Interest is calculated on your account balance, compounded at intervals your bank sets

Banks calculate savings account interest by taking your account balance, explore an interest rate, and compounding the result at regular intervals — usually daily, monthly, or quarterly. The interest rate itself is set by your bank and changes based on what the Federal Reserve does with its benchmark rate. The key variable you control is how much money stays in the account; the compounding schedule and rate are determined by your bank's terms.

The math is straightforward: if you have $1,000 in an account earning 4.5% annual percentage yield (APY), and interest compounds daily, your bank divides that annual rate by 365, applies it to your balance each day, and adds the result back to your account. The next day, interest is calculated on the new, slightly higher balance. Over a year, this daily compounding means you earn slightly more than 4.5% of $1,000 would suggest if interest were calculated once at year-end.

Key Takeaways

  • Interest is calculated on your actual account balance at the time of calculation, so deposits increase your earnings and withdrawals reduce them when ready.
  • The compounding frequency — daily, monthly, or quarterly — determines how often interest is added back to your balance and begins earning interest itself.
  • Your bank's stated APY already accounts for compounding, so you can compare rates between banks directly without doing additional math.
  • The interest rate itself changes when the Federal Reserve adjusts its rates, and most savings accounts pass those changes to customers within days or weeks.

How the daily balance method works in practice

Most savings accounts use the daily balance method, meaning your bank calculates interest based on your balance at the end of each day. If you deposit $500 on Monday and withdraw $200 on Wednesday, the bank uses $1,000 for Monday's calculation, $1,000 for Tuesday's, and $800 for Wednesday's and beyond. Each day's interest is tiny — a fraction of a cent on most balances — but it compounds.

Some banks use the average daily balance method instead, which adds up your balance at the end of each day during the month and divides by the number of days. This smooths out the effect of deposits and withdrawals. A few older accounts or specialty products use the minimum balance method, where interest is based on your lowest balance during the period, which is the least favorable to you. Check your account disclosure or ask your bank which method they use; it will be stated in the terms.

What APY means and why it matters for comparison

Annual Percentage Yield (APY) is the rate your bank publishes, and it already includes the effect of compounding. If a bank advertises 4.5% APY, that means after one year of leaving money untouched, you will have earned 4.5% more than you started with, accounting for daily or monthly compounding. You do not have to do any math yourself — you can compare APY rates between banks directly.

The APY is different from the Annual Percentage Rate (APR), which you will see on credit products like loans. APR does not account for compounding. For savings accounts, always look at APY, not APR. The difference between a 4.25% APY account and a 4.75% APY account is real money: on $10,000, that 0.5% difference adds up to about $50 per year.

How rate changes affect what you earn

When the Federal Reserve raises or lowers its benchmark rate, banks adjust the rates they offer on savings accounts. High-yield savings accounts typically respond within days; traditional bank savings accounts may take weeks or may not move at all. Your bank is not required to pass rate increases to customers, though most do because they compete for deposits. Rate decreases, however, are usually applied quickly.

If you opened an account at 4.5% APY and the Federal Reserve cuts rates, your bank may drop your rate to 3.75% APY. The interest you already earned stays in your account, but future interest is calculated at the new, lower rate. This is why the timing of when you open a savings account matters — opening when rates are high locks in better returns on money you plan to keep there for months or years.

The difference between straightforward and compound interest

straightforward interest is calculated once, on your original balance only. If you had $1,000 earning 4.5% straightforward interest, you would earn $45 in year one, $45 in year two, and so on — always $45, because interest is not added back to the balance before the next calculation.

Compound interest is calculated on your balance plus any interest already earned. In year one, you earn $45 on $1,000. In year two, you earn 4.5% on $1,045, which is $47.03. The extra $2.03 comes from earning interest on your interest. All savings accounts use compound interest, which is why your money grows faster than straightforward math would suggest. The more frequently interest compounds (daily beats monthly beats quarterly), the more you benefit from compounding, though the difference is usually small on typical account balances.

Why your actual earnings may differ from the stated APY

The APY assumes you leave your money untouched for a full year. If you deposit $5,000 and withdraw it after six months, you earn interest for only six months, not twelve. Your actual return is half the stated APY. Similarly, if you make multiple deposits and withdrawals throughout the year, your average balance is lower than it would be if you deposited once and left it alone, so your total interest earned is lower.

Some accounts also charge monthly maintenance fees, which are deducted from your balance and reduce your net earnings. A $10 monthly fee on a $1,000 balance earning 4.5% APY effectively cuts your return in half. Always check whether an account charges fees and factor that into your comparison of rates between banks.

How to calculate your own interest earnings

If you want to estimate what you will earn, use this formula: Balance × APY ÷ 365 × Number of Days. For example, $5,000 × 0.045 ÷ 365 × 365 = $225 in one year. For six months, use 182 days instead of 365. This gives you an approximation; the exact amount will be slightly different because of how your specific bank compounds and the exact number of days in each month.

Most banks also provide an interest calculator on their website where you enter your balance and it shows you the projected earnings. These are more accurate than the formula because they use your bank's exact compounding method. You can also log into your account and look at your statement — banks show the interest earned each month or quarter, so you can see the actual calculation in action.

Frequently Asked Questions

Does interest get added to my account automatically?

Yes. Your bank calculates interest daily or monthly and deposits it directly into your savings account. You do not have to do anything. The interest becomes part of your balance and begins earning interest itself the next compounding period.

Can my interest rate go down after I open the account?

Yes. Savings account rates are variable, meaning your bank can change them at any time. When the Federal Reserve cuts rates, most banks lower their savings rates within days or weeks. Your existing balance keeps the interest already earned, but future interest is calculated at the new rate.

What happens to my interest if I withdraw money mid-month?

Interest is calculated on your balance at the time of calculation. If you withdraw money before interest is posted, that withdrawal reduces the balance used for that period's calculation. You lose interest on the withdrawn amount for that period only; interest already earned stays in your account.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a Form 1099-INT 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 the interest calculation itself — it just affects your taxes.

Why do some banks offer higher APY than others?

Banks set their own rates based on how much they need deposits and what they can earn by lending that money out. Online banks typically offer higher rates than traditional banks because they have lower overhead costs. During periods when the Federal Reserve rate is high, competition for deposits increases and rates go up across the board.