Banks multiply your balance by a daily rate, then add that interest to your account each month
Most savings accounts earn daily compound interest, which means the bank calculates what you owe interest on every single day, then deposits the total once a month. The bank takes your account balance at the end of each day, multiplies it by a tiny fraction (the daily interest rate), and keeps a running total. At the end of the month, that total gets added to your account as a deposit.
The daily interest rate comes from the Annual Percentage Yield (APY) listed in your account terms. If your account shows 4.50% APY, the bank divides that by 365 days to get the daily rate—roughly 0.0123% per day. That rate is then applied to whatever balance sits in your account that day.
Because interest compounds daily, money you earn in early January starts earning interest itself by mid-January. This is why the same $10,000 will earn slightly more over a year in an account that compounds daily than one that compounds monthly—the difference is small but real.
Key Takeaways
- Banks calculate interest daily by dividing your APY by 365, then multiplying that daily rate by your account balance each day.
- Monthly interest deposits are the sum of all those daily calculations added together at the end of the month.
- The APY shown in your account terms is the rate you use to estimate earnings, and it already accounts for daily compounding.
- Your balance changes daily, so deposits and withdrawals during the month change how much interest you earn that month.
- Different banks may use slightly different methods (365 days vs. 360 days), which creates small differences in the final amount.
The formula banks use: daily balance times daily rate
The actual calculation looks like this: Daily Interest = Account Balance × (APY ÷ 365). If you have $5,000 in an account with 4.50% APY, the daily interest is $5,000 × (0.045 ÷ 365) = $5,000 × 0.000123288 = about $0.62 per day.
The bank does this calculation for every day of the month, using whatever balance you actually had that day. If you deposit $2,000 on the 15th, the daily rate applies to $7,000 for the remaining days of the month, not $5,000. If you withdraw $1,000 on the 20th, the rate applies to $4,000 for the last days of the month.
At the end of the month, the bank adds up all those daily interest amounts. If you earned $0.62 per day for 30 days with no changes to your balance, you'd earn about $18.60 for the month. That $18.60 gets deposited into your account, and in the next month, interest is calculated on $5,018.60, not $5,000.
Why your monthly interest varies month to month
The amount of interest you earn each month is not the same because three things change: your balance changes (deposits and withdrawals), the number of days in the month changes (28 to 31 days), and the bank's APY can change.
February has fewer days, so you earn less interest in February than in March, even if your balance is identical. A $5,000 balance earning 4.50% APY earns roughly $18.60 in a 31-day month but only $16.90 in a 28-day month. The difference is small but real.
If the bank raises or lowers its APY during the month, the change usually takes effect on a specific date. Your statement should show the rate that applied to each day. Some banks change rates weekly or even daily based on market conditions, so checking your statement is the only way to know what rate actually earned your interest.
How to estimate your monthly interest before it posts
You can estimate what you'll earn by multiplying your current balance by the APY and dividing by 12. A $10,000 balance at 4.50% APY earns roughly $37.50 per month ($10,000 × 0.045 ÷ 12). This is an estimate because it assumes your balance stays the same all month and ignores the small difference between daily and monthly compounding.
For a more accurate estimate, multiply your average balance for the month by the daily rate. If your balance was $10,000 for the first 15 days and $12,000 for the last 15 days, your average is $11,000. Multiply $11,000 by (0.045 ÷ 365) and then by the number of days in the month to get a closer number.
Your bank's website usually shows your current APY and sometimes includes a calculator. The statement you receive each month shows exactly how much interest posted and what rate was used, so you can verify the calculation yourself if you want to.
The difference between APY and APR in savings accounts
APY (Annual Percentage Yield) is what you see on savings accounts. It includes the effect of compounding—the fact that interest earns interest. APR (Annual Percentage Rate) is used for loans and credit cards and does not include compounding. For savings, APY is always the number you want to use.
If a bank shows 4.50% APY, that is the actual return you will get over a year if you leave the money untouched. The bank has already done the math on daily compounding and given you the final number. You do not need to calculate compounding yourself—the APY already accounts for it.
What happens when interest rates change
Banks change their APY based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks usually raise savings APY within days or weeks. When the Fed cuts rates, banks often cut savings APY just as quickly. Your bank will notify you of rate changes, usually by email or through your online account.
The new rate takes effect on a specific date, which your bank will state in the notice. If your rate drops from 4.50% to 4.25% on the 15th of the month, interest from the 1st to the 14th is calculated at 4.50%, and interest from the 15th onward is calculated at 4.25%. Your monthly statement will show both rates and how many days each one applied.
Why some banks use 360 days instead of 365
A small number of banks use 360 days instead of 365 when calculating the daily rate. This is called the "360-day year" or "ordinary interest" method. It makes the daily rate slightly higher, which sounds good for you, but it is rare in savings accounts—most use 365 days. Your account terms will state which method your bank uses.
The difference is tiny. At 4.50% APY, using 360 days instead of 365 changes the daily rate from 0.000123288 to 0.000125. On a $10,000 balance, that is about $0.02 more per day, or roughly $6 more per year. It is not worth switching banks for, but it is worth knowing if you are comparing two accounts with identical APY.
Frequently Asked Questions
Does interest compound daily or monthly in a savings account?
Interest is calculated daily but deposited monthly. The bank multiplies your balance by the daily rate each day, keeps a running total, and adds the full month's worth of interest to your account in one deposit at the end of the month. From that point forward, interest is calculated on the new, higher balance.
If I deposit money mid-month, when does it start earning interest?
It starts earning interest the day it is deposited. If you deposit $2,000 on the 15th, that $2,000 earns interest from the 15th through the end of the month. The interest from those days gets added to your account when the monthly interest posts.
Why is my monthly interest different from what I calculated?
The most common reasons are that your balance changed during the month (deposits or withdrawals), the APY changed mid-month, or you miscounted the number of days. Check your statement—it shows the exact rate used and the number of days it applied. You can then recalculate using those numbers.
Can I predict exactly how much interest I will earn next month?
Not exactly, because your balance and the APY may change. You can estimate by multiplying your current balance by the APY and dividing by 12, but the actual amount will differ if you make deposits, withdrawals, or if the bank changes the rate.
What does "compounding" mean for savings interest?
Compounding means interest earns interest. When the bank deposits your monthly interest, that interest becomes part of your balance. Next month, the bank calculates interest on the original balance plus the interest you already earned. This creates a snowball effect where your money grows faster over time.