Interest grows your money through daily calculation and periodic deposits
A savings account earns interest because the bank uses your money. When you deposit $1,000, the bank lends that money to other customers through mortgages, car loans, and credit lines. The bank keeps the difference between what it pays you and what it charges borrowers. That difference is your interest.
The bank calculates how much interest you earn based on three things: your account balance, the annual percentage yield (APY) the bank offers, and how often the bank compounds interest. Most banks compound daily, meaning they calculate interest on your balance every single day, then add that daily interest to your account. Some banks compound monthly or quarterly instead.
The actual mechanics are straightforward: the bank divides your APY by 365 (or 360 on some accounts), multiplies that daily rate by your current balance, and adds the result to your account. If you have $10,000 and the APY is 4.5%, the bank calculates roughly $1.23 per day in interest. That amount lands in your account daily, though you typically see it reported as a single monthly or quarterly deposit depending on your bank's schedule.
Key Takeaways
- Interest is calculated daily at most banks by dividing the APY by 365, multiplying by your balance, and adding the result to your account.
- Compounding means the bank pays interest on your interest, so your balance grows faster the longer money sits in the account.
- The APY you see advertised is the rate after accounting for compounding, so it reflects your actual annual return.
- Your balance on the day interest is calculated determines how much you earn that day, so deposits made mid-month earn less than deposits made at the start of the month.
How compounding makes your money grow faster
Compounding is the reason your savings accelerate over time. On day one, the bank pays interest on your original deposit. On day two, the bank pays interest on your original deposit plus the interest you earned on day one. This creates a snowball effect where your balance grows slightly faster each day.
The difference between daily compounding and monthly compounding is real but small for most balances. If you have $5,000 at 4.5% APY, daily compounding earns you about $2.50 more per year than monthly compounding. The larger your balance or the longer you leave money untouched, the more compounding matters. Over 10 years, that $5,000 grows to roughly $7,560 with daily compounding versus $7,557 with monthly compounding—a $3 difference.
The APY you see advertised already includes the effect of compounding. If a bank shows 4.5% APY, that is your actual return after accounting for how often interest compounds. The bank cannot advertise a higher rate and then compound less frequently—federal rules require the APY to reflect the real yield you receive.
When interest actually lands in your account
Banks calculate interest daily, but they deposit it into your account on a schedule. Most banks post interest monthly, some quarterly, and a few daily. The posting schedule does not change how much you earn—the bank still calculates daily—it only changes when you see the money appear.
If your bank posts interest monthly, you might see a single deposit of $12.30 on the first of each month, even though the bank calculated roughly $0.41 per day. If your bank posts daily, you see tiny deposits every single day. Neither approach changes your total earnings; they are just different ways of showing you the same money.
The balance used for interest calculation is usually your average daily balance or your ending daily balance, depending on the bank. Most banks use ending daily balance, meaning they look at what you have at the end of each day and calculate interest on that amount. A few use average daily balance, which smooths out the effect of large deposits or withdrawals mid-month. Check your account agreement or call the bank to confirm which method yours uses.
Why your APY changes and what that means for your earnings
Banks change their APY based on what the Federal Reserve does with interest rates. When the Fed raises rates, banks compete to attract deposits and often raise their APY. When the Fed cuts rates, banks lower their APY because they earn less from lending. Your current APY is not locked in—it can move up or down without notice, and different banks move at different speeds.
A higher APY means more interest in your pocket. Moving $10,000 from a 0.01% APY account to a 4.5% APY account means earning roughly $450 more per year on the same balance. That difference compounds, so over five years you earn roughly $2,300 more. Shopping for a higher APY is one of the few ways to increase your savings without depositing more money.
Some banks offer promotional rates that are higher than their standard rate for a limited time. These rates are real—you earn the higher amount—but they typically last three to six months before dropping to the regular rate. Read the fine print to see when the promotional period ends and what rate applies after.
How deposits and withdrawals affect your interest
The timing of deposits and withdrawals matters because interest is calculated on your balance each day. If you deposit $5,000 on the 15th of the month, you earn interest on that $5,000 for only the remaining 16 days of the month. If you deposit on the 1st, you earn interest for all 31 days. The difference is small but real: depositing early in the month means more interest by month's end.
Withdrawals work the same way. If you withdraw $5,000 on the 20th, you stop earning interest on that amount for the remaining 11 days of the month. The bank calculates interest only on the balance you actually have each day, so your interest shrinks when your balance shrinks.
This is why some people time large deposits to the beginning of a month or quarter—it maximizes the number of days the bank calculates interest on that money. For most people, the difference is negligible, but for very large balances or frequent deposits, it adds up.
What happens if you close your account before interest posts
If you close your account before the bank posts interest, you lose the interest that has been calculated but not yet deposited. If your bank posts interest monthly and you close your account on the 25th, the interest calculated through the 25th is typically forfeited. Some banks will pay accrued interest if you close within a few days of the posting date, but this varies by institution.
To avoid losing interest, check when your bank posts interest and time your closure after that date. Most banks post on the first of the month, so closing on the 2nd or 3rd usually means you receive the previous month's interest. Call your bank before closing to confirm the exact posting date and whether you will receive accrued interest.
Frequently Asked Questions
Does interest compound on interest I have already earned?
Yes. Once interest is added to your account, the bank treats it like any other balance and calculates interest on it the next day. This is why compounding accelerates your growth over time—you earn returns on your original deposit and on all the interest that has accumulated.
What if I withdraw money mid-month—do I lose all the interest for that month?
No. The bank calculates interest on your balance each day, so you earn interest on the money for the days you held it. If you withdraw on the 20th, you earn interest through the 20th. You only lose interest on the money for the days after you withdraw it.
Can a bank change my APY without telling me?
Yes. Banks can change APY at any time unless you have a promotional rate with a specific end date. Most banks notify customers of rate changes, but they are not required to ask permission. Check your account statements or log in online to see if your rate has changed.
Is the interest I earn on a savings account taxable?
Yes. The IRS treats savings account interest as income. Banks report interest over $10 on Form 1099-INT, and you must include it on your tax return. The amount you owe in taxes depends on your tax bracket and total income.
Why do some banks offer much higher APY than others?
Online banks typically offer higher APY than brick-and-mortar banks because they have lower overhead costs. They do not maintain physical branches, so they pass savings to customers through higher rates. The money is equally safe—both types of banks are FDIC insured up to $250,000.