Banks use one of two methods to calculate how much interest you earn: straightforward interest or compound interest

straightforward interest is straightforward math. The bank takes your account balance, multiplies it by the annual interest rate, and divides by the number of days in a year. You earn the same amount of interest each day based on that starting balance, regardless of whether you add or withdraw money. Most savings accounts do not use straightforward interest anymore.

Compound interest is what most banks use now. The bank calculates interest on your balance, adds that interest to your account, then calculates the next period's interest on the new, larger balance. This means you earn interest on your interest. The more often the bank compounds—daily, weekly, monthly—the more you earn, because each compounding adds to the base amount that generates the next round of interest.

The difference matters. On a $10,000 balance at 4% annual interest compounded daily, you earn roughly $408 in a year. With straightforward interest at the same rate, you earn exactly $400. The gap widens as your balance grows or as you keep money in the account longer.

Key Takeaways

  • Banks compound interest daily, monthly, or quarterly depending on the account, and daily compounding produces the highest earnings for you.
  • The annual percentage yield (APY) shown on your account disclosure already includes the effect of compounding, so you can compare accounts directly without doing math yourself.
  • Your actual interest earnings depend on your balance, the interest rate, how often the bank compounds, and how long money stays in the account.
  • Banks calculate interest on the balance at specific times—usually the end of each day—so deposits and withdrawals change what you earn in that period.

How the compounding frequency changes what you earn

A bank that compounds daily calculates interest 365 times per year. A bank that compounds monthly does it 12 times. A bank that compounds quarterly does it 4 times. The more frequently the bank compounds, the more interest you earn, because each compounding event adds earned interest back into the balance that generates the next round of interest.

The difference between daily and monthly compounding is real but not enormous on typical balances. On $5,000 at 4% APY, daily compounding earns you roughly $200 per year, while monthly compounding earns roughly $199. On $50,000, the gap widens to about $2,000 versus $1,990. The larger your balance, the more the compounding frequency matters.

Banks must disclose their compounding frequency in the account terms or on the Truth in Savings disclosure form they give you when you open the account. Look for language like "compounded daily" or "compounded monthly." If you do not see it, call the bank and ask—it is a required disclosure.

Why the APY is the number that actually matters

The annual percentage yield (APY) is the interest rate that already includes the effect of compounding. When a bank advertises 4% APY, that 4% is what you actually earn in a year if you leave the money untouched. You do not have to do any math or figure out the compounding frequency yourself—the APY does that work for you.

The APY is different from the annual percentage rate (APR), which is the raw interest rate before compounding is factored in. Banks are required by law to show you the APY, not the APR, on savings accounts. This makes it straightforward to compare two banks: the one with the higher APY will earn you more money, period.

The APY assumes your balance stays the same for the entire year. If you deposit money partway through the year or withdraw it, your actual earnings will be lower. The bank calculates interest only on the balance you actually have during each period.

How banks measure your balance for interest calculation

Banks use one of three methods to decide what balance they calculate interest on: the daily balance method, the average daily balance method, or the ending balance method. Most banks use the daily balance method for savings accounts.

With the daily balance method, the bank looks at your balance at the end of each day and calculates interest on that amount. If you deposit $1,000 on Monday and withdraw $500 on Wednesday, the bank calculates interest on $10,000 for Monday and Tuesday, then on $9,500 for Wednesday onward. This is the most common method and usually the most favorable to you, because deposits earn interest when ready.

With the average daily balance method, the bank adds up your balance at the end of each day during the month, divides by the number of days, and calculates interest on that average. This method smooths out the effect of deposits and withdrawals but can result in slightly lower interest if you make large deposits late in the month.

With the ending balance method, the bank looks only at your balance on the last day of the month and calculates interest on that. This method is rare for savings accounts and is the least favorable to you, because a large withdrawal on the last day can wipe out interest you earned all month.

What happens to interest when you deposit or withdraw money

When you deposit money, it begins earning interest when ready if the bank uses the daily balance method—which most do. The interest accrues (builds up) daily and is usually credited to your account monthly, though some banks credit it more or less frequently. You do not have to do anything; the bank adds it automatically.

When you withdraw money, you stop earning interest on that amount starting the next day. If you withdraw $5,000 on the 15th of the month, you earn interest on the full balance through the 14th, then on the reduced balance from the 15th onward. The interest you already earned stays in your account.

Some banks have a minimum balance requirement to earn interest at all. If your balance drops below that minimum, the bank may not pay interest for that period, even if you had the minimum earlier in the month. Check your account disclosure to see whether your bank has this rule.

How interest rates change and what that means for your earnings

Banks set their own interest rates and change them whenever they want. Most savings account rates move up or down when the Federal Reserve changes its benchmark interest rate, but the timing and amount of change is up to each bank. Some banks raise rates quickly when the Fed raises; others lag behind. Some cut rates slowly when the Fed cuts; others cut when ready.

When a bank changes your rate, the change applies to new interest earned going forward, not to interest you already received. If you earned $50 in interest at 4% APY and the bank cuts the rate to 3%, you keep the $50. Future interest is calculated at the new 3% rate.

Banks must notify you of a rate change before it takes effect. The notice usually comes by mail or email and includes the new rate and the date it starts. If you disagree with a rate cut, you can move your money to a different bank—there is no penalty for closing a savings account.

How to calculate your own interest to verify the bank's math

You can calculate straightforward interest using this formula: Balance × APY ÷ 365 × Number of Days. If you have $10,000 at 4% APY and want to know how much interest you earn in 30 days, the math is $10,000 × 0.04 ÷ 365 × 30 = $32.88.

This formula gives you an estimate, not an exact number, because it assumes your balance stays the same and does not account for the exact way your bank compounds interest. But it is close enough to spot a major error. If the bank credits you $5 in interest when the formula says you should earn $30, something is wrong and you should call and ask.

Most banks provide an interest calculator on their website where you can enter your balance and see what you would earn at their current rate. This is a faster way to compare banks or to estimate what you will earn before you move money.

Frequently Asked Questions

Does my interest get taxed?

Yes. Interest earned on a savings account is taxable income. Banks report interest of $10 or more to the IRS on a Form 1099-INT, and you must report it on your tax return. The bank does not withhold taxes automatically; you pay when you file.

What if I withdraw money before the interest is credited?

You still earn the interest for the days you held the money. Interest accrues daily and is credited (added to your account) on a schedule set by the bank, usually monthly. You do not have to keep the money in the account until the interest is credited to keep what you earned.

Can a bank change my interest rate without telling me?

No. Banks must notify you of any rate change before it takes effect. The notification usually comes by mail or email. You have the right to close the account if you disagree with a rate cut, and there is no penalty for doing so.

Why do different banks offer different interest rates on savings accounts?

Banks set their own rates based on their costs, competition, and business strategy. Online banks often offer higher rates than brick-and-mortar banks because they have lower overhead. Rates also depend on how much money you deposit and how long you commit to keeping it there.

Is compound interest the same as earning interest on interest?

Yes. Compound interest means the bank adds earned interest to your balance, and then calculates the next period's interest on that larger balance. You are earning interest on the interest you already received, which is why compound interest grows faster than straightforward interest.