How banks calculate the interest you earn

Banks use one of two methods to calculate interest on your savings: simple interest or compound interest. Most savings accounts use compound interest, which means you earn interest on the money you deposited plus interest on the interest you've already earned. This compounds at intervals set by the bank — daily, monthly, quarterly, or annually — and the more often it compounds, the more you earn.

The calculation starts with three numbers: your account balance, the annual interest rate (called APY or Annual Percentage Yield), and how often the bank compounds. Banks divide the annual rate by the number of compounding periods in a year, apply that smaller rate to your balance, add the result back to your account, then repeat the process. Over time, even small rates produce noticeable growth because each compounding period includes the previous period's earnings.

You don't have to do this math yourself. Your bank statement shows the interest posted to your account each month or quarter. But understanding the formula helps you compare accounts and predict what you'll earn over time.

Key Takeaways

  • Compound interest means you earn interest on your interest, and the frequency of compounding (daily, monthly, quarterly) affects your total earnings.
  • APY (Annual Percentage Yield) is the rate banks advertise and is the most useful number for comparing accounts, because it already accounts for compounding.
  • The simple formula is: Interest = Principal × (APY ÷ Number of Compounding Periods) × Number of Periods Elapsed.
  • Daily compounding produces slightly more earnings than monthly or quarterly compounding on the same balance and rate.
  • Your bank posts earned interest to your account automatically; you can verify the amount by checking your statement against the bank's disclosed APY.

The difference between APY and interest rate

Banks list two numbers: the interest rate and the APY. The interest rate is the raw percentage the bank pays on your balance. The APY is the rate you actually earn after compounding is factored in. APY is always equal to or higher than the interest rate because it includes the effect of earning interest on interest.

For example, a bank might advertise 4.50% APY. That APY already includes compounding. If you deposit $10,000 and leave it untouched for one year, you'll earn roughly $450 in interest (the exact amount depends on the compounding frequency and the exact number of days in the year). You should always compare accounts using APY, not the raw interest rate, because APY tells you the true return.

Simple interest versus compound interest

Simple interest is straightforward but rare in savings accounts. You earn interest only on the money you originally deposited, not on previous interest. The formula is: Interest = Principal × Rate × Time. If you deposit $5,000 at 2% simple interest for one year, you earn $100 and end with $5,100. The next year, you earn another $100 on the original $5,000, not on the $5,100.

Compound interest is what most savings accounts use. You earn interest on your principal and on all previously earned interest. Using the same $5,000 at 2% compounded annually, you earn $100 in year one (ending with $5,100). In year two, you earn interest on $5,100, not just $5,000, so you earn $102. The difference grows larger over time, especially with higher rates or more frequent compounding.

How compounding frequency affects your earnings

Banks compound interest at different intervals. Daily compounding is most common for savings accounts, followed by monthly and quarterly. The more frequently interest compounds, the more you earn, because each compounding period adds earned interest back into the balance before the next period's calculation.

The difference between daily and monthly compounding on a typical savings account is small but real. On a $10,000 balance at 4.50% APY, daily compounding might earn you roughly $450 per year, while monthly compounding might earn $449. Over five years, that small difference compounds into noticeably more money. Your bank's disclosure documents (usually the Truth in Savings Act form) state the compounding frequency, so you can compare before opening an account.

The formula you can use to check your earnings

If you want to calculate interest yourself, use this formula for compound interest:

A = P(1 + r/n)^(nt)

Where A is your final amount, P is your principal (starting balance), r is the annual interest rate as a decimal (4.50% becomes 0.045), n is the number of times interest compounds per year, and t is the number of years. Subtract P from A to get the interest earned.

Example: $10,000 principal, 4.50% APY, compounded daily (n=365), for one year. A = 10,000(1 + 0.045/365)^(365×1) = $10,460.17. Interest earned = $460.17. Most online calculators do this for you — you enter the principal, rate, and time, and they show the result. Your bank's website usually has one built in.

Why your actual earnings might differ from the calculation

Several real-world factors can change what you actually earn. If you deposit money partway through the month, the bank may not pay interest on that deposit until the next compounding period. If you withdraw money, you lose interest on that amount going forward. Some banks use a method called "average daily balance," which calculates interest based on your average balance over the period rather than your ending balance.

Banks also use different day-count methods. Some count 365 days per year; others count 360. Some use the exact number of days in each month; others use 30 days per month. These differences are small but real. Your bank's Truth in Savings disclosure explains which method it uses. If you want to verify your interest earnings, compare the amount posted to your statement against the APY your bank advertises — the posted amount should be close to what the formula predicts.

How to track interest earnings on your statement

Your bank statement lists interest posted to your account each period (usually monthly). Look for a line item labeled "Interest Paid" or "Interest Earned." This is the actual money the bank added to your account based on your balance and the APY during that period. Over time, these deposits compound — each month's interest becomes part of your balance for the next month's calculation.

If you notice the interest posted is significantly lower than you expected, check three things: the APY your bank is currently offering (rates change), whether you made deposits or withdrawals during the period (which reduces the average balance), and the compounding frequency. If the posted interest is still much lower than the formula predicts, contact your bank to ask how they calculated it.

Frequently Asked Questions

Do I have to do anything to earn interest on my savings account?

No. Interest accrues automatically based on your balance and the bank's APY. You don't need to take any action. The bank calculates it, compounds it, and posts it to your account on their schedule. You only need to keep money in the account and let time pass.

What happens to interest if I withdraw money before the end of the month?

Interest is calculated on your balance during the compounding period. If you withdraw money partway through, you lose interest on that withdrawn amount for the remainder of the period. Some banks calculate interest on your average daily balance, so a mid-month withdrawal reduces the average and lowers the interest posted that period.

Why does one bank's 4.50% APY earn more than another bank's 4.50% APY?

If both banks truly offer 4.50% APY, the earnings should be nearly identical over a year. Small differences may come from different day-count methods or the exact timing of deposits and withdrawals. If the difference is large, verify that both banks are quoting APY, not just the interest rate, and check whether one compounds more frequently than the other.

Can interest rates change after I open an account?

Yes. Banks can raise or lower the APY on savings accounts at any time. Most banks notify you before a rate drop. If rates fall, your earnings decrease. If rates rise, your earnings increase. You can move your money to a different bank if you find a better rate, though you'll lose any interest that hasn't been posted yet.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on savings accounts is taxable income. Banks 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 amount is small for most savings accounts, but it still counts as income.