Banks multiply your balance by a yearly rate, then divide by the number of days in a year to find what you earn each day

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The next day, you earn interest on the new, slightly larger balance. This compounds—each day's interest earns its own interest going forward.

The actual math is straightforward. The bank takes your account balance, multiplies it by the annual interest rate (called the APY or Annual Percentage Yield), divides by 365 days, and that's what you earn that day. If your balance is $10,000 and the APY is 4.5%, you earn roughly $1.23 per day. Over a year, those daily amounts add up to $450.

The key variable is the APY itself. Banks set this rate and can change it whenever they want—they are not locked in. When the Federal Reserve raises or lowers its benchmark rate, savings account rates usually follow within days or weeks, but the timing and amount vary by bank.

Key Takeaways

  • Daily compounding means interest is calculated on your balance every day, and that interest itself starts earning interest the next day.
  • The formula is your balance multiplied by the APY, divided by 365, and that amount is added to your account each day.
  • APY is the only number you need to compare between banks—it already includes the effect of compounding.
  • Banks can change the APY at any time, so the rate you see today may be lower or higher in three months.
  • The more frequently interest compounds (daily beats monthly), the slightly more you earn, but the difference is small unless balances are very large.

Why APY matters more than the interest rate alone

Banks sometimes advertise an "interest rate" separate from the APY. The interest rate is the raw percentage; the APY is that rate plus the effect of compounding. Because compounding adds real money to your account, APY is the only number worth comparing between banks.

For example, two banks might both advertise 4% interest. But if Bank A compounds daily and Bank B compounds monthly, Bank A's APY will be slightly higher—maybe 4.08% versus 4.07%. Over a year on a $50,000 balance, that tiny difference is about $5. On smaller balances it is invisible; on very large balances it matters more.

When you are shopping for a savings account, ignore the "interest rate" label and look only at the APY. That is the real number.

How compounding frequency changes what you earn

Compounding can happen daily, monthly, quarterly, or annually. The more often interest is added back to your account, the more total interest you earn, because each addition becomes part of the balance that earns interest the next period.

In practice, the difference between daily and monthly compounding is small—usually less than 0.01% APY on the same stated rate. The difference between daily and annual compounding is larger but still modest for most account sizes. A $5,000 balance earning 4% APY compounded daily versus annually might differ by about $2 over a year.

Most online savings accounts compound daily, which is why they tend to offer slightly higher APYs than traditional banks that compound monthly or quarterly. But do not choose a bank based on compounding frequency alone—the APY difference is usually tiny. Choose based on the APY itself and the bank's reputation.

What happens when you deposit or withdraw money mid-month

Interest is calculated on your actual balance each day. If you deposit $5,000 on the 15th of the month, you earn interest on that $5,000 starting the 15th. If you withdraw $2,000 on the 20th, your interest from the 20th onward is calculated on the lower balance.

Some banks use an "average daily balance" method instead, which adds up your balance for each day of the month and divides by the number of days. This smooths out the effect of deposits and withdrawals. Most online banks use daily balance instead, which is simpler and usually slightly better for the account holder.

The timing of deposits and withdrawals does matter if you are moving large sums, but for most people the difference is a few cents per month.

How to find the real interest you will earn

Banks are required to show the APY clearly on their website and in account disclosures. Look for the label "APY" or "Annual Percentage Yield"—that is the number to use.

To estimate your earnings, multiply your balance by the APY and divide by 12 for a monthly estimate. If you have $20,000 at 4.5% APY, you earn roughly $75 per month. This is an approximation because the actual amount depends on the exact number of days in each month and whether your balance changes, but it is close enough for planning.

Many banks also show a "projected earnings" number on their website if you enter your balance. This is a useful sanity check, but the math above works anywhere.

Why your interest earnings may be lower than you expect

The most common reason is that the APY has dropped since you opened the account. If you opened a savings account at 4.5% APY two years ago, that rate may now be 3.75% or lower. Banks lower rates when the Federal Reserve lowers its benchmark rate, and they do this quickly—sometimes within days.

The second reason is that your balance is lower than you think. Interest is calculated on the actual balance in the account on each day. If you withdrew money mid-month or if the balance fluctuates, your average balance for the month is lower than the peak balance.

The third reason is that you are comparing the APY to a different bank's stated interest rate rather than APY. Always compare APY to APY.

How to lock in a higher rate before it drops

You cannot lock in a savings account rate—banks can change APY whenever they want, and most do not require notice. However, you can move your money to a different bank if rates drop significantly.

If you want a may provide rate for a set period, a certificate of deposit (CD) locks in the APY for a fixed term—usually three months to five years. You cannot withdraw the money early without a penalty, but the rate does not change. CDs currently offer higher rates than savings accounts at many banks because you are giving up access to your money.

For money you need to access regularly, a high-yield savings account is more practical. Shop around every few months if rates are changing quickly, and move your balance if another bank offers meaningfully higher APY.

Frequently Asked Questions

Does interest compound on interest I have already earned?

Yes. Once interest is added to your account, it becomes part of your balance and earns interest itself. This is the definition of compounding. Over years, this effect becomes significant—a $10,000 balance at 4.5% APY grows to about $10,460 after one year, and that $460 in interest then earns interest in year two.

What is the difference between APY and APR?

APY includes compounding; APR does not. APY is always higher than APR on the same stated rate. For savings accounts, always use APY. APR is used for loans and credit cards, where you are paying interest rather than earning it.

If I move my money to a different bank, do I lose the interest I earned?

No. Interest that has already been added to your account is yours to keep and move. You only lose future interest—the interest that would have been earned if you had left the money in the original account.

Can a bank change the APY on my existing account without warning?

Yes. Banks can change savings account APY at any time without advance notice. They are required to notify you of the change, usually by email or mail, but the change takes effect when ready. This is why rates on savings accounts fluctuate.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on a savings account is taxable income. Banks send 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 how the interest is calculated—it is a tax matter, not a banking matter.