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 the daily balance method: they calculate interest on the exact balance you hold each day, add those daily amounts together, and deposit the total once a month or once a quarter.
The formula looks like this: (Daily Balance ÷ 365) × Annual Interest Rate = Daily Interest. Your bank repeats this for every day in the month, then adds all those daily interest amounts together to get your monthly interest payment.
Here's a concrete example. Say you have $10,000 in an account earning 4.5% annual interest. On a single day, your interest would be ($10,000 ÷ 365) × 0.045 = $1.23. If your balance stays at $10,000 for the entire month (30 days), you'd earn roughly $36.99 in interest that month. But if you deposit $5,000 on day 15, the second half of the month earns interest on $15,000 instead, so your total for the month would be higher.
Key Takeaways
- Banks calculate daily interest by dividing your balance by 365, multiplying by the annual rate, and repeating this for each day of the month.
- The daily balance method means deposits made mid-month start earning interest when ready, and withdrawals reduce interest earned from that day forward.
- Interest compounds when your bank adds earned interest back into your account, so the next month you earn interest on a larger balance.
- The stated annual percentage yield (APY) already accounts for compounding, so you can compare rates directly without doing extra math.
- Your actual monthly interest will vary if your balance changes, because each day's interest is calculated on that specific day's balance.
Why your balance matters more than the rate
Because interest is calculated daily, the timing and size of deposits and withdrawals change how much you earn. A $1,000 deposit on day 1 of the month earns interest for 30 days. The same deposit on day 20 earns interest for only 11 days, so it generates less than half the interest.
This is why the daily balance method is standard: it's the fairest way to handle accounts where money moves in and out. Your bank tracks your balance at the end of each day, calculates that day's interest, and keeps a running total. At the end of the month, they deposit all that accumulated interest into your account.
If you want to estimate your monthly interest without waiting for the statement, multiply your average daily balance by the annual rate and divide by 12. If your balance was $10,000 for the first 15 days and $15,000 for the last 15 days, your average daily balance is $12,500. At 4.5% annual interest, that's roughly $46.88 for the month.
Compounding: when interest earns interest
Most savings accounts compound interest monthly or daily. Compounding means your bank adds the interest you earned back into your account, so the next period you earn interest on a larger balance—the original amount plus the interest from the previous period.
If you earn $36.99 in interest in month one and your bank compounds monthly, that $36.99 gets added to your balance. In month two, you earn interest on the new, larger balance. Over a year, this compounds 12 times, which is why the annual percentage yield (APY) is slightly higher than the stated interest rate.
The difference is small on savings accounts but real. A 4.5% stated rate with monthly compounding becomes an APY of roughly 4.59%. Banks are required to show you the APY on deposit accounts, so you don't have to calculate this yourself—just compare the APY numbers when choosing between accounts.
How to read your statement and verify the math
Your monthly or quarterly statement shows the interest deposited, but not the day-by-day calculation. To verify the math, you need three pieces of information: your opening balance, your closing balance, the dates and amounts of any deposits or withdrawals, and the stated annual interest rate.
The simplest check is to calculate your average daily balance. Add your opening balance to your closing balance, divide by 2, and multiply by the annual rate divided by 12. This won't be exact—it's an average, not a day-by-day calculation—but it should be close enough to catch a major error. If the interest shown on your statement is significantly lower than this rough estimate, contact your bank.
Some banks offer online calculators or show a detailed breakdown in their online banking portal. If yours does, use it. If not, the average daily balance method gives you a reasonable sanity check without needing to track every single day.
The difference between stated rate and APY
Banks quote two numbers: the interest rate (also called the annual percentage rate or APR) and the annual percentage yield (APY). The rate is the percentage your bank pays. The APY is the rate plus the effect of compounding.
For savings accounts, always compare APY to APY, not rate to rate. A 4.5% APY at one bank is directly comparable to a 4.5% APY at another. But a 4.5% rate with daily compounding (which becomes roughly 4.60% APY) is better than a 4.5% rate with monthly compounding (which becomes roughly 4.59% APY). The APY number does the compounding math for you.
The difference grows larger with higher balances and higher rates, but on typical savings account balances it's usually less than $10 per year. Still, if you're comparing accounts, the APY is the number that matters.
What happens if your rate changes
Savings account rates are variable, meaning your bank can change them at any time. When a rate changes, your interest calculation changes from that day forward. If your bank lowers the rate from 4.5% to 4.0%, the interest earned in the days before the change is calculated at 4.5%, and the interest earned after the change is calculated at 4.0%.
Your statement will show the rate that was in effect during each period. If your rate changed mid-month, the statement should show two different rates and the interest earned at each one. If it doesn't, ask your bank to explain the breakdown.
You can't control when rates change, but you can control where you keep your money. If your bank's rate drops significantly below what other banks are offering, moving your savings to a higher-rate account will increase your interest earnings when ready.
Frequently Asked Questions
Do I earn interest on interest?
Yes, if your account compounds interest. When your bank adds the interest you earned to your account balance, you earn interest on that interest in the next period. This is called compounding. Most savings accounts compound monthly or daily, so you do earn interest on interest, though the effect is small on typical balances.
Why is my interest lower than I calculated?
The most common reason is that your balance changed during the month. Interest is calculated on your daily balance, so deposits made mid-month earn less interest than deposits made at the start. Withdrawals reduce the balance when ready, so they reduce interest earned from that day forward. Check your statement for the dates of any deposits or withdrawals.
How often does interest get added to my account?
Most banks deposit interest monthly, though some do it quarterly or daily. Your account agreement or statement will show the frequency. Even if interest is calculated daily, it may not be deposited until the end of the month, so you won't see it in your balance until then.
Is the APY may provide?
No. Banks can change savings account rates at any time. The APY shown when you open the account is current as of that date, but it may be different next month. If you want a may provide rate, you would need a certificate of deposit (CD), which locks in a rate for a set period.
Can I calculate interest on a savings account without knowing the exact daily balance?
You can estimate it using your average daily balance: add your opening and closing balance, divide by 2, multiply by the annual rate, and divide by 12 for a monthly estimate. This won't be exact, but it's close enough to verify that your statement is in the right ballpark.