Banks multiply your balance by an interest rate, then divide by the number of days in a year

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance × the annual interest rate ÷ 365 (or 360, depending on the bank) = the interest you earn that day. That daily interest then becomes part of your balance the next day, so you earn interest on your interest.

The rate you see advertised—called the Annual Percentage Yield (APY)—already accounts for compounding. So if your account shows 4.50% APY, that is the actual return you will receive over a year if you leave the money untouched. The bank does the compounding math behind the scenes; you do not have to.

The timing of deposits and withdrawals matters because interest is calculated on your daily balance. If you deposit $5,000 on the 15th of the month, you earn interest on that $5,000 starting the 15th. If you withdraw it on the 20th, you stop earning interest on it after the 20th. Banks post the accumulated interest to your account monthly, quarterly, or annually depending on the account terms.

Key Takeaways

  • Interest is calculated daily on your actual balance, but posted to your account on a schedule set by your bank—usually monthly or quarterly.
  • The APY shown on the account already includes the effect of compounding, so you do not need to calculate it yourself.
  • Deposits earn interest starting the day they are credited; withdrawals stop earning interest the day they leave your account.
  • Different banks use slightly different day counts (365 or 360 days per year), which creates small differences in the final amount.
  • A higher APY means more interest earned, but only if you compare accounts with the same compounding frequency.

Why APY matters more than the interest rate alone

Banks sometimes list two numbers: the interest rate and the APY. The interest rate is the base percentage. The APY is what you actually earn because it includes compounding. If a bank compounds interest daily, the APY will be slightly higher than the stated rate. If it compounds monthly, the difference is smaller.

When you compare two savings accounts, always look at the APY, not the rate. A 4.40% rate compounded daily might deliver more money than a 4.50% rate compounded quarterly. The APY tells you the true annual return without doing the math yourself.

How compounding works over time

On day one, you have $10,000 and a 4.80% APY. The bank calculates: $10,000 × 0.048 ÷ 365 = $1.31 in interest that day. Your balance becomes $10,001.31. On day two, the bank calculates interest on $10,001.31, not the original $10,000. The extra $1.31 earns interest too.

Over a year, this compounding effect adds up. On a $10,000 balance at 4.80% APY, you earn roughly $480 in the first year. But in year two, you earn interest on $10,480, so you earn slightly more. The longer money sits in the account, the more noticeable the compounding becomes. This is why leaving money untouched produces more growth than moving it in and out frequently.

The difference between daily, monthly, and quarterly compounding

Compounding frequency determines how often the bank adds earned interest back to your balance. Daily compounding (the most common for savings accounts) means interest is calculated and added every day. Monthly compounding means it happens once a month. Quarterly means four times a year.

The difference in final dollars is small but real. On $10,000 at 4.80% APY, daily compounding might earn you $480.33 over a year, while monthly compounding might earn $479.50. The daily version wins because interest starts earning interest more often. Most online banks and credit unions use daily compounding because it is better for the customer and helps them compete on rates.

What happens when you deposit or withdraw mid-month

Banks track your balance every day, so deposits and withdrawals change how much interest you earn. If you deposit $5,000 on the 10th and withdraw it on the 20th, you earn interest for 10 days on that $5,000. The interest earned is small—roughly $1.30 at 4.80% APY—but it is real.

Some banks use the "average daily balance" method instead of daily compounding. This method adds up your balance for each day of the month, divides by the number of days, and calculates interest on that average. It produces nearly the same result as daily compounding for most customers, but the math is slightly different behind the scenes. Your account statement will tell you which method your bank uses.

Why your APY can change

The Federal Reserve sets a benchmark interest rate that influences what banks pay on savings. When the Fed raises rates, banks usually raise the APY on savings accounts within days or weeks. When the Fed cuts rates, banks cut APY more slowly—sometimes not at all if they are trying to keep customers. Your bank can also change your rate whenever they want, though they must notify you in advance.

This means the 4.80% APY you see today might be 4.50% in three months. Online banks tend to change rates faster than brick-and-mortar banks because they compete heavily on rate. If you want to lock in a higher rate, some banks offer certificates of deposit (CDs), which may provide a fixed rate for a set period—usually three months to five years.

How to find the actual interest earned on your account

Your bank statement shows the interest posted each month or quarter. Look for a line item labeled "Interest Paid" or "Interest Earned." This is the actual dollars added to your account. If you want to calculate what you should have earned, use this formula: (Your Average Daily Balance × APY ÷ 365) × Number of Days in the Period.

Most online banking platforms also show a running total of interest earned year-to-date. If your bank does not display this, you can add up the interest from each statement. At tax time, your bank sends a 1099-INT form if you earned $10 or more in interest during the year. You report this on your tax return.

Frequently Asked Questions

Does interest compound on weekends and holidays?

Yes. Banks calculate interest every calendar day, including weekends and holidays. The daily interest is added to your balance even if the bank is closed. You will see the total posted to your account on the next business day, but the interest accrued every day.

What is the difference between APY and APR?

APY (Annual Percentage Yield) includes compounding and shows your actual return. APR (Annual Percentage Rate) does not include compounding and is used mainly for loans and credit cards. For savings accounts, always use APY to compare rates.

Can I lose money if interest rates drop?

No. The interest you have already earned stays in your account. If rates drop, you simply earn less interest going forward on new balances. Your principal and all previously earned interest are safe.

Why do some banks offer higher APY than others?

Online banks have lower overhead costs than physical branches, so they can pay more interest to attract customers. Credit unions sometimes pay higher rates because they are member-owned and return profits to members. Banks also adjust rates based on competition and how much they need deposits at any given time.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on a savings account is taxable income. If you earn $10 or more in a calendar year, your bank sends you a 1099-INT form for tax filing. You report this interest on your federal tax return. Some states also tax savings account interest.