The basic formula: multiply your balance by the rate and the time

Bank interest on a savings account is calculated by multiplying three numbers: your account balance, the annual interest rate (APY), and how long your money sits in the account. The formula is Interest = Balance × APY ÷ 365 × Number of Days. If you have $5,000 in an account earning 4.5% APY and leave it untouched for one year, you earn $225. If you leave it for 30 days, you earn about $18.49.

Banks calculate interest daily but typically credit it monthly or quarterly. This means your balance changes slightly each day as interest accrues, and that new balance earns interest the next day—a process called compounding. The more often interest compounds, the more you earn, because you're earning interest on your interest.

The APY (Annual Percentage Yield) shown on your account already includes the effect of compounding, so you don't have to calculate it separately. That's the number you use in the formula above. The APR (Annual Percentage Rate) does not include compounding and is not what you use for savings accounts.

Key Takeaways

  • Interest earned equals your balance multiplied by the APY, divided by 365, multiplied by the number of days your money was in the account.
  • Banks compound interest daily but credit it monthly or quarterly, meaning you earn interest on interest you've already earned.
  • The APY shown on your account statement already accounts for compounding, so use that number in your calculation, not the APR.
  • Your actual interest payment varies if you deposit or withdraw money during the month, because the balance changes.
  • Most banks show your earned interest on your statement, so you can verify the calculation yourself using the formula.

How daily compounding works in practice

When a bank says it compounds interest daily, it means it calculates how much you've earned each day and adds that to your balance. The next day, the interest calculation includes that new, slightly higher balance. Over a month or year, this compounding effect adds real money to your account.

Here's a concrete example: You deposit $10,000 in an account earning 4.5% APY. On day one, the bank calculates $10,000 × 0.045 ÷ 365 = $1.23 in interest. Your balance is now $10,001.23. On day two, the bank calculates interest on $10,001.23, earning you $1.23 plus a fraction of a cent more. By the end of 30 days, you've earned about $36.99 instead of the $36.99 you'd earn if interest were not compounded. The difference grows larger over months and years.

The APY already reflects this compounding. If a bank advertises 4.5% APY, that 4.5% is the total return you'll receive after one year of daily compounding. You don't need to recalculate the compounding yourself—the APY does that work for you.

What happens when you deposit or withdraw money mid-month

If your balance changes during the month, the interest calculation changes too. Banks use the daily balance method, which means they calculate interest on each day's actual balance, then add those daily amounts together at the end of the month.

Suppose you start the month with $5,000 at 4.5% APY. For the first 15 days, you earn about $9.32. On day 16, you deposit $5,000, bringing your balance to $10,000. For the remaining 15 days, you earn about $18.49. Your total interest for the month is about $27.81. If you'd kept $10,000 in the account the whole month, you'd have earned about $36.99.

Withdrawals work the same way. If you withdraw money mid-month, the interest calculation drops for the remaining days. Some banks have minimum balance requirements, and if your balance falls below that threshold, they may reduce your APY or charge a fee, which cuts into your interest earnings.

Reading your bank statement to verify the calculation

Your bank statement shows the interest credited to your account each month or quarter. You can use this number to verify the bank's calculation. Look for a line item labeled "Interest Paid," "Interest Credited," or "Dividend Paid" (credit unions sometimes use "dividend" instead of "interest").

To check the math, gather three pieces of information from your statement: the opening balance, the closing balance, and the APY. If your balance stayed the same all month, use the formula: Interest = Balance × APY ÷ 365 × Number of Days. If your balance changed, the calculation is more complex, but your statement should show the daily balance method used.

If the interest shown on your statement is significantly lower than your calculation, check whether a fee was charged or whether your APY changed during the period. Some banks reduce rates without notice, and some charge monthly maintenance fees that reduce your net interest. If the discrepancy is large, contact your bank and ask them to explain the calculation.

Why APY matters more than the advertised rate

Banks sometimes advertise a lower number called the APR (Annual Percentage Rate) instead of the APY. The APR does not include the effect of compounding, so it understates what you'll actually earn. Always look for the APY, which is the true annual return.

The difference between APR and APY is small on savings accounts but real. A 4.5% APR compounded daily becomes about 4.60% APY. On $10,000, that's the difference between earning $450 and earning $460 over a year. On larger balances or longer time periods, the gap widens.

Federal law requires banks to disclose the APY prominently on savings account disclosures, so you should always see it. If you're comparing accounts, compare the APY numbers, not the APR. The APY is what you'll actually receive.

How interest rates change and what that means for your earnings

Banks change their savings rates frequently, especially when the Federal Reserve changes its benchmark rate. When rates rise, new deposits earn more. When rates fall, your earnings drop. Your bank will notify you of rate changes, usually by email or on your statement.

If your rate drops mid-month, the interest calculation uses the old rate for the days before the change and the new rate for the days after. Your statement should show both rates and the interest earned at each rate. If you're unhappy with a lower rate, you can move your money to a different bank offering a higher rate—there's no penalty for moving savings between banks.

Some accounts offer a promotional rate for a limited time, then drop to a lower standard rate. Read the fine print when opening an account to see when a promotional rate expires. If you want to lock in a higher rate, consider a certificate of deposit (CD), which guarantees a fixed rate for a set period, usually three months to five years.

Frequently Asked Questions

How often do banks credit interest to my account?

Banks calculate interest daily but credit it monthly, quarterly, or annually depending on the account. Most savings accounts credit monthly. Check your account disclosure or statement to see the schedule. The APY already accounts for how often interest is credited, so you don't need to adjust your calculation.

If I withdraw money before interest is credited, do I lose the interest I earned?

No. Interest accrues daily, and you own it as soon as it's calculated. If you withdraw money before the monthly or quarterly credit date, you still receive the interest earned up to that point. Some accounts have minimum balance requirements that trigger a fee if you fall below the threshold, which can offset your interest, but the interest itself is yours.

Why does my interest payment vary from month to month?

Interest varies because the number of days in each month is different (28 to 31), and your balance may change if you deposit or withdraw money. February earns less interest than July simply because it has fewer days. If you add or withdraw money mid-month, that month's interest reflects the time your money was actually in the account.

Can I calculate interest earned on money I haven't deposited yet?

Yes. Use the formula Interest = Balance × APY ÷ 365 × Number of Days with the balance you plan to deposit and the number of days you plan to keep it there. This gives you an estimate of what you'll earn. The actual amount may vary slightly if the APY changes or if you deposit or withdraw money during the period.

What's the difference between simple interest and compound interest on a savings account?

Simple interest is calculated only on your original balance. Compound interest is calculated on your balance plus any interest already earned. Banks use compound interest on savings accounts, which is why you earn more over time. The APY already includes the compounding effect, so you don't have to calculate it separately.