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

Most savings accounts use daily compound interest. That means the bank looks at how much money you have at the end of each day, multiplies it by the annual interest rate, divides by 365 (or 360, depending on the bank), and adds that amount to your account. The next day, interest is calculated on the new, slightly larger balance. This compounding repeats every single day.

The formula banks use is: Daily Interest = (Account Balance × Annual Interest Rate) ÷ 365. If you have $10,000 and the rate is 4.5% per year, one day's interest is ($10,000 × 0.045) ÷ 365 = $1.23. That $1.23 gets added to your account, so the next day's calculation starts with $10,001.23.

Interest is usually credited monthly — meaning the bank adds up all those daily calculations and deposits the total once a month — but the daily compounding is what makes the difference between a good rate and a mediocre one.

Key Takeaways

  • Daily compound interest means the bank calculates interest on your balance every single day, and that interest itself earns interest the next day.
  • The annual percentage yield (APY) already accounts for compounding, so you can compare rates directly without doing the math yourself.
  • Moving money in or out changes your balance mid-month, which changes how much interest you earn that month.
  • Banks may use 360 days instead of 365 in their calculation, which slightly reduces what you earn — always check the account disclosure.

Why the interest rate shown is not the same as what you actually earn

Banks advertise two different numbers: the interest rate (also called APR, or annual percentage rate) and the annual percentage yield (APY). The APY is always higher because it includes the effect of daily compounding. If a bank shows 4.5% APY, you will earn closer to 4.5% over a year. If it shows 4.5% APR, you will earn less because compounding is not factored in.

Always look for the APY when comparing accounts. That is the real number. A 4.50% APY account will earn you more than a 4.45% APY account, even if the APR looks close. The difference grows larger the longer your money sits in the account.

How deposits and withdrawals mid-month affect your interest

Interest is calculated on your daily balance, so when you deposit money, the next day's calculation includes that new amount. When you withdraw, the balance drops immediately. This means the timing of deposits and withdrawals matters.

If you deposit $5,000 on the first of the month, that money earns interest for the full 30 days. If you deposit it on the 28th, it only earns interest for 2 or 3 days that month. The bank still compounds daily, but the total earned that month is smaller because the balance was lower for most of the month. Some accounts have a minimum balance requirement — if your balance drops below it, you may lose the advertised interest rate or pay a fee.

The difference between simple and compound interest

Simple interest calculates once per year on your original balance only. Compound interest calculates repeatedly on your balance plus any interest already earned. Savings accounts use compound interest because it grows your money faster.

With simple interest on $10,000 at 4.5% for one year, you earn $450 and end with $10,450. With daily compound interest at the same rate, you earn about $460 and end with $10,460. The difference seems small in year one, but compounds over time. After five years, simple interest gives you $12,250; daily compound interest gives you about $12,500. Banks use compound interest because it is standard in the industry and because it benefits both the bank and the account holder.

Why some banks use 360 days instead of 365

A small number of banks calculate daily interest using 360 days instead of 365. This is called the 360-day year or ordinary interest. The effect is tiny but measurable: it reduces your earnings by about 1.4% compared to a 365-day calculation.

On a $10,000 balance at 4.5% APY, using 360 days instead of 365 costs you roughly $6 per year. Over time, this adds up. Check your account's disclosure document (usually called the "Truth in Savings" form or account agreement) to see which method your bank uses. Most online banks and credit unions use 365 days.

How interest rates change and what that means for your earnings

Banks change their interest rates based on what the Federal Reserve does. When the Fed raises rates, banks usually raise savings account rates within days or weeks. When the Fed cuts rates, banks cut savings account rates just as fast. Your rate can change at any time unless you have a fixed-rate account, which is rare for savings accounts.

If your bank lowers the rate, your daily interest calculation gets smaller immediately. If it raises the rate, your daily calculation gets larger. You do not have to do anything — the bank handles the change automatically. The best time to open a savings account is when rates are high, because you lock in that rate for as long as you keep the account open (even though the rate can still change).

How to calculate what you will earn over time

You can estimate your earnings using the APY and a simple formula: Final Amount = Starting Balance × (1 + APY)^Years. If you have $10,000 at 4.5% APY for one year, the calculation is $10,000 × (1.045)^1 = $10,450. For five years, it is $10,000 × (1.045)^5 = $12,462.

Most banks and financial websites have savings calculators that do this math for you. You enter your starting balance, the APY, and how long you plan to keep the money there, and the calculator shows you the final amount. This is useful for comparing accounts: a 4.5% APY account will show you exactly how much more you earn than a 4.25% account over the same time period.

Frequently Asked Questions

Does interest compound daily even if it is only credited monthly?

Yes. The bank calculates interest every day and adds it to your balance, but you only see the total deposited once a month on your statement. The daily compounding still happens behind the scenes, which is why the APY is higher than the simple annual rate.

What happens to my interest if I withdraw money before the month ends?

You only earn interest on the balance you actually had each day. If you withdraw $5,000 mid-month, the interest earned for the days before the withdrawal is still yours, but you earn no interest on that $5,000 for the remaining days of the month.

Can a bank change my interest rate without telling me?

Banks can change rates at any time, but they must notify you before the change takes effect. You will receive notice by mail, email, or in your online account. You are not locked into a rate unless you have a special fixed-rate product, which is uncommon for regular savings accounts.

Why is the APY higher than the interest rate?

The APY includes the effect of daily compounding — interest earning interest. The interest rate (APR) does not. APY is always the number to use when comparing accounts because it shows what you actually earn.

Do I pay taxes on savings account interest?

Yes. Interest earned on a savings account is taxable income. Your bank will send you 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. The interest is taxed at your ordinary income tax rate, not as a capital gain.