The basic formula: multiply your daily balance by the rate, then by the number of days

To calculate annual interest on a savings account, you need three numbers: your account balance, the annual percentage yield (APY) your bank publishes, and the number of days in the year. The formula is: Balance × APY ÷ 365 = Annual Interest Earned.

Most banks compound interest daily or monthly, which means they calculate interest on your balance and then add that interest back into your account, so the next calculation includes the new total. When a bank publishes an APY, that rate already accounts for compounding—so you can use it directly without adjusting for how often the bank compounds.

Example: You have $5,000 in a savings account with a 4.5% APY. The calculation is $5,000 × 0.045 ÷ 365 = $0.62 per day, or roughly $226 per year. If your balance changes during the year, the interest earned on each portion depends on how long that money sat in the account.

Key Takeaways

  • The annual interest formula is Balance × APY ÷ 365; the APY your bank shows already includes the effect of compounding.
  • Banks calculate interest daily but may credit it monthly, so your actual balance grows in steps rather than smoothly.
  • If your balance changes, only the money that was in the account for the full year earns the full annual amount; deposits and withdrawals change the calculation.
  • The difference between APY and APR matters: APY includes compounding, APR does not, so always use APY for savings accounts.
  • Your bank's monthly statement shows interest actually credited; comparing that to your own calculation reveals whether the bank's math matches the published rate.

Why APY is not the same as the interest rate your bank quotes

Banks publish two different rates: APR (annual percentage rate) and APY (annual percentage yield). APR is the raw interest rate without compounding. APY is the rate you actually earn after the bank compounds interest—adds earned interest back into your balance so you earn interest on the interest.

For savings accounts, always use APY in your calculation. If a bank quotes only APR, you can convert it to APY using the formula: APY = (1 + APR ÷ compounding periods) ^ compounding periods − 1. But most banks publish APY directly on their website and statements, so you rarely need to do this conversion yourself.

The difference between APR and APY grows larger as the rate rises and as compounding happens more often. At a 0.01% APR with daily compounding, the APY is nearly identical. At a 5% APR with daily compounding, the APY is about 5.13%—a meaningful gap if you have a large balance.

How to account for deposits and withdrawals during the year

If your balance stays the same all year, the calculation is straightforward. But most people deposit or withdraw money, which changes how much interest they earn. Banks handle this by calculating interest on the actual balance each day, then adding those daily amounts together.

To do this yourself, track your balance at the end of each day (or each statement period if daily tracking is impractical). Multiply each daily balance by the APY, divide by 365, and add all the daily interest amounts together. This is tedious by hand but is exactly what your bank's system does automatically.

A simpler approximation: calculate the average balance across the year, then use the basic formula. If you started with $5,000, added $1,000 in June, and withdrew $2,000 in September, your average balance is roughly $4,667. Multiply that by the APY and divide by 365 to estimate annual interest. This will not match your bank's exact calculation but gives you a ballpark figure to check against your statement.

What happens when the bank changes the interest rate mid-year

Banks change savings rates frequently, sometimes weekly. When the rate changes, your interest calculation splits into two periods: the old rate applies to the days before the change, and the new rate applies to the days after.

Example: Your account earns 4.5% APY for the first six months, then the bank drops the rate to 3.0% APY for the remaining six months. Calculate interest for each period separately: (Balance × 0.045 ÷ 365 × 182 days) + (Balance × 0.030 ÷ 365 × 183 days). Your bank does this automatically, but knowing how it works helps you understand why your annual interest does not match a simple calculation using only the current rate.

If you are shopping for a savings account, ask whether the rate is may provide for a set period or can change at any time. High-yield savings accounts often advertise a rate but reserve the right to lower it without notice, so the interest you earn in month one may not match month twelve.

Reading your bank statement to verify the calculation

Your monthly or quarterly statement shows the interest actually credited to your account. Add up all the interest deposits for the year to see your total annual interest earned. Compare this to your own calculation using the formula above.

Small differences (a few cents) are normal and usually come from rounding or the exact number of days the bank counts in each period. Larger gaps—more than a dollar or two on a typical balance—suggest either a math error on your part, a rate change you missed, or (rarely) an error by the bank.

If you spot a discrepancy, contact your bank with your statement in hand. Ask them to walk you through the calculation for one month. Banks are required to calculate interest correctly, and most will explain the math if you ask. If they cannot justify the difference, ask them to review the account for errors.

The difference between simple and compound interest

Simple interest is calculated only on your original balance; earned interest does not earn interest. Compound interest is calculated on your balance plus all previously earned interest, so you earn returns on your returns.

Savings accounts use compound interest, which is why the APY (which includes compounding) is always higher than the APR (which does not). The more often interest compounds, the higher the effective return. Daily compounding beats monthly compounding, which beats annual compounding—though the difference is small at typical savings rates.

Over a long period, compounding creates a noticeable gap. On $10,000 at 4% APY compounded daily, you earn about $408 in the first year. In year five, you earn about $488 on the same account, even though the rate has not changed, because you are earning interest on a larger balance. This is why starting early and leaving money untouched matters for savings.

Using online calculators versus doing the math yourself

Most banks and financial websites offer savings calculators that do the formula for you. You enter your balance, the APY, and how long you plan to keep the money, and the calculator returns the projected interest. These are accurate as long as you enter the correct APY and assume the rate does not change.

Doing the calculation yourself—even roughly—teaches you how interest actually works and helps you spot when a bank's published rate does not match what you actually earn. A five-minute manual calculation once a year is worth the insight, especially if you are comparing accounts or trying to decide whether a higher-rate account is worth switching to.

If you use a calculator, verify that it asks for APY, not APR. Some calculators ask for the compounding frequency separately, which means they are calculating APY from an APR you provide—make sure you have the right number going in.

Frequently Asked Questions

Do I use 365 or 366 days in the calculation for a leap year?

Banks typically use 365 days even in leap years, because the APY is an annualized rate designed to be consistent year to year. Some banks use 360 days as a standard, which slightly inflates the interest earned. Check your account agreement or ask your bank which method they use; the difference is usually less than a dollar on typical balances.

What if my bank compounds interest monthly instead of daily?

If your bank publishes an APY, use that rate directly regardless of compounding frequency. The APY already reflects how often compounding happens. If you only have the APR and know compounding is monthly, divide the APR by 12 to get the monthly rate, then use the compound interest formula: APY = (1 + monthly rate)^12 − 1.

Why does my actual interest earned not match my calculation?

The most common reason is a rate change you did not account for. Check your statements for the exact dates the rate changed and recalculate using the old rate for the first period and the new rate for the second. Deposits and withdrawals also change the calculation; only money in the account for the full period earns the full annual amount.

Can I use the APY to calculate interest for just three months?

Yes. The APY is an annual rate, so divide it by 4 to get the approximate quarterly rate, or divide by 12 for a monthly rate. Multiply your balance by that fraction and divide by 365 (or use 90 days for a quarter, 30 for a month). This is approximate because it does not account for compounding within the shorter period, but it is close enough for planning.

Does the interest I earn count as income for taxes?

Yes. Interest earned on 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. This is why the after-tax return on a savings account is lower than the APY suggests—the interest is taxed at your ordinary income rate.