The basic formula: multiply your balance by the APY, then divide by 365
Interest on a high yield savings account accrues daily but is usually paid monthly. To find out how much you'll earn in a single day, take your account balance, multiply it by the annual percentage yield (APY), and divide by 365. That gives you one day's interest. To find monthly interest, multiply that daily amount by the number of days in the month.
Example: You have $10,000 in an account with a 4.50% APY. One day's interest is ($10,000 × 0.045) ÷ 365 = $1.23. Over a 30-day month, that's $1.23 × 30 = $36.99 before the bank deposits it.
Banks use this daily calculation method because your balance changes throughout the month as you deposit and withdraw money. Each day's interest is based on whatever balance sits in the account that morning.
Key Takeaways
- Daily interest = (Account balance × APY) ÷ 365; multiply by the number of days to get monthly interest.
- Banks calculate interest on your actual daily balance, so deposits and withdrawals change how much you earn each month.
- Interest compounds when the bank deposits your earnings back into the account, meaning next month you earn interest on the interest.
- The APY already accounts for compounding, so you do not need to calculate compound interest separately—the stated rate is what you'll actually earn.
- Your account statement or online banking dashboard shows the interest posted each month, so you can verify the bank's math.
How daily balance tracking affects your total
Because interest is calculated daily, the exact amount you earn depends on when money enters or leaves your account. If you deposit $5,000 on the 15th of a 30-day month, you earn interest on your original balance for 14 days, then on the larger balance for the remaining 16 days.
Most banks use the "average daily balance" method, which adds up each day's balance and divides by the number of days in the month. Some use the "daily balance" method, which calculates interest on each day's actual balance separately. The difference is usually small—a few cents per month—but it matters if your balance swings widely.
Check your account's terms or call the bank to confirm which method they use. Your monthly statement should show the interest earned and the average daily balance they used to calculate it.
Compounding: earning interest on your interest
When the bank deposits your monthly interest back into the account, that interest becomes part of your balance. Next month, you earn interest on both your original deposit and the interest from the previous month. This is compounding, and it's why high yield savings accounts build wealth faster than regular savings accounts.
The APY you see advertised already includes the effect of monthly compounding. If a bank quotes 4.50% APY, that 4.50% is the total you'll earn over a year after interest compounds monthly—you do not need to calculate compounding yourself. The stated rate is the real rate.
Over time, compounding adds up. On a $10,000 balance at 4.50% APY compounded monthly, you earn roughly $450 in year one. In year two, you earn interest on $10,450, so you earn slightly more. After 10 years, the difference between simple and compound interest becomes substantial.
What happens when the APY changes
Banks adjust APY rates frequently, sometimes weekly. When your rate changes, the new rate applies to interest earned going forward—it does not retroactively change what you already earned. If your account earns 4.50% for three months, then the bank drops it to 4.00%, you keep the 4.50% interest already posted, and future interest uses 4.00%.
To recalculate your expected earnings after a rate change, use the new APY with your current balance and the number of days remaining in the year. If you started the year at 4.50% for six months and switched to 4.00% for the remaining six months, you'd calculate interest in two parts: six months at the old rate, six months at the new rate.
Check your bank's website or app regularly to see the current APY. Many banks display the rate prominently on the account page. If you see it drop and you have other options, you can move your money to a bank offering a higher rate—there's no penalty for transferring between savings accounts.
Using a calculator to verify your earnings
Most banks provide an online savings calculator on their website. You enter your starting balance, the APY, and how long you plan to keep the money, and it shows you the projected interest. These calculators assume you do not add or withdraw money and that the APY stays constant, so they give you a rough estimate rather than an exact figure.
To verify what the bank actually paid you, compare the interest shown on your monthly statement to your own calculation. Take your average daily balance for the month (your statement should list it), multiply by the APY, and divide by 365, then multiply by the number of days in that month. If the numbers match within a dollar, the bank's math is correct.
If you want to project earnings over several years with a changing balance, a spreadsheet works better than a calculator. List each month's starting balance, multiply by the monthly interest rate (APY ÷ 12), add that interest to the balance, and repeat. This shows you how compounding builds over time and lets you adjust for deposits or withdrawals.
The difference between APY and APR
APY (annual percentage yield) is what you earn on savings; it includes compounding. APR (annual percentage rate) is what you pay on loans; it does not include compounding in the same way. Banks advertise savings accounts using APY because it's the real number you'll earn. Never compare a savings account's APY to a loan's APR—they measure different things.
When you see a high yield savings account advertised at "4.50% APY," that 4.50% is the total you'll earn over a year after monthly compounding. A loan advertised at "4.50% APR" is the interest rate before compounding, and the actual cost to you is higher if interest compounds more than once a year.
Why your actual earnings might differ from the calculation
If you calculate interest and it doesn't match your statement exactly, the most common reasons are rounding, the number of days the bank counted, or a rate change mid-month. Banks round interest to the nearest cent, so a calculation showing $36.987 becomes $36.99 on your statement. If the APY changed during the month, the bank calculates interest in two parts: the old rate for days before the change, the new rate for days after.
Some banks also hold deposits for a day or two before crediting them to your account, which delays when interest starts accruing on that money. Check your account's terms to see if there's a holding period. If your calculation is off by more than a few cents, contact the bank and ask them to walk you through their math—they can show you the exact daily balance they used and the rate applied each day.
Frequently Asked Questions
Do I need to do anything to earn interest, or does it happen automatically?
Interest accrues and deposits automatically. You do not need to take any action. As long as money sits in the account, the bank calculates and posts interest each month based on your balance and the APY. You'll see the interest appear on your statement without doing anything.
If I withdraw money mid-month, do I lose all the interest for that month?
No. Interest is calculated on your daily balance, so you earn interest on the money for the days it was in the account. If you withdraw $5,000 on the 20th of a 30-day month, you earn interest on that $5,000 for 19 days and on the remaining balance for all 30 days. You do not forfeit the month's interest.
What's the difference between monthly and daily compounding?
Monthly compounding means interest is calculated and added to your balance once a month. Daily compounding means interest is calculated each day but added monthly. For savings accounts, the difference is tiny—usually less than a dollar per year on a $10,000 balance. The APY accounts for whichever method the bank uses, so the advertised rate is what you'll actually earn.
Can I earn more interest by moving money between accounts?
No. Interest is based on your balance and the APY; moving money between accounts at the same bank does not change the rate or how much you earn. Moving to a different bank with a higher APY will earn you more, but moving within the same bank will not. The bank calculates interest on your total balance regardless of how many accounts you hold.
What happens to my interest if the bank fails?
Interest that has already been posted to your account is protected by FDIC insurance up to $250,000 per depositor per bank, just like your principal. Interest that has accrued but not yet posted is also protected. If you're concerned about exceeding the insurance limit, open accounts at different banks rather than multiple accounts at the same bank.