Interest is calculated on your balance, compounded at intervals your bank sets

Banks calculate savings account interest by taking your account balance, multiplying it by the annual interest rate, and dividing by the number of times per year they compound. Compounding means the bank adds earned interest back into your account, and then calculates next period's interest on that larger balance. Most banks compound daily or monthly, which means your interest earns interest.

The actual dollar amount you receive depends on three things: how much money sits in your account, what annual percentage yield (APY) the bank offers, and how often the bank compounds. A bank advertising 4.50% APY compounds that rate according to its schedule—usually daily—so you earn slightly more than 4.50% would suggest if it were straightforward interest paid once a year.

The timing matters because interest accrues (builds up) continuously but posts (actually appears in your account) on the bank's schedule. You might earn interest daily but see it credited monthly. Withdrawals reduce your balance when ready, so pulling money out mid-month lowers the balance used to calculate that month's interest.

Key Takeaways

  • Banks multiply your balance by the APY and divide by the compounding frequency to calculate how much interest you earn each period.
  • Compounding means interest earned gets added back to your balance, so the next calculation includes that interest as part of the principal.
  • Daily compounding earns you more than monthly compounding at the same APY because interest accrues more frequently.
  • Withdrawals reduce your balance when ready, lowering the amount the bank uses to calculate interest for that period.
  • The bank's compounding schedule (daily, monthly, quarterly) is set in the account agreement and does not change based on your balance.

The basic formula: balance times rate divided by time

The standard interest calculation is: Interest = (Balance × APY) ÷ Number of Compounding Periods. If you have $10,000 in an account earning 4.50% APY compounded daily, the bank divides 4.50% by 365 days to get a daily rate of about 0.0123%. It then multiplies $10,000 by 0.000123 to calculate that day's interest: about $1.23. The next day, if your balance is still $10,000, it calculates interest on $10,000 again—but if you made a deposit, the new balance is used instead.

This is why the compounding frequency matters. A $10,000 balance at 4.50% APY compounded daily earns roughly $450 per year. The same balance at the same rate compounded monthly earns slightly less because interest accrues fewer times. The difference is small—perhaps $2 to $5 per year on a $10,000 balance—but it compounds over time, especially on larger balances.

Banks do not always use 365 days in their calculation. Some use 360 days (called the "ordinary interest" method), which slightly increases the interest you earn. Your account agreement specifies which method the bank uses, though most online banks and large institutions use 365.

How compounding multiplies your money over time

Compounding is powerful because earned interest becomes part of your principal. In month one, you earn interest on your original deposit. In month two, you earn interest on your original deposit plus the interest from month one. This creates exponential growth rather than linear growth.

On a $10,000 balance at 4.50% APY compounded daily, after one year you would have approximately $10,460. After five years, approximately $12,400. After ten years, approximately $15,400. The longer the money sits, the more the compounding effect accelerates. This is why banks advertise APY rather than a straightforward annual rate—APY reflects the actual return you receive after compounding is factored in.

The difference between daily and monthly compounding becomes visible over years, not months. On $50,000 at 4.50% APY, daily compounding earns roughly $25 more per year than monthly compounding. Over ten years, that difference grows to several hundred dollars because the extra interest itself compounds.

When interest posts versus when it accrues

Interest accrues (accumulates) on the bank's schedule, usually daily, but it posts (actually credits to your account) on a different schedule, often monthly. This distinction matters if you withdraw money mid-month. If your bank accrues interest daily but posts monthly, and you withdraw $5,000 on the 15th of the month, the bank calculates interest for days 1–15 using your full balance, then calculates days 16–31 using the reduced balance. You do not lose accrued interest—it still posts at month-end—but future accrual uses the lower balance.

Some banks post interest on the last day of the month; others post on the first day of the following month. Check your account agreement or online banking portal to see your bank's schedule. The posting date is when the interest officially becomes part of your balance and starts earning interest itself in the next compounding cycle.

If you close an account mid-month, the bank typically posts accrued interest before closing. You receive the interest earned up to the closing date, even if the regular posting date has not arrived yet.

How your balance changes the interest you earn

Interest is calculated on your daily balance or average daily balance, depending on the bank's method. Daily balance means the bank uses whatever balance you have on each day to calculate that day's interest. Average daily balance means the bank adds up your balance for each day of the month and divides by the number of days, then uses that average to calculate the month's interest.

Most savings accounts use daily balance, which benefits you if your balance fluctuates. If you have $20,000 for 20 days and $5,000 for 10 days, daily balance calculates interest on $20,000 for 20 days and $5,000 for 10 days separately. Average daily balance would calculate interest on the average of those balances, which would be lower. Online banks almost always use daily balance because it is simpler to track and more transparent.

Deposits increase your balance when ready, so interest accrues on the new amount starting the next day (or the same day, depending on the bank's posting time). Withdrawals reduce your balance when ready. If you withdraw on a Friday, interest accrues on the reduced balance starting Saturday, even if the withdrawal does not post until Monday.

Why different banks offer different rates on the same product

Banks set their own APY based on market conditions, their funding costs, and competition. Online banks typically offer higher rates than brick-and-mortar banks because they have lower overhead. A savings account at a large national bank might offer 0.01% APY while an online bank offers 4.50% APY on the same account type. Over a year, on $10,000, that is a difference of $1 versus $450.

The APY is fixed for the account type but can change at any time. Banks usually notify customers of rate changes by email or through the online banking portal. A rate decrease might happen within days; a rate increase might take weeks. Your account agreement specifies how much notice the bank must give, but there is no federal minimum—some banks change rates when ready.

The rate you see advertised is the rate new customers receive. Existing customers sometimes receive a different rate, especially if they opened their account years ago. Check your account statement or login to your bank's website to see your actual current rate, not the advertised rate.

What happens to interest if you withdraw money early

Savings accounts have no penalty for withdrawals, so you keep all interest earned up to the withdrawal date. If you withdraw $5,000 on the 15th of the month and the bank posts interest on the 30th, you receive interest on your full balance through the 14th, then on the reduced balance from the 15th onward. The interest already accrued is yours; you do not forfeit it.

This is different from certificates of deposit (CDs), which charge an early withdrawal penalty if you take money out before the term ends. Savings accounts impose no such penalty. You can withdraw any amount at any time and keep every cent of interest earned.

If you move money to a different bank, the old bank calculates and posts accrued interest before the account closes. You receive that interest in the form of a final deposit or a check, depending on the bank's process.

Frequently Asked Questions

Does my interest rate stay the same forever?

No. Banks can change savings account rates at any time without advance notice, though most provide notification by email or through online banking. Rates typically decrease when the Federal Reserve lowers its benchmark rate and increase when the Fed raises rates. Your rate is not locked in like it would be with a CD.

Why does my bank show different interest amounts each month?

Your balance changes throughout the month as you deposit and withdraw money. Interest is calculated on whatever balance you have each day, so months with higher average balances earn more interest. If you deposited a large sum mid-month, that month's interest will be higher than the previous month's.

If I earn interest on interest, does that mean I owe taxes on it?

Yes. The IRS taxes all interest earned, including interest that compounds. Your bank sends you a 1099-INT form each January showing total interest earned in the previous year. You report this on your tax return even if the interest was reinvested into the account.

What is the difference between APY and APR on a savings account?

APY (annual percentage yield) includes the effect of compounding, while APR (annual percentage rate) does not. Banks advertise APY for savings accounts because it shows the actual return you receive. APR is typically used for loans and credit cards. Always compare APY when shopping for savings accounts.

Can I earn interest on money I just deposited?

Usually yes, but timing depends on when the deposit posts. If you deposit money and it posts the same day, interest accrues starting the next day (or the same day, depending on the bank). If the deposit posts the next day, accrual starts then. Check your bank's deposit posting times to know when interest begins.