Banks use your account balance, the interest rate they set, and the number of days in the period to calculate what you earn

Most banks calculate savings account interest using one of two methods: simple interest or compound interest. Simple interest is straightforward—the bank multiplies your balance by the annual rate and divides by the number of days in a year. Compound interest is more common and works in your favor: the bank calculates interest on your balance plus any interest you've already earned, then adds that to your account. The more often interest compounds (daily, monthly, or quarterly), the more you earn.

The actual dollar amount you receive depends on three things: your account balance, the annual percentage yield (APY) the bank offers, and how often the bank compounds interest. A $10,000 balance at 4.5% APY compounded daily will earn more than the same balance at 4.5% compounded monthly, even though the stated rate is identical. Banks are required to disclose their compounding frequency and APY in writing before you open the account.

Key Takeaways

  • Simple interest multiplies your balance by the annual rate; compound interest adds earned interest back into your balance so you earn interest on interest.
  • The annual percentage yield (APY) already accounts for how often interest compounds, so comparing APY between accounts tells you which will actually earn more.
  • Interest is usually calculated daily but posted to your account monthly, quarterly, or annually depending on the bank's schedule.
  • Your balance on the day interest is calculated matters—deposits made mid-month may not earn interest until the next period, and withdrawals reduce what you earn.

How simple interest works and why banks rarely use it

Simple interest is the easiest method to understand. The bank takes your balance, multiplies it by the annual interest rate, and divides by 365 (or sometimes 360, depending on the bank's method). If you have $5,000 in an account earning 3% simple interest, you earn $150 per year, or about $12.50 per month.

Almost no retail banks use simple interest anymore because compound interest earns them more money and keeps customers' balances growing faster. You will see simple interest mainly in certificates of deposit (CDs) from very small banks or credit unions, and in some older savings accounts. When you see it, the math is transparent and easy to verify on paper.

Compound interest: how interest earns interest

Compound interest means the bank calculates interest on your balance plus any interest already in the account. After each compounding period, that new interest is added to your balance, and the next calculation includes it. Over time, this creates a snowball effect—your money grows faster than it would with simple interest alone.

The compounding frequency matters. If a bank compounds daily, it calculates interest 365 times per year and adds it to your account each time. If it compounds monthly, it does this 12 times. Daily compounding earns you more than monthly compounding at the same stated rate, because you earn interest on interest more often. A $10,000 balance at 4% compounded daily will earn roughly $408 in a year; the same balance at 4% compounded monthly will earn roughly $407. The difference is small in this case, but it grows with larger balances and higher rates.

Annual Percentage Yield (APY) already includes compounding

Banks are required to show you the Annual Percentage Yield (APY), not just the interest rate. The APY is the actual return you will earn in a year after accounting for how often interest compounds. This makes it easy to compare accounts: if one bank offers 4.5% APY and another offers 4.4% APY, the first one will earn you more money, period. You do not have to do any math yourself.

The difference between the stated interest rate and the APY shows the effect of compounding. A bank might advertise "4.50% interest rate, 4.60% APY." The 0.10% difference is what you gain from compounding. The higher the compounding frequency, the larger the gap between the rate and the APY.

When interest is calculated versus when it appears in your account

Banks usually calculate interest daily but post it to your account on a different schedule. Your bank might calculate interest every single day based on your balance that day, but only add the total to your account once a month, once a quarter, or once a year. This matters if you are watching your balance closely or planning a withdrawal.

Interest is typically calculated on the balance you hold at the end of each day. If you deposit $5,000 on the 15th of the month, that $5,000 starts earning interest immediately (or the next business day). If you withdraw $2,000 on the 20th, the interest calculation for that day uses the lower balance. The bank then sums all the daily interest earned during the month and posts it once, usually on the last day of the month or the first day of the next month.

How your balance affects what you earn

The higher your balance, the more interest you earn—this is obvious, but the timing of deposits and withdrawals changes the calculation. If you keep $10,000 in the account all month, that full amount earns interest every day. If you deposit $10,000 on the last day of the month, it only earns interest for one day before the month ends and interest is posted.

Some banks use the "average daily balance" method instead of the ending balance. They add up your balance at the end of each day during the month and divide by the number of days. This smooths out the effect of large deposits or withdrawals mid-month. A bank using this method will pay you interest on a portion of a deposit even if you only held it for part of the month. Always ask your bank which method they use—it is in the account disclosure document they give you when you open the account.

Why different banks pay different amounts on the same balance

Two banks might both offer savings accounts, but one pays significantly more interest than the other. The reasons are the bank's cost structure, competition, and business model. Online banks typically pay higher APY because they have lower overhead costs than brick-and-mortar branches. Large national banks often pay lower rates because they have more customers and do not need to compete as aggressively for deposits.

The interest rate environment also matters. When the Federal Reserve raises its benchmark rate, banks gradually increase the APY they offer on savings accounts. When rates fall, banks lower their rates. Your existing account rate may not change immediately when the Fed moves—some banks adjust quickly, others lag by weeks or months. If you have held an account for years at a low rate, you may be earning significantly less than new customers at the same bank.

Frequently Asked Questions

Does my bank round down the interest I earn?

Banks calculate interest to the penny and post the full amount. They do not round down or keep the fractional cents. If your daily interest calculation results in $0.47 for the month, you receive $0.47. Some banks display interest in your online portal rounded to the nearest cent, but the actual deposit is precise.

If I withdraw money mid-month, do I lose all the interest I earned?

No. Interest is calculated on your balance each day, so you keep the interest earned on the days you held the money. If you had $5,000 for 20 days and $3,000 for 10 days, you earn interest on both amounts for their respective periods. The bank sums the daily interest and posts it at the end of the month.

Why does my bank show a different APY than the rate advertised on their website?

The rate on the website is usually current as of that day, but your account may have been opened when rates were different. Banks are not required to change the rate on existing accounts when they change the rate for new customers. If you opened your account six months ago at 3.5% APY and the bank now advertises 4.5% APY, your rate stays at 3.5% unless you close the account and open a new one.

Can I predict exactly how much interest I will earn?

You can estimate it, but not predict it exactly, because your balance will likely change during the month. Use your bank's interest calculator (most banks have one on their website) or multiply your average expected balance by the APY and divide by 12 for a monthly estimate. The actual amount will be close but not exact unless your balance never changes.