The formula is straightforward: multiply your balance by the annual rate, then divide by the number of days in a year

Interest earned on a savings account comes down to three numbers: your balance, the annual percentage yield (APY), and how long your money sits in the account. Banks calculate what you earn using a daily method, which means they work out interest on whatever balance you have each day, then add those daily amounts together.

The basic calculation is: Daily Interest = (Account Balance × APY) ÷ 365. If you have $10,000 in an account earning 4.5% APY, you earn roughly $1.23 per day. Over a month, that's about $37. Over a year, it's $450. The catch is that your balance changes—deposits add to it, withdrawals reduce it—so banks recalculate daily interest every single day.

Most banks compound interest daily or monthly, which means they add earned interest back into your account, and then you earn interest on that interest. This compounding effect is small in the short term but meaningful over years. A $10,000 deposit earning 4.5% APY compounds to $10,460 after one year, not $10,450, because of the daily compounding.

Key Takeaways

  • Daily interest is calculated by multiplying your balance by the APY and dividing by 365, then repeating this for each day your money is in the account.
  • Banks use your actual daily balance, so deposits and withdrawals change how much interest you earn that day and every day after.
  • Compounding means interest earned gets added back to your account, so you earn interest on your interest—this happens daily or monthly depending on the bank.
  • The APY shown on the bank's website already accounts for compounding, so you do not need to calculate it separately.
  • Your actual earnings will be lower than the APY suggests if you withdraw money partway through the year or if the rate changes.

How banks track your daily balance

Your bank does not calculate interest once a year. It calculates it every single day based on what you have in the account that day. If you deposit $5,000 on the 15th of the month, you earn interest on $5,000 starting the 15th. If you withdraw $2,000 on the 20th, your daily interest drops on the 20th.

Banks use one of two methods to track this: the average daily balance method or the daily balance method. Most savings accounts use the daily balance method, which is simpler. They add up the interest earned each day, then credit it to your account. If your balance is $10,000 on day one and $12,000 on day two, you earn interest on $10,000 for one day and $12,000 for one day—two separate calculations.

The timing of when interest posts matters too. Some banks add interest monthly, others daily. If your bank compounds daily but posts monthly, you are still earning interest every day—it just shows up in your account once a month. This does not change how much you earn, only when you see it.

What happens when the rate changes

Savings account rates move frequently. Your bank may raise or lower the APY, and when it does, your daily interest calculation changes when ready. If your account earns 4.5% APY and the bank drops it to 4.0%, your daily interest drops that same day.

Banks must notify you before lowering a rate, usually by email or through your online account. The notification often comes a few days before the change takes effect. If you have $10,000 earning 4.5% and the rate drops to 4.0%, your daily interest falls from $1.23 to $1.10—a difference of about $0.13 per day, or roughly $47 per year.

Rate increases work the same way in reverse. When rates rise, your daily interest rises when ready. This is why checking your account's current APY matters—the rate you opened with may not be the rate you are earning now.

Calculating interest over different time periods

If you want to know how much you will earn over a specific period—say, six months—you need to know whether your rate will stay the same. Assuming it does, the math is straightforward: multiply your balance by the APY, then multiply by the fraction of the year.

For six months: (Balance × APY) × 0.5. For three months: (Balance × APY) × 0.25. If you have $10,000 at 4.5% APY and want to know earnings over three months, the calculation is ($10,000 × 0.045) × 0.25 = $112.50. This assumes your balance stays at $10,000 and the rate does not change.

In reality, your balance probably changes. If you deposit $1,000 per month, your interest compounds on different amounts each month. Month one earns interest on roughly $10,500 (your opening balance plus half the month's deposit). Month two earns on roughly $11,500. Month three earns on roughly $12,500. The total is higher than if you had left $10,000 untouched, because you are earning interest on the deposits too.

How compounding affects your total earnings

Compounding is the reason the APY (annual percentage yield) is higher than the APR (annual percentage rate). The APY already includes the effect of compounding, so when your bank shows you 4.5% APY, that is the real rate you earn after compounding is factored in.

If a bank compounds daily, it adds a tiny bit of interest to your account each day, and tomorrow you earn interest on that interest. Over a year, this adds up. On $10,000 at 4.5% APY with daily compounding, you earn $450 in interest, and that $450 itself earns a small amount of interest. The total is $10,460.68, not $10,450.

The difference between daily and monthly compounding is small for savings accounts. Monthly compounding on $10,000 at 4.5% APY yields $10,460.41 after one year—$0.27 less than daily compounding. Over longer periods or larger balances, the gap widens slightly, but for most people the difference is negligible.

Using online calculators versus doing it yourself

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 assume your balance stays the same and your rate does not change, so they are useful for rough estimates but not precise predictions.

If you want to calculate by hand, the formula is: Final Balance = Starting Balance × (1 + APY)^(days ÷ 365). For $10,000 at 4.5% APY over one year, this is $10,000 × (1.045)^1 = $10,450. For six months, it is $10,000 × (1.045)^0.5 = $10,221.80. This method accounts for compounding automatically.

The online calculator is faster and less error-prone. Use it to compare accounts or estimate earnings. Use the formula if you want to understand exactly how the math works or if you are comparing accounts with different compounding schedules.

Why your actual earnings may differ from the estimate

The biggest reason actual earnings differ from estimates is balance changes. If you deposit money partway through the year, you earn less than the full APY on that deposit because it was not in the account for the whole year. If you withdraw money, you lose the interest you would have earned on it.

Rate changes also affect the total. If your bank lowers the rate halfway through the year, you earn the higher rate for six months and the lower rate for six months. Your total earnings are the average of the two rates, weighted by how long each rate was in effect.

Fees can reduce earnings too. Some savings accounts charge monthly maintenance fees or fees for excessive withdrawals. These fees come out of your interest earnings or your balance, lowering your net gain. Always check the fee schedule before opening an account.

Frequently Asked Questions

Do I need to do anything to earn interest, or does it happen automatically?

Interest accrues automatically. As long as your money is in the account, the bank calculates and compounds interest every day. You do not need to take any action. Interest posts to your account on a schedule set by the bank—usually monthly—but it is being earned daily.

If I withdraw money mid-month, do I lose all the interest for that month?

No. You lose interest only on the amount you withdraw, starting the day you withdraw it. If you have $10,000 on the 1st and withdraw $5,000 on the 15th, you earn interest on $10,000 for 14 days and $5,000 for the remaining days of the month. You keep the interest earned on the $10,000 for those first 14 days.

What is the difference between APY and APR for savings accounts?

APR is the annual rate before compounding. APY is the annual rate after compounding is included. Banks show you APY because it is the real rate you earn. For savings accounts, APY is always higher than APR, though the difference is small. You should always use the APY when comparing accounts.

Can I calculate interest if the rate changes during the year?

Yes, but you need to break the year into periods. Calculate interest for the months the old rate was in effect, then calculate interest for the months the new rate was in effect, and add them together. If your rate was 4.5% for six months and 4.0% for six months on $10,000, you earn roughly $225 at the first rate and $200 at the second rate, for a total of $425.

Why do different banks show different earnings on the same balance and rate?

The main reason is compounding frequency. A bank that compounds daily earns slightly more than one that compounds monthly. The difference is small—usually under $1 per year on a $10,000 balance—but it adds up over time. Always check whether the bank compounds daily or monthly when comparing accounts.