The basic formula banks use

Banks calculate savings account interest using this formula: Interest = Principal × Annual Interest Rate × Time Period. The principal is the amount of money you have in the account. The annual interest rate is the APY (annual percentage yield) the bank advertises. The time period is how long your money stays in the account, expressed as a fraction of a year.

For example, if you have $5,000 in an account earning 4.5% APY and leave it untouched for one full year, you earn $225 in interest ($5,000 × 0.045 × 1). If you only keep the money there for six months, you earn $112.50 ($5,000 × 0.045 × 0.5).

The catch is that most banks don't calculate interest once a year. They calculate it daily or monthly, then add it to your account on a schedule. This is where compounding comes in—and it's the part that actually makes your money grow faster than the straightforward formula suggests.

Key Takeaways

  • The basic interest calculation is principal multiplied by the annual rate multiplied by the time period, but this only works if interest compounds once per year.
  • Most savings accounts compound interest daily or monthly, meaning interest gets added to your balance and then earns interest itself in the next period.
  • The APY shown on the bank's website already accounts for compounding, so you don't have to do the math yourself to compare accounts.
  • The more frequently interest compounds, the more total interest you earn on the same principal and rate, though the difference is usually small for savings accounts.
  • Your actual interest earnings depend on your lowest balance during the compounding period, not your average balance—check your bank's rules.

How compounding changes the calculation

When a bank compounds interest, it adds the interest you've earned to your principal, and then the next calculation includes that new, larger balance. This means you earn interest on your interest. Over time, this creates exponential growth instead of linear growth.

If your bank compounds monthly, it divides the annual rate by 12, calculates interest on your current balance, adds it to the account, and repeats the next month with the new balance. If it compounds daily, it does this 365 times per year. The formula for compounded interest is: Final Amount = Principal × (1 + Rate/Compounding Periods)^(Compounding Periods × Years).

Using the same $5,000 example at 4.5% APY for one year: with daily compounding, you'd earn about $230.89 instead of $225. The difference seems small, but it grows larger with bigger balances and longer time periods. With $50,000 over five years, daily compounding versus annual compounding adds up to several hundred dollars in extra earnings.

Why the APY number matters more than the interest rate

Banks are required to show you the APY, not just the interest rate, because APY already includes the effect of compounding. When you see "4.5% APY" on a savings account, that number already reflects how often the bank compounds your interest. You don't need to do any math to compare two accounts—the higher APY wins.

The interest rate (sometimes called the nominal rate) is the raw percentage before compounding is factored in. A bank might advertise "4.45% interest rate, 4.5% APY"—the difference is compounding. The APY is what you actually earn.

This is why you should always look at APY when comparing savings accounts, not the advertised interest rate. Two banks might show different rates but the same APY, or vice versa, depending on how often they compound.

What happens when you deposit or withdraw money mid-period

Most banks use the daily balance method to handle deposits and withdrawals. They calculate interest based on your balance at the end of each day, then compound it on a set schedule (usually monthly or daily). If you deposit $2,000 on the 15th of the month, that $2,000 only earns interest from the 15th onward, not for the whole month.

Some banks use the average daily balance method, which adds up your balance at the end of each day during the month and divides by the number of days. This smooths out the effect of deposits and withdrawals. A few banks still use the minimum balance method, where you only earn interest on the lowest balance you held during the entire period—this is rare and usually a sign to move your money elsewhere.

Check your bank's disclosure documents or account terms to see which method they use. The daily balance method is most common and usually works in your favor if you're making regular deposits.

How to calculate your actual earnings

If you want to know exactly how much interest you'll earn without waiting for your statement, use the APY and the daily balance method. Multiply your current balance by the APY, then multiply by the fraction of the year that has passed. For a balance of $10,000 at 4.5% APY after three months: $10,000 × 0.045 × (3/12) = $112.50.

This gives you an estimate. The actual amount will be slightly different because your balance probably changed during those three months, and the bank compounds on a specific schedule you may not know the exact timing of. But it's close enough to plan with.

Most online banks show you a running interest total in your account dashboard. You can also call your bank or log into your account to see how much interest you've earned year-to-date. This is the most reliable way to track your actual earnings without doing the math yourself.

The difference between savings accounts and money market accounts

Money market accounts use the same interest calculation as savings accounts—principal, rate, time, and compounding. The difference is that money market accounts often pay higher APY in exchange for requiring a larger minimum balance (often $2,500 or more) and limiting how many withdrawals you can make per month.

Certificates of deposit (CDs) also use the same formula, but the rate is fixed for the entire term (three months, one year, five years, etc.), and you pay a penalty if you withdraw early. The interest calculation itself works the same way: the bank compounds on a schedule and pays you the total when the CD matures.

For the purposes of calculating interest, treat all three the same way: look at the APY, multiply by your principal, and account for how long your money will actually stay in the account.

Common mistakes people make when calculating interest

The most common mistake is using the advertised interest rate instead of the APY. A 4.45% rate might become 4.5% APY after compounding, and that 0.05% difference costs you money if you're comparing accounts. Always use APY.

The second mistake is assuming interest compounds once a year. If you calculate interest as $5,000 × 0.045 × 1 and expect $225, but the bank compounds daily, you'll actually earn about $230.89. The difference is small on small balances but meaningful on larger ones.

The third mistake is not accounting for deposits and withdrawals. If you deposit money mid-month, that deposit doesn't earn a full month's interest. If you withdraw money, you lose interest on that amount for the rest of the period. Banks handle this automatically, but if you're trying to predict your earnings, you need to account for the timing of your transactions.

Frequently Asked Questions

Does my interest get taxed?

Yes. Interest earned in 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. You report this on your tax return. The interest is taxed at your ordinary income tax rate, not as capital gains.

Why do some banks show different APY for different balance tiers?

Banks often offer higher APY on larger balances to encourage bigger deposits. You might see 4.0% APY on balances under $25,000 and 4.5% APY on balances of $25,000 or more. The interest calculation is the same; you just earn a higher rate on the portion of your balance that meets the threshold.

Can the APY change after I open the account?

Yes. Banks can change the APY on savings accounts at any time, usually with a few days' notice. If rates drop, your APY drops. If rates rise, your APY may or may not rise—it depends on the bank's decision. CDs lock in a fixed rate for the entire term, so the APY cannot change once you open the CD.

What's the difference between APY and APR?

APY (annual percentage yield) includes compounding and is used for savings and deposit accounts. APR (annual percentage rate) does not include compounding and is used for loans and credit cards. For savings, always look at APY. For borrowing, APR is the relevant number.

If I move my money to a different bank mid-year, do I lose the interest I earned?

No. Interest you've already earned stays in your account and transfers with you. You only lose future interest—the interest you would have earned if you'd left the money in the original account for the rest of the year. Some banks may also charge a penalty if you close the account within a certain timeframe, so check the terms before you move.