Interest is calculated on your balance, compounded at intervals your bank sets
Banks calculate savings account interest by explore a percentage rate to the money you have on deposit. The rate is called the Annual Percentage Yield (APY), and it tells you what percentage of your balance you'll earn over a year. The actual amount you receive depends on three things: how much money sits in the account, what APY 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.
The compounding schedule matters more than most people realize. A bank might compound interest daily, monthly, or quarterly. Daily compounding means the bank calculates what you've earned each day and adds it to your balance, so the next day's calculation includes yesterday's earnings. Monthly or quarterly compounding spreads those calculations further apart, which means your money grows more slowly. The APY already accounts for the compounding schedule, so you don't have to do the math yourself—but understanding the difference helps you compare accounts accurately.
Key Takeaways
- The APY shown on a savings account already includes the effect of compounding, so it represents your actual yearly return if you leave the money untouched.
- Banks compound interest on different schedules—daily, monthly, or quarterly—and daily compounding grows your money faster than less frequent compounding at the same APY.
- The interest you earn is calculated only on money that was in the account during the period being compounded, so deposits and withdrawals change what you earn.
- Interest rates change over time, and banks can lower your APY without notice, so the rate you see today may not be the rate you earn next month.
How the calculation actually works
The formula banks use is straightforward: Interest = Balance × (APY ÷ 365) × Number of Days. If you have $10,000 in an account with a 4.5% APY and the bank compounds daily, you earn roughly $1.23 per day ($10,000 × 0.045 ÷ 365). That daily amount gets added to your balance, so the next day the calculation includes that $1.23 plus your original $10,000.
The catch is that your balance changes whenever you deposit or withdraw money. If you deposit $5,000 on day 15 of a month, the bank only calculates interest on the original $10,000 for the first 14 days, then on $15,000 for the remaining days. Some banks use the "average daily balance" method instead, which adds up your balance at the end of each day and divides by the number of days in the period. This smooths out the effect of deposits and withdrawals but produces roughly the same result.
Most banks show you the interest earned in your monthly or quarterly statement. You can verify the calculation by checking your opening balance, closing balance, and the interest posted. If the numbers don't match the APY, contact the bank—errors happen, though they're uncommon.
Why APY matters more than the interest rate
Banks sometimes advertise an "interest rate" separate from the APY. The interest rate is the raw percentage applied to your balance; the APY is that rate plus the effect of compounding. A bank might advertise a 4.4% interest rate with a 4.5% APY because of daily compounding. The APY is the number that matters for comparing accounts, because it shows you the real return you'll receive.
When you see two savings accounts side by side, always compare the APY, not the interest rate. A 4.5% APY compounded daily will earn you more than a 4.5% APY compounded monthly, even though the APY is the same. The compounding frequency is usually listed near the APY on the account details page.
How often interest is credited to your account
Compounding frequency and crediting frequency are not the same thing. A bank might compound interest daily but credit it to your account only once a month. This doesn't change the total amount you earn—the daily compounding still happens—but it means you see the interest appear in your balance only once a month. Some banks credit interest monthly, others quarterly, and a few credit it daily.
The timing of credits doesn't affect your earnings, but it does affect when you can withdraw the interest. If interest is credited monthly and you need the money on day 20 of the month, you'll have access to the interest earned through day 30 of the previous month, but not the interest earned so far in the current month.
What happens when interest rates change
The Federal Reserve sets a benchmark interest rate that influences what banks offer on savings accounts. When the Fed raises rates, banks typically raise the APY on new accounts and sometimes on existing ones. When the Fed lowers rates, banks lower APY quickly—sometimes within days. Your bank can change your APY at any time without asking permission, though they must notify you before the change takes effect.
High-yield savings accounts tend to change rates more frequently than traditional savings accounts because they're designed to track market conditions closely. If you opened an account at 4.5% APY and rates drop, your APY will drop too. This is why the interest you earn on a savings account is not locked in—it's variable unless you move money to a certificate of deposit (CD), which does lock in a rate for a set period.
How minimum balances affect interest earned
Some savings accounts require a minimum balance to earn the advertised APY. If your balance falls below the minimum, the bank might pay a lower rate, charge a monthly fee, or both. A few banks calculate interest only on the amount above the minimum, so if the minimum is $1,000 and you have $5,000, you earn interest only on $4,000.
Read the account terms carefully to understand how your bank handles minimum balances. If you can't maintain the minimum consistently, a no-minimum account will earn you more money even if the APY is slightly lower, because you won't lose earnings or pay fees.
Interest earned on money market accounts and CDs
Money market accounts and certificates of deposit use the same compounding and calculation methods as savings accounts, but with different terms. A money market account works like a savings account but may offer a higher APY in exchange for a larger minimum balance or limits on withdrawals. A CD locks in an APY for a fixed period—typically three months to five years—and you pay a penalty if you withdraw before the term ends.
The interest calculation is identical: your balance times the APY, compounded at the frequency the bank specifies. The difference is that a CD's rate doesn't change during the term, so you know exactly what you'll earn. A savings account's rate can change at any time, so the interest you earn next month might be different from this month.
Frequently Asked Questions
Does interest compound on interest I've already earned?
Yes, that's what compounding means. Once the bank adds interest to your balance, the next compounding period includes that interest in the calculation. This is why daily compounding grows your money faster than monthly compounding—you earn interest on the interest more often.
What's the difference between APY and APR?
APY (Annual Percentage Yield) is used for savings accounts and shows your real return including compounding. APR (Annual Percentage Rate) is used for loans and credit cards and does not include compounding. For savings, always look at APY.
If I withdraw money mid-month, do I lose all the interest for that month?
No. Interest is calculated based on how long your money was in the account. If you withdraw on day 15, you earn interest only for the 15 days the money was there. The interest earned through day 15 is credited on the regular schedule.
Can a bank lower my interest rate without telling me?
Banks must notify you before lowering your rate, though the notification can come by mail, email, or through your online account. They cannot lower your rate without notice, but they can lower it with just a few days' notice.
Why do some banks offer much higher APY than others?
Online banks typically offer higher APY than brick-and-mortar banks because they have lower overhead costs. Banks also adjust rates based on how much money they need to attract. During periods when banks have plenty of deposits, rates drop. When deposits are scarce, rates rise to attract more customers.