.045 ÷ 12). This is an approximation because it does not account for the exact days in each month or changes to your balance, but it is close enough for planning.

For a more exact calculation, use the compound interest formula: Final Balance = Starting Balance × (1 + APY)^(number of years). If you start with $10,000 at 4.50% APY for one year, the math is $10,000 × (1.045)^1 = $10,450. The $450 is your interest earned.

Most banks provide an interest calculator on their website where you can enter your balance and see what you will earn. Your bank statement also shows interest earned to date, so you can track whether the actual amount matches what you expected.

What happens to interest if you close your account

Interest accrues (builds up) every day, but the bank only deposits it into your account on a set schedule—usually monthly or quarterly. If you close your account before the next deposit date, you lose the interest that has accrued but not yet posted. Some banks will pay out accrued interest when you close, but others will not, so check your account terms before you close.

If your account requires a minimum balance and you drop below it, the bank may stop paying interest entirely or charge a monthly fee that wipes out your earnings. This is why it matters to read the fine print on your account—a $25 monthly fee on a $5,000 balance earning 4.50% APY means you are actually losing money.

Frequently Asked Questions

Does interest compound on money I just deposited?

Yes. Interest starts accruing the day your deposit clears. If you deposit $1,000 on Monday and the bank compounds daily, that $1,000 earns interest starting Monday. The amount is tiny—less than a penny on most balances—but it counts.

Why does my bank statement show less interest than I calculated?

The most common reason is that your balance changed during the month. If you withdrew money mid-month, the bank calculated interest on the lower balance for the rest of the month. Also check whether your bank compounds daily or monthly—monthly compounding produces less interest. Finally, confirm you are looking at the right time period; interest may post on the 15th of each month, so a statement dated the 10th will not show the full month's interest yet.

Can I earn interest on interest?

Yes, that is what compounding does. When the bank adds interest to your balance, the next interest calculation includes that added interest. Over time, this creates a snowball effect. The longer your money sits untouched, the more noticeable the compounding becomes.

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

Money market accounts often offer higher interest rates than regular savings accounts, but they usually require a larger minimum balance and limit how many withdrawals you can make per month. Both use the same compounding methods. The higher rate is the main advantage, but it comes with more restrictions.

Does my bank owe me interest if the rate drops after I open the account?

No. Banks can lower rates on variable-rate accounts at any time. If you locked in a fixed rate, that rate is may provide for the stated period. Once that period ends, the bank can change it. This is why some people move money to a different bank when rates drop—the new bank might offer a higher rate on a new account.