The basic formula: your balance, the rate, and the time
Banks calculate savings account interest by multiplying three things: the money you have in the account, the interest rate the bank is paying, and how long that money sits there. The result is the interest you earn. Most banks use a method called daily compounding, which means they calculate interest on your balance every single day, then add that interest back into your account so the next day's calculation includes it.
Here is the simplest version: if you have $1,000 in an account earning 4% annual interest, and the bank compounds daily, you do not earn exactly $40 at the end of the year. You earn slightly more, because each day the bank adds a tiny bit of interest, and the next day it calculates interest on that larger amount. That is compounding — earning interest on your interest.
The actual daily calculation is small enough that you will not see it happen. A bank divides the annual rate by 365 days, then multiplies that daily rate by your current balance. That amount gets added to your account. Tomorrow, your balance is slightly higher, so tomorrow's interest is slightly higher too.
Key Takeaways
- Banks calculate daily interest by dividing the annual rate by 365, multiplying by your current balance, and adding the result to your account each day.
- Compounding means you earn interest on the interest already added to your account, which is why the total at year-end is more than the straightforward annual rate suggests.
- The interest rate a bank advertises is the annual percentage yield (APY), which already accounts for daily compounding, so you do not have to do the math yourself.
- Your balance matters: deposits made mid-month earn less interest that month than deposits made on the first day, because they sit in the account fewer days.
- Interest is usually added to your account monthly, even though it is calculated daily, so you see the total once a month in your statement.
Why the advertised rate is not the same as what you calculate
When a bank advertises an interest rate, it shows you the annual percentage yield, or APY. This number already includes the effect of daily compounding. It is the actual percentage you will earn over a year if you leave the money untouched.
This matters because if you tried to calculate interest yourself using only the annual rate and straightforward multiplication, you would get a lower number than what actually appears in your account. The bank is not hiding anything — the APY is the honest number. It is just that compounding makes the real earnings higher than a straightforward calculation would suggest.
Different banks offer different APYs, and the rate changes over time. A high-yield savings account at an online bank might offer 4.5% APY, while a traditional bank down the street might offer 0.01% APY on the same type of account. The difference in what you earn is enormous, even on the same $1,000, over the same year.
How often interest gets added to your account
Banks calculate interest daily, but they do not add it to your account every day. Instead, they add it once a month, usually on the last day of the month or the first day of the next month. Your statement will show the total interest posted that month as a single deposit.
This does not change how much you earn — the daily calculations still happened, and the compounding still worked. It just means you see the result once a month instead of 365 times a year. Some banks post interest quarterly (every three months) instead of monthly, but this is less common for savings accounts.
Why your balance on different days matters
Because interest is calculated daily, the exact day you deposit money changes how much interest you earn that month. A deposit made on the first of the month earns interest for 30 or 31 days. A deposit made on the 15th earns interest for only 15 or 16 days. A withdrawal on the 20th means you lose interest on that money for the rest of the month.
Banks use what is called the average daily balance method to handle this. They add up your balance at the end of each day of the month, then divide by the number of days. That average is what they use to calculate the month's interest. So if you had $1,000 for 15 days and $2,000 for 15 days, your average daily balance for the month is $1,500.
This is why timing matters for large deposits or withdrawals. Moving money in or out near the end of the month affects less interest than moving it near the beginning.
The difference between savings accounts and money market accounts
Savings accounts and money market accounts both use daily compounding and post interest monthly, but money market accounts usually offer a higher APY in exchange for requiring a larger minimum balance and limiting how often you can withdraw. The calculation method is identical — the difference is in the account rules, not the math.
Certificates of deposit (CDs) work differently. You lock your money away for a set time — three months, one year, five years — and the bank pays a fixed rate for that entire period. The interest is still calculated daily and compounded, but you cannot touch the money without a penalty. Because the bank knows exactly how long it will have your money, it can offer a higher rate.
What happens if you withdraw money before the interest posts
If you withdraw money on the 25th of the month, the bank has already calculated interest on that money for the first 25 days. That interest stays in your account — you do not lose it. What you lose is the interest that would have been calculated for the remaining days of the month on that withdrawn amount.
This is why the timing of a withdrawal matters. Taking money out on the 1st of the month costs you interest for the entire month. Taking money out on the 28th costs you interest for only a few days. The interest already earned and posted to your account is yours to keep.
How to find your account's actual APY
Your bank is required by law to disclose the APY in writing before you open an account. You will see it on the account disclosure form, on the bank's website, or on the account agreement. It is usually labeled "APY" or "Annual Percentage Yield." Do not confuse it with "APR" (annual percentage rate), which is used for loans and credit cards, not savings.
The APY can change at any time, and banks often lower it when overall interest rates fall. You can check your current APY by logging into your online account, calling the bank, or visiting a branch. Your monthly statement also shows the APY that was in effect during that month.
If your bank lowers the APY and you are unhappy with the new rate, you can move your money to a different bank. There is no penalty for closing a savings account and opening one elsewhere, unlike with CDs or some other products.
Frequently Asked Questions
If I deposit $500 on the 15th, do I earn interest on it right away?
Yes. The bank begins calculating daily interest on that $500 starting the day it is deposited. However, you will not see that interest in your account until the end of the month when the bank posts it. The interest earned for those 15 or 16 days will appear as a single deposit on your statement.
Why is my interest lower than I expected?
The most common reason is that the APY is lower than you thought. Check your account disclosure or statement to confirm the actual rate. Another reason is that you withdrew money partway through the month, reducing the average daily balance. Large deposits or withdrawals near the beginning of the month have more impact than those near the end.
Does interest compound if I withdraw it every month?
No. If you withdraw the interest as soon as it posts, you lose the compounding effect on that interest. Compounding only works if you leave the interest in the account so the next month's calculation includes it. This is why leaving money untouched for a full year earns noticeably more than withdrawing interest monthly.
What is the difference between APY and the interest rate the bank advertises?
The advertised interest rate and the APY are the same thing for savings accounts. Banks must show you the APY, which already includes the effect of daily compounding. For loans and credit cards, APR and APY are different, but for savings, they are one number.
Can a bank change my APY without telling me?
Banks can change APY at any time, but they must notify you before the change takes effect. You will receive notice by mail, email, or through your online account. If you disagree with a rate cut, you have the right to close the account and move your money elsewhere without penalty.