The basic formula: balance times rate times time

Interest on a savings account is calculated by multiplying your account balance by the annual interest rate, then dividing by the number of days in a year. Most banks use what's called daily compounding, which means they calculate interest on your balance every single day, then add that interest back into your account so tomorrow's calculation includes today's interest.

The formula looks like this: Daily Interest = (Account Balance × Annual Interest Rate) ÷ 365. If you have $10,000 in an account earning 4.5% annual interest, your daily interest is ($10,000 × 0.045) ÷ 365 = $1.23 per day. That $1.23 gets added to your balance, so the next day the bank calculates interest on $10,001.23 instead.

The reason this matters is that compounding creates a snowball effect. Over a year, daily compounding at 4.5% on $10,000 earns you $460.45, not $450. That extra $10.45 comes entirely from interest earning interest on itself.

Key Takeaways

  • Banks calculate interest daily by multiplying your balance by the annual rate and dividing by 365, then add that amount back to your account.
  • Daily compounding means interest starts earning its own interest when ready, which is why the total earned over a year exceeds the straightforward annual rate.
  • The Annual Percentage Yield (APY) on your account statement already includes the effect of compounding, so it shows what you'll actually earn.
  • Your balance changes every time you deposit or withdraw money, so interest calculations restart based on the new balance.
  • Different banks compound at different frequencies — daily is most common for savings accounts, but some use monthly or quarterly.

Why APY matters more than the interest rate

Your bank statement shows two numbers: the interest rate (sometimes called the APR) and the Annual Percentage Yield (APY). The interest rate is what the bank uses in the daily calculation. The APY is what you actually earn after compounding happens all year.

If a bank advertises 4.5% APY, that's the number you should use to estimate your earnings. It already accounts for daily compounding. If you see only an interest rate without an APY, the bank is required by law to show you the APY somewhere on the account disclosure document — usually a page titled "Truth in Savings" or "Account Terms and Conditions."

For example, a 4.5% interest rate with daily compounding becomes 4.60% APY. The difference looks small until you're comparing accounts. One bank offering 4.5% APY and another offering 4.0% APY will earn you meaningfully different amounts over time, especially on larger balances.

How deposits and withdrawals reset the calculation

Every time you deposit money, your balance goes up, so the next day's interest calculation is larger. Every time you withdraw, your balance drops, and so does the next day's interest. The bank recalculates based on your actual balance at the end of each day.

This is why the timing of deposits matters slightly. If you deposit $5,000 on the first of the month, that money earns interest for the entire month. If you deposit it on the 28th, it earns interest for only a few days. Over a year, the difference is small, but it's real.

Some banks use what's called the "average daily balance" method instead of calculating on the ending balance each day. This is less common for savings accounts but more common for checking accounts. If your bank uses this method, it will be stated in your account agreement. The effect is usually negligible for most account holders.

The difference between straightforward and compound interest

straightforward interest calculates once per year on your original balance only. If you put $10,000 in an account earning 4.5% straightforward interest, you earn $450 the first year, $450 the second year, and $450 every year after. Your balance grows in a straight line.

Compound interest calculates on your balance plus all the interest that's already been added. After year one, you have $10,460.45. In year two, the bank calculates interest on that larger amount, so you earn $469.21. The growth accelerates because you're earning interest on interest.

Savings accounts use compound interest, usually daily. Certificates of Deposit (CDs) also use compound interest, though the compounding frequency varies — some compound daily, some monthly, some at maturity. The account disclosure will specify which one applies to you.

Working through a real example

Let's say you open a savings account with $5,000 on January 1st, and the APY is 4.5%. Here's what actually happens:

January 1–31: Your balance is $5,000. Daily interest is ($5,000 × 0.045) ÷ 365 = $0.62 per day. Over 31 days, that's $19.18 added to your account. Your balance is now $5,019.18.

February 1–28: Your balance is now $5,019.18. Daily interest is ($5,019.18 × 0.045) ÷ 365 = $0.62 per day. Over 28 days, that's $17.35. Your balance is now $5,036.53.

Notice that February's interest is slightly less than January's, even though the rate is the same, because February has fewer days. By the end of the year, if you made no deposits or withdrawals, your balance would be approximately $5,230.69. The difference between that and $5,225 (which is what straightforward interest would give you) is the compounding effect.

How to find your bank's compounding frequency

Your bank's compounding method is stated in the account disclosure document you received when you opened the account. Look for a section titled "Interest" or "How Interest Is Calculated." It will say something like "Interest is compounded daily and credited monthly" or "Interest is compounded and credited quarterly."

"Credited" means the interest is actually added to your balance. Some banks calculate daily but credit monthly, meaning they do the daily math but only add the total to your account once a month. This doesn't change your earnings — the compounding still happens — but it does mean you won't see the interest appear in your account every day.

If you can't find the disclosure document, call your bank's customer service line or log into your online banking portal. Most banks have a link to account terms or disclosures in the account settings. You can also ask directly: "How often do you compound interest on this account?"

Why rates change and what that means for your calculation

Banks change their interest rates frequently, especially savings accounts. When the Federal Reserve raises or lowers its benchmark rate, banks typically adjust their savings account rates within days or weeks. When your rate changes, the bank will notify you in writing, and the new rate applies to your balance going forward.

If your rate drops from 4.5% to 4.0% on the 15th of the month, your interest for the first 14 days is calculated at 4.5%, and interest from the 15th onward is calculated at 4.0%. The bank handles this automatically — you don't need to do anything.

This is why comparing APY across banks matters most when you're opening a new account. Once you're in an account, the rate will move with the market. If you want to chase higher rates, you can move your money to a different bank, but there's no penalty for doing so with a savings account (unlike a CD, which charges you to withdraw early).

Frequently Asked Questions

Does my interest get taxed?

Yes. Interest earned on a savings account is taxable income. If you earn $100 or more in interest during the year, your bank will send you a 1099-INT form in January showing how much you earned. You report this on your tax return. The interest is taxed at your ordinary income tax rate, not at a special rate.

What's the difference between APR and APY?

APR (Annual Percentage Rate) is the interest rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding. For savings accounts, always compare using APY, because that's what you'll actually earn. The difference between the two grows larger as the interest rate increases.

Can I calculate my interest manually, or should I trust the bank?

You can calculate it manually using the formula provided here, but the bank's calculation is what matters legally. Banks are required to calculate correctly, and you can verify your earnings by checking your monthly statement. If the numbers don't match your calculation, contact the bank — errors do happen, though they're rare.

Does my interest calculation change if I have multiple savings accounts at the same bank?

No. Each account is calculated separately based on its own balance and rate. If you have a regular savings account earning 4.5% and a high-yield savings account earning 5.0%, the bank calculates interest on each one independently. Your total earnings are the sum of both.

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

You keep all the interest earned up to the day you withdraw. The bank calculates interest on your balance each day, so if you withdraw on the 15th, you've earned interest on your full balance for 14 days and a smaller balance for the rest of the month. You don't lose any interest you've already earned.