Interest grows your balance by a percentage of what you hold, compounded at intervals your bank sets

Interest in a savings account works like this: your bank pays you a percentage of your balance at regular intervals—usually daily, monthly, or quarterly. That percentage is called the annual percentage yield, or APY. If your account earns 4.5% APY and you hold $1,000, the bank calculates how much interest you've earned based on how many days have passed since the last interest payment, then adds that amount to your account.

The key mechanic is compounding: once interest is added to your balance, the next interest calculation includes that new, larger balance. This means you earn interest on your interest. The more frequently your bank compounds—daily is better than monthly, monthly is better than quarterly—the more total interest you accumulate over time, even at the same APY rate.

The actual dollar amount you earn depends on three things: how much money sits in the account, what APY the bank offers, and how long the money stays there. A $10,000 balance at 4.5% APY earns roughly $450 per year if left untouched. A $1,000 balance at the same rate earns roughly $45 per year. Move money in or out, and the calculation adjusts for the days that money was actually in the account.

Key Takeaways

  • Banks calculate interest as a percentage of your balance (the APY) at intervals they set, usually daily or monthly.
  • Compounding means interest earned gets added to your balance, so the next interest payment is calculated on a larger amount.
  • Daily compounding produces more total interest than monthly or quarterly compounding at the same APY, because interest accrues more frequently.
  • The actual dollars you earn depend on your balance size, the APY rate, and how long money stays in the account.
  • APY rates vary by bank and change over time, so the interest you earn this month may differ from next month if the rate shifts.

How banks calculate the interest payment each period

When your bank compounds interest daily, it divides the annual rate by 365 (or 366 in a leap year) to get a daily rate, then multiplies that by your balance at the end of each day. That daily interest accrues—meaning it's tracked but not yet added to your account. At the end of the month or quarter, depending on your bank's schedule, all those daily accruals are added at once.

For example: if your APY is 4.5% and your balance is $5,000, the daily rate is roughly 0.0123% (4.5% ÷ 365). On a day when your balance is exactly $5,000, you earn about $0.62 in interest that day. If your balance drops to $4,500 the next day, that day's interest is calculated on $4,500 instead. At month-end, all those daily amounts are summed and posted to your account.

Banks that compound monthly or quarterly use the same principle but calculate less frequently. Monthly compounding divides the APY by 12; quarterly divides by 4. The result is that you earn slightly less total interest over a year than you would with daily compounding, because interest sits in a "pending" state longer before being added to your balance and beginning to earn its own interest.

Why the same APY can produce different results at different banks

Two banks offering 4.5% APY will produce slightly different balances after one year if one compounds daily and the other compounds monthly. The difference is small—on a $10,000 balance, it might be $5 to $10 over a year—but it's real. This is why the APY figure matters more than the interest rate alone: APY already accounts for compounding frequency, so comparing APYs between banks tells you the true earning power.

Banks also differ in when they start counting interest. Some begin accruing interest the day you deposit money; others start the next business day. Some stop accruing the day you withdraw; others include that day. These timing differences are usually small, but they add up over months. Always check your bank's specific terms if you're comparing accounts.

How deposits and withdrawals change your interest earnings

Interest is calculated on your actual balance each day, so deposits and withdrawals change how much you earn. Deposit $5,000 on the 15th of the month, and interest accrues only on that $5,000 for the remaining days of the month. Withdraw $2,000 on the 20th, and interest for the 20th onward is calculated on the smaller balance.

If you make multiple deposits throughout the month, each one begins earning interest when ready (or the next business day, depending on your bank). The same applies to withdrawals: they reduce the balance on which interest is calculated going forward. This is why the interest you earn in month two might differ from month one if your balance changed—the average balance was different.

APY rates change, and so does your interest income

The APY your bank offers is not fixed forever. Banks raise and lower rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise savings account APYs within days or weeks. When the Fed cuts rates, banks usually cut APYs more slowly, but they do cut them eventually.

This means the interest you earn in January might be 4.5% APY, but in March it could drop to 3.8% APY if your bank lowers its rate. Your balance doesn't change, but your monthly interest payment shrinks. Conversely, if rates rise, your interest income increases. Over a year, these shifts can add or subtract hundreds of dollars from what you earn, depending on how much is in the account and how much rates move.

The difference between stated rate and actual earnings

Banks advertise APY because it's the honest way to compare accounts. But the APY is an annual figure, so you don't earn that full amount each month. If your APY is 4.5%, you earn roughly 0.375% per month (4.5% ÷ 12), which on a $10,000 balance is about $37.50 per month. Over 12 months, that compounds to roughly $450, which is close to 4.5% of $10,000.

The word "roughly" matters because compounding means the actual total is slightly higher than straightforward multiplication. On $10,000 at 4.5% APY compounded daily, you'd earn closer to $460 over a year, not $450, because each month's interest starts earning interest itself. This difference grows larger with bigger balances and higher rates, but it's always in your favor.

What happens to interest if you close the account or move money

Interest accrues only while money is in the account. Close the account or transfer the balance to another bank, and interest stops accruing on that money. However, interest that has already been posted to your account—added to your balance—is yours to keep. If you withdraw on the 10th of the month and your bank posts interest on the 30th, you don't receive that month's interest payment.

Some banks have minimum balance requirements tied to interest rates. If your balance drops below the minimum, the bank may lower your APY or stop paying interest altogether. Read your account agreement to know whether your rate depends on maintaining a certain balance.

Frequently Asked Questions

Does interest compound automatically, or do I have to do something?

Interest compounds automatically. Your bank calculates it, accrues it daily or monthly depending on their schedule, and posts it to your account without any action from you. Once it's posted, it becomes part of your balance and earns interest itself going forward.

If I withdraw money mid-month, do I lose all the interest I earned that month?

No. Interest accrues daily based on your balance each day. If you held $5,000 for the first 15 days and $3,000 for the last 15 days, you earn interest on both amounts for the days they were in the account. You only lose interest on the money after you withdraw it.

Why do some savings accounts earn more interest than others if they have the same APY?

They shouldn't earn significantly different amounts if the APY is truly the same. Small differences come from compounding frequency (daily beats monthly) and timing of when interest is posted. If one account earns noticeably more, check whether the APY is actually the same or whether one compounds more frequently.

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

You can estimate it, but not predict exactly, because your balance may change and your bank may change the APY. If your balance stays constant and the rate doesn't change, multiply your balance by the APY and divide by 12 for a rough monthly figure. The actual amount will be slightly higher due to compounding.

What's the difference between APY and the interest rate my bank lists?

The interest rate is the base percentage; APY includes the effect of compounding. APY is always equal to or higher than the stated rate. When comparing accounts, use APY because it shows what you'll actually earn.