Banks use one of two methods to calculate interest: simple interest or compound interest, and the frequency matters more than you might think

Most banks calculate savings account interest using compound interest, which means you earn interest on your interest. The bank takes your account balance, multiplies it by the interest rate, and adds that amount to your account. The next time interest is calculated, the bank uses your new, higher balance—so you earn a tiny bit more. This repeats daily, monthly, quarterly, or annually depending on your bank's compounding frequency.

A smaller number of accounts use simple interest, which only pays interest on your original deposit, never on accumulated interest. Simple interest is rare for savings accounts today because compound interest is more attractive to customers. You will see simple interest more often on certificates of deposit (CDs) or money market accounts, though many of those use compound interest too.

The difference between daily compounding and annual compounding can add up to real money over time, even though the stated interest rate is the same. A bank that compounds daily will pay you slightly more than one that compounds annually, because your balance grows faster and you earn interest on that growth sooner.

Key Takeaways

  • Banks multiply your balance by the annual interest rate and divide by the number of times interest is calculated per year to find each payment.
  • Daily compounding means the bank recalculates and adds interest every day, so you earn interest on interest more often than with monthly or annual compounding.
  • The Annual Percentage Yield (APY) shown on your account disclosure already includes the effect of compounding, so comparing APY between banks tells you which will actually pay more.
  • Interest is usually posted to your account monthly, but some banks calculate it daily and some quarterly—check your account agreement to know when to expect deposits.
  • Your balance on the day interest is calculated determines how much you earn that period, so deposits made late in the month may not earn interest until the next calculation cycle.

The formula banks use to calculate each interest payment

The basic calculation is: Interest = Balance × (Annual Interest Rate ÷ Number of Compounding Periods Per Year). If your account balance is $10,000, the annual interest rate is 4.5%, and the bank compounds interest daily, the calculation for one day is: $10,000 × (0.045 ÷ 365) = $1.23. That $1.23 is added to your account, and tomorrow's calculation uses $10,001.23 as the new balance.

The number of compounding periods per year varies by bank. Daily compounding divides by 365 (or 366 in a leap year). Monthly compounding divides by 12. Quarterly compounding divides by 4. Annual compounding divides by 1. The more frequently the bank compounds, the more times this multiplication happens, and the more total interest you receive by year's end.

Banks do not always calculate interest on your exact balance. Some use the average daily balance method, which adds up your balance at the end of each day during the month and divides by the number of days. Others use the minimum balance method, which only counts your lowest balance during the period. A few use the ending balance method, which only looks at what you have on the last day of the month. Your account agreement or disclosure statement will tell you which method your bank uses.

Why the Annual Percentage Yield (APY) matters more than the interest rate

Banks are required to show you two numbers: the Annual Percentage Rate (APR) and the Annual Percentage Yield (APY). The APR is the raw interest rate before compounding. The APY is what you will actually earn after compounding is factored in. APY is the number to compare between banks because it reflects the real return on your money.

For example, two banks might both advertise a 4.5% interest rate. Bank A compounds daily, and Bank B compounds annually. Bank A's APY will be slightly higher—around 4.60%—while Bank B's APY will be exactly 4.5%. Over a year, that small difference means Bank A pays you more money, even though both advertise the same rate. The APY difference grows larger the longer your money sits in the account.

Your account disclosure statement (sometimes called a Truth in Savings disclosure) must show both the APR and APY. If you are comparing accounts online, the APY is usually displayed prominently. If you only see an interest rate and no APY, contact the bank and ask for the APY before opening the account.

When interest is actually added to your account

The bank calculates interest on a schedule, but posting—the moment the money actually appears in your account—happens on a different schedule. A bank might calculate interest daily but post it monthly. This means interest accrues (builds up) every day, but you do not see it in your balance until the end of the month when it is all added at once.

Some banks post interest monthly, some quarterly, and a few post annually. High-yield savings accounts often post monthly. Traditional savings accounts at large banks sometimes post quarterly. Your account agreement will state the posting schedule. The calculation frequency matters for how much you earn; the posting frequency only matters for when you see the money appear.

If you close your account before interest is posted, you may lose accrued interest that has not yet been added. If you withdraw money on the day before interest posts, you earn interest on the lower balance. Timing a withdrawal to capture interest is difficult because you do not know the exact balance the bank will use for that period's calculation.

How deposits and withdrawals affect your interest calculation

When you deposit money, it usually begins earning interest immediately if the deposit is made before the bank's cutoff time that day. Deposits made after the cutoff may not earn interest until the next calculation period. Withdrawals reduce your balance right away, so you stop earning interest on the withdrawn amount immediately.

If your bank uses the average daily balance method, a deposit made on the 15th of the month will increase your average for the rest of the month, raising the interest you earn that period. A withdrawal on the 15th will lower your average. If your bank uses the minimum balance method, a withdrawal that drops your balance below your previous low point will reduce the interest you earn that entire period, even if you deposit the money back the next day.

This is why some people keep a separate emergency fund outside their savings account: withdrawals for unexpected expenses can reduce the interest earned that month. If you know you will need to withdraw money, ask your bank which calculation method they use so you can time the withdrawal to minimize the impact on your interest.

The difference between stated rate and what you actually earn

Banks advertise interest rates that change frequently, especially for high-yield savings accounts. The rate you see today might be different from the rate you locked in when you opened your account. Most savings accounts have variable rates, meaning the bank can change the rate at any time without notice (though they must notify you of the change). Some accounts, like CDs, have fixed rates that do not change for the term of the account.

Your actual earnings also depend on how long your money stays in the account. If you deposit $5,000 on January 15th and withdraw it on February 1st, you earn interest for only about two weeks, not a full month. The interest you receive will be much smaller than if you left the money for a full year. Banks calculate interest based on the number of days the money is actually in the account.

Fees can also reduce your actual earnings. If your account charges a monthly maintenance fee, that fee is subtracted from your interest. Some banks waive the fee if you maintain a minimum balance or set up direct deposit. Over a year, a $5 monthly fee can wipe out most of the interest on a small account, so check the fee schedule before opening an account.

How to find out your bank's specific calculation method

Your bank's account agreement or Truth in Savings disclosure will state the compounding frequency, posting frequency, and balance calculation method. You can find this document online by searching your bank's name plus "account agreement" or "disclosure statement." If you cannot find it online, call your bank's customer service line and ask for the savings account disclosure.

The disclosure will also show the current APY and any conditions that affect the rate (such as minimum balance requirements). If the disclosure is unclear, ask the bank directly: "How often do you compound interest?" and "When is interest posted to my account?" These are straightforward questions that any bank employee can answer.

If you are comparing two banks, request the disclosure from both and compare the APY, not the interest rate. The APY already includes the compounding effect, so it is the fairest way to compare. A high-yield savings account at an online bank often has a higher APY than a traditional savings account at a brick-and-mortar bank, even though both may advertise similar interest rates.

Frequently Asked Questions

Does my bank round down interest to the nearest penny?

Yes, most banks round interest to the nearest cent when posting. If your daily interest calculation comes to $1.234, the bank rounds it to $1.23. The rounding rules vary slightly by bank, but the difference is negligible over time. Your account agreement will state the rounding method if you want to verify it.

What happens to my interest if I have a negative balance?

If your account goes negative (overdraft), you do not earn interest on the negative balance. Instead, you owe the bank overdraft fees and possibly interest on the amount you owe. Overdraft interest rates are much higher than savings interest rates. Avoid overdrafts by monitoring your balance and setting up alerts.

Can I earn interest on money I just deposited today?

It depends on your bank's cutoff time and calculation method. Most banks begin calculating interest on deposits made before the cutoff time on the day of deposit. Deposits made after the cutoff may not earn interest until the next calculation period. Check your bank's website or call to confirm the cutoff time.

Why is my interest lower than I expected based on the APY?

The APY assumes your money stays in the account for a full year without deposits or withdrawals. If you withdrew money during the month, your average balance was lower, so you earned less interest. If you opened the account mid-month, you only earned interest for part of the month. Fees can also reduce your earnings.

Do I pay taxes on savings account interest?

Yes, savings account interest 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 must report this interest on your tax return. The interest is taxed at your ordinary income tax rate, not at a special rate.