Banks multiply your balance by an annual rate, then divide by the number of days in a year

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 formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360, depending on the bank), then multiplied by the number of days the money sat in the account.

The result is that interest compounds — you earn interest on your interest. If you deposit $1,000 at 4.5% annual rate and leave it untouched for a year, you don't earn exactly $45. You earn slightly more, because each day the bank adds a small amount of interest, and the next day's calculation includes that new, slightly larger balance.

The actual amount you see in your account depends on three things: how much money you have, what rate the bank is paying, and how long it stays there. Banks are required to disclose the Annual Percentage Yield (APY), which is the real return you'll get after compounding is factored in. The APY is always higher than the stated interest rate because it accounts for daily compounding.

Key Takeaways

  • Banks calculate interest daily by multiplying your balance by the annual rate and dividing by 365, then adding that amount back to your account each day.
  • Compounding means you earn interest on the interest already added to your account, so your balance grows faster than simple math would suggest.
  • The Annual Percentage Yield (APY) shown on your account is the real return you'll receive, and it's always higher than the stated interest rate because it includes compounding.
  • Different banks compound at different frequencies — daily is most common for savings accounts, but some use monthly or quarterly compounding, which results in slightly lower returns.
  • Your actual interest earnings depend on your balance, the rate offered, and how long the money stays in the account without withdrawal.

Why the APY matters more than the interest rate

When you compare savings accounts, the interest rate advertised is not what you'll actually earn. The bank must show you the APY instead, because that's the number that reflects reality. A 4.5% interest rate with daily compounding becomes roughly 4.6% APY. The difference grows larger with higher rates and more frequent compounding.

If you're choosing between two banks, always compare APY to APY, not rate to rate. A bank offering 4.3% APY compounded daily will pay you more than a bank offering 4.4% interest rate compounded quarterly, even though the second number looks bigger.

How compounding works in practice

Start with $10,000 at 4% APY compounded daily. On day one, the bank calculates $10,000 × 0.04 ÷ 365 = $1.10 in interest. That $1.10 gets added to your account, so your new balance is $10,001.10. On day two, the calculation is $10,001.10 × 0.04 ÷ 365 = $1.10 (slightly more, because the balance is slightly larger). This repeats every day for the year.

After 365 days, your balance is $10,400 instead of $10,400 exactly. That extra $0.40 came from earning interest on interest. Over decades, compounding creates substantial differences. A $10,000 deposit at 4% APY grows to $14,866 in 10 years. At 2% APY, it grows to $12,190. The difference is $2,676 — all from the compounding effect of a higher rate.

The difference between daily, monthly, and quarterly compounding

Most savings accounts compound daily, but some older accounts or accounts at smaller banks compound monthly or quarterly. The difference in your payout is real but small. On a $10,000 balance at 4% annual rate, daily compounding pays roughly $408 per year, while quarterly compounding pays roughly $406 per year. Over a year, that's a $2 difference — not huge, but it adds up over time.

Banks that compound less frequently often offer higher stated rates to compensate, so the APY ends up similar. Always check the APY disclosure, which the bank is required to provide. It will tell you the true return regardless of how often compounding happens.

What happens to interest when you make withdrawals

If you withdraw money from your savings account, the interest calculation stops on that portion. If you had $10,000 earning interest daily and withdrew $3,000 on day 100, the remaining $7,000 continues to earn interest at the full rate, but the $3,000 you withdrew stops earning anything (unless you deposit it elsewhere).

Some banks calculate interest based on your minimum balance during the month, while others use your average daily balance. This matters if you're moving money in and out frequently. A bank using minimum balance will pay interest only on the lowest amount your account held during that period. A bank using average daily balance will pay based on the mean of your daily balances. Most modern banks use average daily balance, which is fairer to customers who make regular deposits and withdrawals.

How interest rates change and what that means for your earnings

Banks change their savings rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise savings rates within days or weeks. When the Fed cuts rates, banks cut savings rates just as quickly. Your existing balance doesn't get locked into an old rate — the new rate applies immediately to all money in the account.

This means your interest earnings can fluctuate month to month. If your bank was paying 4.5% APY and cuts it to 3.8% APY, your monthly interest payment drops right away. Conversely, if rates rise, your earnings increase. Some people move money between banks to chase higher rates, but the difference is usually small enough that the hassle isn't worth it unless you're moving a very large balance.

How to find your actual interest earnings on your statement

Your bank statement shows interest paid in a line item, usually labeled "Interest Paid" or "Interest Earned." This is the actual money the bank added to your account that month or quarter. If your statement shows $3.42 in interest paid and your balance is $10,000, you can work backward: $3.42 × 12 months = $41.04 per year, which is roughly 0.41% APY. That would be a very low rate — most savings accounts today pay between 3% and 5% APY.

If your interest earnings seem low compared to the APY the bank advertised, check whether your balance fluctuated during the period. If you had $10,000 for half the month and $5,000 for the other half, your average balance was $7,500, and interest is calculated on that lower average, not on your current balance.

Frequently Asked Questions

Does interest compound on money I just deposited?

Yes, interest starts accruing immediately on new deposits. If you deposit $5,000 on the 15th of the month, that money earns interest from the 15th onward. The bank calculates daily interest on whatever balance you have, including deposits made that same day.

What's the difference between APY and APR?

APY (Annual Percentage Yield) is for savings and includes compounding. APR (Annual Percentage Rate) is for loans and does not include compounding. For savings accounts, always look at APY. For loans, APR is the relevant number. They are calculated differently and cannot be compared directly.

Can I lose money if interest rates drop?

No. Your balance never decreases because of a rate drop. If rates fall, you simply earn less interest going forward. Your existing money stays in the account and continues to grow, just at a slower rate than before.

Why do some banks offer much higher rates than others?

Online banks typically offer higher rates than brick-and-mortar banks because they have lower overhead costs. Credit unions sometimes offer competitive rates to members. Banks also raise rates to attract new deposits during periods when the Fed is raising its benchmark rate. The highest rates are usually available at online savings accounts and money market accounts.

Is interest taxable?

Yes. Interest earned on savings accounts is taxable income. Banks send you a 1099-INT form at tax time if you earned $10 or more in interest during the year. You report this on your tax return as ordinary income. This is separate from how the bank calculates the interest itself — it's a tax matter between you and the IRS.