Banks multiply your balance by an interest rate, then divide by the number of days in a year
The basic formula is straightforward: your bank takes the money you have on deposit, multiplies it by the annual interest rate they've promised you, and divides by 365 (or sometimes 360). The result is how much interest you earn that day. They repeat this calculation every single day, adding each day's interest to your balance, so tomorrow's calculation includes today's interest. This is called compounding—earning interest on interest.
The catch is that your balance changes constantly. Every deposit adds to it. Every withdrawal reduces it. So the bank doesn't use your opening balance or your closing balance. They use your daily balance—the actual amount in the account at the end of each day—and calculate interest on that specific number.
At the end of the month, the bank adds up all those daily interest amounts and deposits the total into your account. This is why two accounts with the same annual rate can earn different amounts: the one with more money in it, or money in it for longer, will earn more.
Key Takeaways
- Banks calculate interest daily by multiplying your daily balance by the annual rate and dividing by 365, then add all daily amounts together at month's end.
- Your daily balance is what you actually have on deposit at the end of each day, not your opening or closing balance for the month.
- Compounding means you earn interest on the interest already credited to your account, which is why longer holding periods and higher balances earn more.
- The stated annual percentage yield (APY) already includes the effect of compounding, so you can compare rates directly between banks without doing the math yourself.
Why your daily balance matters more than your monthly average
Some banks calculate interest on your average daily balance instead of your actual daily balance. The difference is real money. If you deposit $10,000 on the first of the month and withdraw it on the 15th, the average daily balance method treats you as if you had $5,000 for the whole month. The daily balance method counts the full $10,000 for 14 days and zero for the remaining days—which is what actually happened.
Most savings accounts use the daily balance method, but some older accounts or promotional rates use average daily balance. Check your account agreement or ask your bank directly which method they use. The disclosure should be in the section labeled "How Interest Is Calculated" or "Interest Calculation Method."
The practical impact: if you move money in and out frequently, daily balance calculation rewards you more than average daily balance does. If your balance is stable, the difference is negligible.
How the annual percentage yield (APY) relates to the interest rate
Banks advertise two different numbers: the interest rate and the annual percentage yield (APY). The interest rate is the raw percentage—say, 4.50% per year. The APY is that rate plus the effect of compounding, expressed as a single number.
Because interest compounds daily, you earn slightly more than the stated rate. If a bank offers 4.50% compounded daily, the actual APY might be 4.60%. The difference grows larger the higher the rate. This is why the law requires banks to show you the APY: it's the true annual return you'll receive if you hold the money for a full year without touching it.
When you compare savings accounts between banks, compare the APY, not the interest rate. The APY already accounts for how often the bank compounds, so you're comparing apples to apples.
What happens when you deposit or withdraw mid-month
The daily balance method handles deposits and withdrawals cleanly. If you deposit $5,000 on the 10th, that $5,000 earns interest starting on the 10th. If you withdraw $2,000 on the 20th, that $2,000 stops earning interest on the 20th. The bank calculates interest on whatever balance you actually have each day.
Some banks credit interest on the day of deposit; others credit it the next business day. This matters only if you're depositing very large amounts or moving money between accounts frequently. For most people, the difference is a few cents per month. Check your account agreement for the exact timing, or ask your bank's customer service line.
Withdrawals are usually processed the same day you request them during business hours, so they reduce your balance when ready for interest calculation purposes. This is why withdrawing money early in the month costs you more interest than withdrawing it late in the month—you lose interest on that money for more days.
How promotional rates and tiered rates change the calculation
Some banks offer a higher rate for the first few months, then drop to a lower rate. The calculation doesn't change—the bank just uses the promotional rate for those days, then switches to the standard rate. If you had a 5.00% promotional rate for 60 days and then 4.00% for the rest of the year, the bank calculates interest at 5.00% for the first 60 days and 4.00% for the remaining days.
A few banks use tiered rates: you earn one rate on the first $10,000, a higher rate on the next $50,000, and an even higher rate on anything above that. The bank calculates interest separately for each tier and adds them together. This is rare in savings accounts but common in money market accounts. Your statement should show the breakdown if your account uses tiered rates.
Read the terms carefully when you open an account. Promotional rates usually expire on a specific date, and the bank will notify you before the switch, but the lower rate will explore automatically. There's no action required on your part.
The difference between straightforward and compound interest
straightforward interest means the bank calculates interest only on your original deposit, never on the interest already earned. Compound interest means the bank calculates interest on your balance plus all previously earned interest. Every savings account uses compound interest; straightforward interest is mainly a teaching tool.
With compound interest, your money grows faster the longer you leave it alone. After one year at 4.50% APY, $10,000 becomes $10,450. After two years, it becomes $10,920.25—not $10,900. That extra $20.25 is interest earned on the first year's interest. The longer the timeline, the more dramatic the difference.
This is why banks compound daily instead of monthly or yearly: daily compounding means you earn interest on interest more often, which benefits the customer. It's also why moving money out of a savings account and into a checking account costs you money—checking accounts earn little or no interest, so you lose the compounding benefit.
Reading your statement to verify the calculation
Your monthly statement should show the interest credited to your account. It may also show the daily balance, the rate used, and the number of days in the period. Not all banks show all of this information, but the interest amount itself is always there.
To spot-check the calculation, find the APY on your statement, divide it by 365, and multiply by your average daily balance. The result should be close to the interest shown—it won't be exact because the bank calculates daily, not on an average, but it should be within a few cents. If the interest is significantly lower than this rough calculation suggests, contact the bank and ask them to explain the discrepancy.
If your account has a promotional rate that expired during the month, the statement should show when the rate changed and how much interest was earned at each rate. If it doesn't, ask for a detailed breakdown. Banks are required to provide this information if you request it.
Frequently Asked Questions
Does the bank calculate interest on money I deposit on the last day of the month?
Yes. If you deposit money on the last day of the month, it earns interest starting that day. The amount will be tiny—one day's worth of interest—but it counts. The interest posts to your account at the end of the following month, so you won't see it when ready.
What happens to interest if I close my account mid-month?
You receive all interest earned up to the day you close the account. The bank calculates it through your closing date and deposits it before closing the account. You'll see this interest on your final statement.
Why is my interest lower than I expected based on the advertised APY?
The APY assumes you hold the money for a full year without deposits or withdrawals. If you withdrew money partway through the month, your average balance was lower, so you earned less. Also, if the rate changed during the month, the APY shown on your statement is a blended rate, not the rate for the entire year.
Can a bank change the interest rate on my savings account?
Yes. Banks can change rates at any time unless you have a fixed-rate account (rare for savings accounts). They must notify you before the change takes effect, usually by email or mail. The new rate applies to interest earned after the change date.
Is interest calculated the same way at all banks?
The basic formula is the same everywhere, but the compounding frequency and the method for handling deposits and withdrawals can vary slightly. Most banks compound daily and credit interest monthly. Some older accounts or special products may differ. Always check your account agreement or ask your bank directly.