Banks use one of two methods to calculate interest: simple interest or compound interest
Most savings accounts use compound interest, which means the bank pays you interest on your interest. Your balance grows faster than it would with simple interest because each time interest is added to your account, the next interest payment is calculated on that larger amount.
Simple interest, used less often, pays interest only on your original deposit. If you put $1,000 in an account earning 5% simple interest annually, you earn $50 per year, every year. With compound interest at the same rate, your earnings grow each year because the interest itself starts earning interest.
The difference between the two methods becomes larger over time. After five years, simple interest on $1,000 at 5% gives you $1,250. Compound interest at the same rate gives you roughly $1,276. The longer your money sits, the wider that gap becomes.
Key Takeaways
- Compound interest means you earn interest on your interest, while simple interest only pays on your original deposit.
- The compounding frequency—daily, monthly, or quarterly—affects how much you actually earn, even when the stated rate is the same.
- Annual Percentage Yield (APY) shows your real earnings after compounding is factored in, while Annual Percentage Rate (APR) does not.
- Banks calculate interest based on your daily balance, your average balance, or your ending balance, depending on the account terms.
- A higher stated interest rate matters less than how often it compounds and what balance method your bank uses.
How compounding frequency changes what you actually earn
Banks compound interest at different intervals: daily, monthly, quarterly, or annually. The more often interest compounds, the more you earn. A bank compounding daily adds interest to your account 365 times per year. A bank compounding monthly does it 12 times. Even with the same stated interest rate, daily compounding produces more money in your pocket.
This is why Annual Percentage Yield (APY) matters more than the stated rate. APY shows what you actually earn after compounding is included. If a bank advertises 4.5% APY, that number already accounts for how often they compound. The stated rate (called the Annual Percentage Rate or APR) does not. A 4.5% APR compounded daily produces a different APY than 4.5% APR compounded monthly.
When you compare savings accounts, look at the APY, not the APR. Two banks might advertise the same APR, but the one compounding daily will pay you more by the end of the year.
The three ways banks calculate your balance for interest
Banks use three different methods to decide what balance they pay interest on. The method matters because it changes how much you earn, especially if your balance moves around during the month.
Daily balance method: The bank calculates interest on your balance each day, then adds all those daily interest amounts together at the end of the month. This is the most common method and usually pays you the most, because even small deposits earn interest immediately.
Average daily balance method: The bank adds up your balance for each day of the month, then divides by the number of days. Interest is paid on that average. If you had $1,000 for 15 days and $2,000 for 15 days, your average balance is $1,500. This method is less generous than daily balance but more generous than the next one.
Ending balance method: The bank pays interest only on whatever balance you have on the last day of the month. This method pays you the least. If you withdraw money on the 28th, you lose interest on that money for the entire month, even though you had it for most of the month.
What happens when you deposit or withdraw money mid-month
The timing of deposits and withdrawals affects your earnings because of how banks calculate your balance. With the daily balance method, a deposit on the 15th starts earning interest immediately. With the ending balance method, a deposit on the 15th does not help you at all that month—only next month's interest will include it.
Withdrawals work the opposite way. With daily balance, money you withdraw on the 15th stops earning interest immediately. With ending balance, you lose interest on that money for the entire month, even though you only had it for half the month.
This is why the daily balance method is better for savers who move money in and out. If your balance is stable, the method matters less.
How interest rates change and when banks recalculate
Banks set their own interest rates and can change them whenever they want. Most savings accounts have variable rates, meaning the rate can go up or down. When the Federal Reserve raises or lowers its benchmark rate, banks usually adjust their savings rates within days or weeks, though they are not required to.
Banks recalculate interest at least monthly, sometimes daily. If your rate changes mid-month, the bank applies the old rate to the days before the change and the new rate to the days after. You see the combined interest payment at the end of the month.
High-yield savings accounts often change rates more frequently than traditional savings accounts because they compete on rate. If you want to track whether your bank is keeping up with market rates, check what other banks are offering. If you are earning significantly less, you may want to move your money.
The math behind the interest calculation
If you want to see the actual formula, banks use this: Interest = Principal × Rate × Time. Principal is your balance, Rate is the daily interest rate (the annual rate divided by 365), and Time is the number of days.
For example, if you have $5,000 in an account earning 4% APY, compounded daily, the daily rate is 4% ÷ 365 = 0.01096%. On day one, you earn $5,000 × 0.0001096 = $0.55 (rounded). On day two, if your balance is still $5,000, you earn another $0.55. If you deposit $1,000 on day two, your new balance is $6,000, and day three's interest is $6,000 × 0.0001096 = $0.66.
Most people do not need to do this math themselves. Your bank statement shows the interest you earned each month. But understanding the formula helps you see why compounding frequency and balance method matter.
Why your statement shows interest different from what you calculated
If you try to calculate your own interest and it does not match your statement, the most common reasons are: the bank changed the rate mid-month, your balance changed on a day you forgot about, or the bank rounds differently than you did.
Banks also sometimes hold deposits for a day or two before they are available for interest calculation. A deposit you made on the 15th might not start earning interest until the 16th or 17th, depending on the bank's processing time. This is separate from when the money appears in your account—it may show up immediately but not earn interest immediately.
If the difference is small (a few cents), it is almost always due to rounding or timing. If it is large, contact your bank and ask them to walk you through the calculation. They can show you the exact balance they used each day and the rate they applied.
Frequently Asked Questions
Does my bank pay interest on money I just deposited?
It depends on when the deposit clears and your bank's policy. Money that appears in your account immediately may not earn interest until the next business day. Some banks start interest on the day of deposit; others wait until the deposit fully clears. Check your account terms or ask your bank directly.
If I withdraw money, do I lose all the interest I earned that month?
No. You keep the interest you already earned. You only lose interest on the money you withdrew, and only for the days after the withdrawal. The daily balance method minimizes this loss because interest stops accruing the day you withdraw.
Why does my bank's APY look lower than other banks' APY?
Banks set their own rates based on their costs and competition. Some banks pay less because they have lower overhead or less competition in their market. Others pay more to attract deposits. Compare APY across several banks to find the best rate for your situation.
Can I predict exactly how much interest I will earn next month?
Not precisely, because your balance may change and the rate may change. You can estimate by multiplying your average balance by the APY and dividing by 12, but the actual amount will vary slightly based on the exact days your money sits in the account and any rate changes.
What is the difference between APR and APY?
APR is the stated annual rate before compounding. APY is the actual rate you earn after compounding is included. APY is always equal to or higher than APR. When comparing accounts, use APY because it shows what you actually make.