Interest is calculated on your balance, compounded at intervals set by your bank

Banks calculate savings account interest by taking your account balance, multiplying it by the annual interest rate (called the Annual Percentage Yield or APY), and dividing by the number of times per year interest is added to your account. Most banks compound interest daily or monthly, meaning they calculate and add interest more than once per year — and each time, the new interest is calculated on your previous balance plus the interest already earned.

The timing and frequency matter. A bank that compounds daily will pay you slightly more than one that compounds monthly, even if both advertise the same APY. The APY already accounts for compounding, so it is the number to compare across banks. Your actual earnings depend on three things: how much money sits in the account, what APY the bank offers, and how often interest compounds.

Key Takeaways

  • Annual Percentage Yield (APY) is the rate to compare across banks because it already includes how often interest compounds.
  • Interest compounds when the bank adds earned interest to your balance, then calculates next period's interest on the larger amount.
  • Daily compounding pays slightly more than monthly or quarterly compounding, but only if the APY is the same.
  • Your balance on the day interest is calculated determines how much you earn that period — deposits made mid-period usually earn interest starting the next day.

How the math works: balance, rate, and time

The basic formula is: Interest = Balance × APY ÷ Number of Compounding Periods Per Year. If you have $10,000 in an account with 4.50% APY that compounds monthly, the bank divides 4.50% by 12 to get the monthly rate (0.375%), then multiplies $10,000 by 0.375% to calculate that month's interest. That comes to $37.50.

The next month, the bank calculates interest on $10,037.50 — your original balance plus the interest earned. This is compounding. Over a year, that extra $37.50 earning its own interest adds up. With daily compounding at the same 4.50% APY, you would earn slightly more because interest is calculated and added 365 times instead of 12, giving each day's interest a chance to earn interest sooner.

The difference between monthly and daily compounding on $10,000 at 4.50% APY is roughly $15 to $20 per year — not huge, but real. The difference between 4.50% APY and 0.01% APY is thousands of dollars per year on the same balance, which is why shopping for the highest APY matters far more than the compounding frequency.

When your deposits and withdrawals affect interest calculations

Banks use different methods to decide which balance earns interest. The most common is the daily balance method: the bank tracks your balance at the end of each day, adds up all those daily balances for the month, divides by the number of days, and calculates interest on that average. A deposit made on the 15th of the month counts as part of your balance for the remaining 16 days of that month.

Some banks use the minimum balance method, where only your lowest balance during the period earns interest. This is rare for savings accounts but common for money market accounts. If you had $10,000 on day 1, withdrew $5,000 on day 15, and kept $5,000 for the rest of the month, only the $5,000 would earn interest that period.

A few banks use the ending balance method, calculating interest only on what you have at the end of the period. This is the least common and usually the worst for savers. Check your account agreement or call your bank to find out which method they use — it is listed in the fine print, often under "how interest is calculated" or "interest calculation method."

The difference between APY and APR on savings accounts

You may see both APY and APR listed. APR (Annual Percentage Rate) does not include compounding — it is the simple interest rate. APY (Annual Percentage Yield) includes the effect of compounding and is always higher than APR on a savings account. Banks are required to show you the APY when advertising savings rates, so that is the number to use when comparing accounts.

For example, a bank might offer 4.50% APR with daily compounding. The actual APY would be around 4.60% because of compounding. The difference grows larger as the APR rises and as compounding happens more often. When you see a savings rate advertised, it is almost always the APY, which is what you will actually earn.

How interest frequency affects your earnings over time

Compounding frequency matters most when you leave money in the account for years. Over one year, daily versus monthly compounding on $10,000 at 4.50% APY makes a difference of roughly $15 to $20. Over five years, that difference compounds itself and grows to $80 to $100. Over ten years, it can reach $200 or more.

The real power of compounding comes from leaving the interest in the account. If you withdraw interest as it is paid, you lose the compounding effect — you earn interest only on your original balance. If you leave it alone, you earn interest on the interest, and that accelerates over time. This is why a savings account with 4.50% APY left untouched for ten years will grow significantly more than one where you withdraw interest each month.

However, the difference between a bank offering daily compounding and one offering monthly compounding is small compared to the difference between banks offering different APY rates. A bank with 4.50% APY and monthly compounding will pay you far more than a bank with 0.50% APY and daily compounding.

Why your bank's APY changes and how it affects new interest

Banks adjust their APY based on the Federal Reserve's interest rate decisions. When the Fed raises rates, banks typically raise APY on savings accounts within days or weeks. When the Fed cuts rates, banks lower APY, sometimes immediately. The change applies to new interest going forward, not to interest already paid.

If your bank offered 4.50% APY last month and drops it to 3.75% this month, the interest you already earned stays in your account. Only the interest calculated from this point forward uses the new 3.75% rate. This is why some savers move money to a different bank when rates drop — they can lock in a higher rate elsewhere. Banks do not penalize you for moving savings between accounts or to a different bank, though some require a minimum balance to earn the advertised rate.

How to track what you are actually earning

Your bank statement shows interest paid each month or quarter, depending on how often your bank compounds. The statement lists the amount added and the date it was added. Over time, you can add up all the interest payments to see your actual earnings and compare them to what you expected based on the APY.

Most online banks show a running total of interest earned year-to-date on your account dashboard. You can also calculate expected earnings yourself: multiply your average balance by the APY and divide by 12 for a monthly estimate. If your actual interest is significantly lower than expected, check whether your balance was lower than you thought, whether the APY changed during the period, or whether the bank is using a different calculation method than you assumed.

If you notice an error — interest that seems too low or a rate that does not match what was advertised — contact your bank's customer service. Banks make mistakes, and they will correct them if you catch them.

Frequently Asked Questions

Does interest start earning interest right away?

Yes, but only after the first interest payment is made. When the bank adds interest to your account for the first time, that interest becomes part of your balance. The next time interest is calculated, it includes the previous interest. If your bank compounds daily, this happens every day. If it compounds monthly, you wait until the end of the month for the first interest payment, then it starts earning interest immediately.

What happens to my interest if I withdraw money mid-month?

It depends on your bank's calculation method. With the daily balance method (most common), you earn interest on the money for the days you held it. If you deposit $5,000 on the 1st and withdraw it on the 15th, you earn interest on $5,000 for 15 days. With the minimum balance method, you would earn nothing because your balance dropped. Check your account agreement to know which method applies.

Is the APY may provide to stay the same?

No. Banks can change APY at any time, and most do when the Federal Reserve changes interest rates. Your current rate is may provide only for the current period — usually one month or one quarter. When the period ends, the bank can offer a new rate. You are not locked in, and you can move your money to a different bank if rates drop.

Why do different banks offer different APY rates?

Banks set their own rates based on their costs, competition, and business strategy. Online banks typically offer higher APY than brick-and-mortar banks because they have lower overhead. Credit unions sometimes offer higher rates to members. Shopping around for the highest APY is the most direct way to earn more interest on the same balance.

Does my interest get taxed?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest, and you report that amount on your tax return. The interest is taxed at your ordinary income tax rate, not at a special rate. This is why high-yield savings accounts are more valuable in high-tax situations — the higher interest can offset the tax burden.