The basic formula: multiply your balance by the APY, then divide by 365

To find out how much interest you'll earn in a high-yield savings account, you need three pieces of information: your account balance, the annual percentage yield (APY), and the number of days the money sits in the account. The formula is: Balance × APY ÷ 365 = Annual Interest. If you want to know what you'll earn in a month or a week, divide that annual number by 12 (for months) or 52 (for weeks).

Most banks compound interest daily, meaning they calculate what you owe every single day and add it to your balance. This is why the daily calculation matters—you earn interest on your interest. The difference between daily compounding and monthly compounding can add up to several dollars per year on a $10,000 balance, depending on the APY.

Let's use a real example. Say you have $5,000 in an account with a 4.50% APY. Multiply $5,000 by 0.045 (the decimal form of 4.50%), which gives you $225. That's your annual interest. If you leave the money untouched for one year, you'll earn $225. If you want to know what you'll earn in three months, divide $225 by 4, which is about $56.25.

Key Takeaways

  • The basic calculation is your balance multiplied by the APY (as a decimal), divided by 365 days, then multiplied by the number of days your money stays in the account.
  • Daily compounding means the bank recalculates your interest every day and adds it to your balance, so you earn interest on interest.
  • The APY already includes the effect of compounding, so you don't need to calculate it separately—just use the APY number the bank shows you.
  • Your actual earnings will be slightly lower than the calculation if you withdraw money during the period, because the balance changes.

Why APY matters more than the interest rate

Banks sometimes show two numbers: the interest rate and the APY. The APY is the number you should use for your calculation, because it already includes compounding. The interest rate alone does not. If a bank shows you a 4.50% APY, that's the true annual return you'll get—you don't need to do anything else to account for compounding.

The difference between the two can be small (usually less than 0.05%), but it matters over time. A bank might advertise a 4.48% interest rate that compounds daily to a 4.59% APY. If you calculate using the interest rate instead of the APY, you'll underestimate what you earn. Always look for the APY on the account details page or the disclosure document the bank sends you.

Calculating interest for a partial year or month

If you're not keeping the money for a full year, the math changes slightly. The formula becomes: Balance × APY × (Number of Days ÷ 365) = Interest Earned. This tells you how much you'll earn for any time period.

For example, if you deposit $10,000 at 4.50% APY and leave it for 90 days, the calculation is: $10,000 × 0.045 × (90 ÷ 365) = $110.96. For six months (roughly 182 days): $10,000 × 0.045 × (182 ÷ 365) = $224.66.

Banks count days differently depending on their system. Some use a 360-day year instead of 365, which slightly increases your earnings. Most online banks use 365 days. Check your account agreement or ask your bank which method they use—it won't change the calculation much, but it's good to know.

What happens when your balance changes during the month

If you deposit money partway through the month or withdraw some, your interest calculation becomes more complex because the balance isn't constant. Banks handle this by calculating interest on the average daily balance—they add up what you had each day and divide by the number of days in the period.

You don't need to do this calculation yourself. Your bank does it automatically and shows you the interest earned on your statement. But if you want to estimate, add up your balance at the end of each day for the month, divide by the number of days, and use that average in the formula: Average Daily Balance × APY ÷ 365 = Monthly Interest.

For example, if you had $5,000 for 20 days and $7,000 for 10 days in a 30-day month, your average daily balance is (($5,000 × 20) + ($7,000 × 10)) ÷ 30 = $5,667. At 4.50% APY, that earns about $70.83 for the month.

The difference between stated APY and what you actually earn

Banks can change the APY at any time, and they often do. The APY you see today might be different next month. If the rate drops, your future interest earnings will be lower. If it rises, you'll earn more. Your bank will notify you of rate changes, usually by email or a notice in your account.

Also, the APY is only may provide for the period stated—usually the current month or quarter. After that period ends, the bank can offer a new rate. This is why high-yield savings accounts are different from certificates of deposit (CDs), where the rate is locked in for a specific term.

Your actual earnings will also be lower than your calculation if you withdraw money before the interest is credited. Most banks credit interest monthly, on the last day of the month or the first day of the next month. If you withdraw the day before interest posts, you lose that month's earnings.

Using a calculator to check your math

You can calculate this by hand, but most banks provide an interest calculator on their website. You enter your balance, the APY, and how long you plan to keep the money, and it shows you the projected earnings. These calculators use the same formula described here, so the results should match your manual calculation.

Spreadsheet software like Excel or Google Sheets also works well. Create a simple formula: =Balance*APY*(Days/365). This lets you test different scenarios—what if you deposit $15,000 instead of $10,000? What if the APY drops to 4.00%? You can see the impact instantly.

Frequently Asked Questions

Do I need to do this calculation myself, or does the bank do it for me?

Your bank calculates and credits the interest automatically. You don't need to do anything. The calculation is useful if you want to estimate your earnings before opening an account or compare two banks' offers.

What if the APY changes in the middle of the month?

Banks calculate interest based on the APY in effect each day. If the rate changes mid-month, they use the old rate for the days before the change and the new rate for the days after. Your statement will show the total interest earned under both rates.

Is the interest I earn taxable?

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. This is separate from calculating how much you earn—it's about what you owe the IRS.

Why do some banks show a different number than my calculation?

The most common reason is that you used a different APY than what was actually in effect during the period, or the bank uses a 360-day year instead of 365. Check your account statement to see the exact APY applied and the number of days the bank counted.

Can I earn more interest by moving money between accounts?

No. Interest is based on how long your money sits in the account, not how many times you move it. Moving money between accounts doesn't change the calculation—you still earn interest only on the balance you hold each day.