Interest grows your money through daily calculation and periodic deposits
A bank pays you interest on the money you keep in a savings account by calculating a percentage of your balance and adding that amount back to your account at set intervals. The percentage is your annual percentage yield (APY), and the bank compounds it—meaning it calculates interest on your interest—usually every day, then deposits the total monthly or quarterly. If you have $10,000 in an account earning 4.50% APY, the bank divides that rate by 365, applies it to your balance each day, and after a month you might see roughly $37 added to your account. That money then earns interest too in the next period.
The actual mechanics depend on how often the bank compounds and how often it credits the interest to your account. Daily compounding with monthly crediting is standard at most online banks. A few banks still compound and credit quarterly or annually, which means your money grows more slowly because you wait longer to earn interest on the interest. The difference compounds over years—literally.
Key Takeaways
- Banks calculate interest daily by dividing your APY by 365 and multiplying by your current balance, then add the total to your account monthly or quarterly.
- Compounding means the bank pays interest on the interest you have already earned, so your balance grows faster than straightforward multiplication would suggest.
- The frequency of compounding and crediting matters: daily compounding with monthly crediting grows your money faster than quarterly or annual crediting.
- Your actual interest earned depends on your balance throughout the month, not just your ending balance, because the bank recalculates every day.
- Moving money in or out changes how much interest you earn that month because the calculation is based on your daily balance.
How the daily calculation actually works
Each day, the bank takes your account balance at the end of that day, divides the APY by 365, and multiplies the result by your balance. That is your interest for that one day. If your APY is 4.50% and your balance is $10,000, the daily rate is 4.50% ÷ 365 = 0.0123%. Multiply $10,000 by 0.0123% and you earn $1.23 that day. The next day, if your balance is still $10,000, you earn another $1.23. If you deposit $5,000, the next day's calculation uses $15,000, so you earn about $1.85.
The bank adds up all these daily amounts and deposits them into your account once a month (or once a quarter, depending on the bank). That deposit is when you actually see the interest appear. Some banks show you a running total of "interest earned this month" in your account details, even though the money has not been credited yet. Once the interest is credited, it becomes part of your balance and earns interest itself in the next period.
Why compounding makes a difference over time
Compounding is the reason your money grows faster than you might expect from a straightforward percentage. In month one, you earn interest on your original $10,000. In month two, you earn interest on $10,000 plus the interest from month one. In month three, you earn interest on all of that. Over a year, the effect is noticeable. Over five years, it is substantial.
The difference between daily compounding and annual compounding is real but not dramatic for most balances. On $10,000 at 4.50% APY, daily compounding with monthly crediting earns you roughly $461 in a year. Annual compounding earns you $450. The gap widens with larger balances and higher rates, but for most people the bigger factor is the APY itself—a 4.50% account grows your money much faster than a 0.01% account, regardless of compounding frequency.
What happens when you deposit or withdraw money
Your interest for any given month is based on your balance on each day of that month, not your balance at the end. If you deposit $5,000 on the 15th, you earn interest on the higher balance for the remaining 16 days of the month. If you withdraw $5,000 on the 15th, you earn interest on the lower balance for those 16 days. Banks call this the average daily balance method, though most modern banks actually calculate interest on each day's ending balance and sum them up—the result is nearly identical.
Timing matters slightly. A deposit on the 1st of the month earns interest for the full month. A deposit on the 30th earns interest for only one or two days. For most people, this difference is small—a few cents—but it is why some people move money into savings accounts early in the month rather than late. Withdrawals work the same way in reverse: the sooner you withdraw, the less interest you lose.
How different crediting schedules change your earnings
Most online banks credit interest monthly. Some credit quarterly. A few older banks or money market accounts credit annually. The difference is when the interest you have earned actually gets added to your account and starts earning interest itself.
| Crediting Schedule | When Interest Is Added | Effect on Your Money |
|---|---|---|
| Monthly | Last day of each month | Interest earns interest 11 more times that year |
| Quarterly | Last day of March, June, September, December | Interest earns interest 3 more times that year |
| Annual | Last day of December | Interest does not earn interest until next year |
On a $10,000 balance at 4.50% APY, monthly crediting earns you about $461 in a year. Quarterly crediting earns you about $459. Annual crediting earns you about $450. The difference grows with larger balances and higher rates, but for most people it is not the deciding factor when choosing an account. The APY itself matters far more than the crediting schedule.
Why your APY can change and what that means for your interest
Banks set their own APY and can change it at any time. When the Federal Reserve raises or lowers its benchmark rate, banks usually adjust their savings rates within days or weeks. A rate that is 4.50% today might be 4.25% next month if the Fed cuts rates. Your existing balance continues to earn at the new rate once the change takes effect—there is no grace period or lock-in.
Some accounts offer a promotional rate for a limited time, then drop to a standard rate. Read the account terms to see whether your rate is promotional and when it expires. A few banks offer tiered rates, where higher balances earn higher APY. If you have $100,000 in the account, you might earn 4.75% on the first $50,000 and 5.00% on the amount above that. The bank calculates interest on each tier separately and credits the total.
How to estimate your interest earnings before opening an account
Most banks publish an interest calculator on their website. You enter your balance, the APY, and how long you plan to keep the money, and it shows you the projected interest. These calculators assume your balance stays constant and the APY does not change, so the real number will differ if either assumption is wrong. But they give you a reasonable estimate for comparison.
For a rough mental math check: divide the APY by 12 to get the monthly rate, then multiply by your balance. A 4.50% APY is roughly 0.375% per month. On $10,000, that is about $37 per month, or $450 per year. This ignores compounding, so the real number is slightly higher, but it is close enough to compare accounts quickly.
Frequently Asked Questions
Does the bank pay interest on the interest I already earned?
Yes. Once the bank credits interest to your account, that money becomes part of your balance and earns interest in the next period. This is compounding. The more frequently interest is credited, the more times it can earn interest during the year.
What if I withdraw money before the month ends?
You still earn interest on the balance for the days you held the money. If you withdraw on the 15th, you earn interest on your full balance for the first 15 days, then on the lower balance for the remaining days. The interest is calculated daily, so you do not lose the interest you already earned.
Can a bank change my interest rate after I open the account?
Yes. Banks can change APY at any time without notice. When rates change, the new rate applies to your balance going forward. There is no lock-in period for savings accounts, unlike certificates of deposit.
Why do different banks offer different interest rates?
Banks set their own rates based on their funding costs, competition, and business strategy. Online banks typically offer higher rates than brick-and-mortar banks because they have lower overhead. During periods when the Fed raises rates, online banks usually raise their savings rates faster than traditional banks do.
Does the interest I earn count as income for taxes?
Yes. Interest earned on a savings account 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 that counts is the interest credited to your account during the year, not the interest you withdrew.