Banks multiply your balance by an interest rate, then divide by the number of days in a year
Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 days. That gives you one day's worth of interest. The bank repeats this calculation each day, and each time it adds the new interest to your balance, so tomorrow's calculation includes today's interest.
The result is that your money grows faster than it would with simple interest, because you earn interest on the interest itself. A $10,000 balance at 4.5% annual interest earns roughly $450 per year with daily compounding, but the exact amount depends on how long the money stays in the account and whether you add or withdraw funds during that time.
The interest rate itself—called the Annual Percentage Yield (APY)—is what the bank advertises. This rate already accounts for compounding, so you do not need to do any math yourself. You can compare APYs directly between banks to see which one pays more.
Key Takeaways
- Daily compounding means the bank calculates interest on your balance every day and adds it back, so you earn interest on your interest.
- The Annual Percentage Yield (APY) is the rate banks advertise and already includes the effect of compounding.
- Your actual interest earned depends on your balance, how long money stays in the account, and whether you make deposits or withdrawals.
- Banks use 365 days per year for the calculation, even in leap years, though some use 360 days—check your account terms.
Why the APY matters more than the stated interest rate
Banks sometimes list two different rates: the interest rate and the APY. The interest rate is the raw percentage, while the APY is the rate after compounding is factored in. Because of daily compounding, the APY is always slightly higher than the interest rate itself.
For example, a bank might advertise 4.5% interest, but the APY might be 4.60%. That difference comes from compounding—you earn interest on the interest you already earned. When you compare savings accounts between banks, always look at the APY, not the interest rate, because that is the true measure of what your money will earn.
When interest posts to your account
Even though the bank calculates interest daily, it does not add the money to your balance every day. Most banks post interest monthly, meaning they add up all the daily interest from the entire month and deposit it once, usually on the last day of the month or the first day of the next month.
Some banks post interest quarterly (four times per year) or even annually. Check your account agreement or call the bank to find out when interest posts. The timing does not change how much you earn—only when you see it in your account—but it matters if you are watching your balance closely or planning a withdrawal.
How withdrawals and deposits affect your interest
Because interest is calculated on your daily balance, any money you withdraw stops earning interest immediately. If you have $10,000 on January 1st and withdraw $5,000 on January 15th, the bank calculates interest on $10,000 for 14 days and $5,000 for the remaining days of the month.
Deposits work the same way in reverse. Money you deposit starts earning interest the next day (or sometimes the same day, depending on the bank). If you deposit $2,000 on the 20th of the month, that $2,000 earns interest for the remaining 10 or 11 days of the month.
This is why moving money in and out frequently can reduce your total interest earned—you are lowering your average daily balance. Keeping a steady balance earns more than making many small withdrawals and deposits.
The difference between compounding periods
Most savings accounts compound daily, but some older or lower-tier accounts compound monthly or quarterly. The more frequently interest compounds, the more you earn, because you earn interest on your interest more often.
| Compounding Period | How Often Interest Is Calculated | Effect on Your Earnings |
|---|---|---|
| Daily | Every day (365 times per year) | Highest earnings; standard for most savings accounts |
| Monthly | Once per month (12 times per year) | Lower earnings than daily; less common now |
| Quarterly | Four times per year | Lowest earnings; rare in modern savings accounts |
The difference is small on small balances but becomes noticeable on larger amounts. A $50,000 balance at 4.5% APY earns roughly $2,250 per year with daily compounding. With monthly compounding, it might earn $2,245—a difference of about $5. With quarterly compounding, the difference grows larger.
What happens if you close your account mid-month
If you close your account before interest posts, you still receive the interest you earned up to that point. The bank calculates it through your closing date and either deposits it to your account before closing or sends it to you separately. You do not lose accrued interest by closing early.
However, some banks charge a fee to close an account within a certain time period (often 90 days to six months after opening). Check your account agreement before opening to see if an early closure fee applies. That fee could eat into the interest you earned, so factor it in if you think you might need to close the account soon.
How to calculate your own interest (if you want to)
You do not need to calculate interest yourself—your bank does it and shows you the total in your account—but understanding the math can help you compare accounts or estimate earnings.
The basic daily interest formula is: (Balance × APY) ÷ 365 = Daily Interest. If you have $10,000 at 4.5% APY, your daily interest is ($10,000 × 0.045) ÷ 365 = $1.23 per day. Multiply that by the number of days your money stays in the account to estimate total interest.
For a more precise calculation that accounts for changing balances, you would need to track your daily balance for the entire month and add up the daily interest for each day—which is exactly what your bank's computer does automatically. Most online banks show you a running interest total in your account dashboard, so you can watch it accumulate.
Frequently Asked Questions
Does my interest rate change?
Yes. Banks adjust savings account rates based on Federal Reserve decisions and market conditions. Your rate may go up or down, and the bank will notify you of changes. Rates can change weekly or even daily at some banks, so check your account terms to see how often your bank updates rates.
Why is my interest so low?
Savings account rates vary widely between banks and change constantly. Online banks typically offer higher rates than brick-and-mortar banks because they have lower overhead costs. If your current rate is below 4%, you may earn more by moving your money to a different bank. Compare APYs across several banks before deciding.
What if I have multiple savings accounts at the same bank?
Interest is calculated separately for each account based on its individual balance and rate. If you have a regular savings account earning 0.5% and a high-yield savings account earning 4.5%, each account earns interest only on its own balance. Combining them into one account would not change your total interest unless the rates are different.
Can interest rates go negative?
In the United States, savings account rates have not gone negative, though they have come close to zero during certain periods. Negative rates are more common in some other countries. If rates drop significantly, your bank will notify you of the change before it takes effect.
Is interest taxable?
Yes. Interest earned on savings accounts is taxable income. Banks send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. The amount of tax you owe depends on your tax bracket and total income.