The basic formula: multiply your balance by the rate and the time

Interest earned on a savings account is calculated by taking your account balance, multiplying it by the annual percentage yield (APY), and multiplying that result by the fraction of the year the money sat in the account. The formula is: Interest = Balance × APY × Time. If you keep $1,000 in an account with a 4.5% APY for one full year, you earn $45. If you keep it for six months, you earn $22.50.

Most banks compound interest daily or monthly, which means they calculate and add interest more frequently than once per year. When compounding happens, you earn interest on the interest already added to your account. This is why the actual amount you earn may be slightly higher than a simple calculation suggests, especially over longer periods.

The difference between simple interest and compound interest grows over time. With simple interest, you earn the same amount each period. With compound interest, each calculation includes the previous interest payments, so your earnings accelerate. Banks disclose their compounding frequency in the account terms, usually found on their website or in the account agreement you receive when you open the account.

Key Takeaways

  • Interest earned equals your balance multiplied by the APY multiplied by the time period, though most banks compound daily or monthly so the actual amount is usually slightly higher.
  • APY already includes the effect of compounding, so you do not need to calculate it separately—use the APY figure the bank provides.
  • Your balance changes throughout the month as you deposit and withdraw money, so banks calculate interest on the average daily balance or the ending balance, depending on their method.
  • You can estimate interest by dividing the APY by 12 for a monthly rate, then multiplying by your balance, though the exact amount depends on how many days are in each month.
  • Banks report interest earned on Form 1099-INT if the amount exceeds $10 in a calendar year, and that interest is taxable income.

How compounding changes the amount you earn

When a bank compounds interest daily, it divides the annual rate by 365, calculates interest on your current balance, and adds that amount to your account. The next day, it calculates interest on the new, slightly higher balance. Over a year, this daily compounding produces more interest than if the bank calculated once at year-end.

The relationship between APY and the stated interest rate matters here. Banks publish the APY specifically because it reflects compounding. If a bank offers a 4.5% APY with daily compounding, the actual annual interest rate is slightly lower—around 4.39%—but the APY accounts for the compounding effect. When you see an APY advertised, you can use that number directly in your calculation without adjusting for compounding yourself.

For example, $10,000 at 4.5% APY compounded daily earns about $461 over one year, not exactly $450. The extra $11 comes from earning interest on the interest. Over five years, the difference becomes more noticeable: compound interest produces roughly $2,432 in total earnings versus $2,250 with simple interest.

Calculating interest when your balance changes

Most people do not keep the same balance in a savings account all month. You deposit paychecks, withdraw cash, and move money between accounts. Banks handle this in two ways: they calculate interest on the average daily balance, or they calculate on the ending balance at the end of the statement period.

With the average daily balance method, the bank adds up your balance for each day of the month and divides by the number of days. If your balance was $1,000 for 15 days and $2,000 for 15 days, your average daily balance is $1,500. Interest is then calculated on $1,500. This method rewards you for deposits made early in the month.

With the ending balance method, the bank only looks at what you have on the last day of the statement period. If you had $1,000 most of the month but deposited $5,000 on day 30, interest is calculated on $6,000. This method can work in your favor if you deposit money late in the month, but it ignores the time your money actually sat in the account. Your account agreement specifies which method your bank uses.

A practical example with real numbers

Say you open a savings account on January 1 with $5,000 and the bank offers 4.5% APY compounded daily. You make no other deposits or withdrawals for the full year. Using the simple formula: $5,000 × 0.045 × 1 year = $225. But because the bank compounds daily, you actually earn about $230.68 by December 31.

Now assume you deposit an additional $2,000 on July 1. For the first six months, you earn interest on $5,000. For the second six months, you earn interest on $7,000. A rough calculation: ($5,000 × 0.045 × 0.5) + ($7,000 × 0.045 × 0.5) = $112.50 + $157.50 = $270. With daily compounding, the actual amount is closer to $273. The bank's statement at year-end will show the exact figure.

If you want to estimate monthly interest without waiting for the statement, divide the APY by 12. At 4.5% APY, that is 0.375% per month. On a $5,000 balance, monthly interest is roughly $18.75. On a $7,000 balance, it is roughly $26.25. These are approximations because months have different numbers of days, but they give you a sense of what to expect.

Where to find your interest earnings on statements and tax forms

Your bank sends a monthly or quarterly statement showing the interest earned during that period. The statement lists deposits, withdrawals, and the interest added to your account. Some banks show interest as a separate line item; others include it in the running balance. If you cannot find it, search the statement for "interest paid" or "interest earned."

At the end of each calendar year, if you earned $10 or more in interest across all your accounts at that bank, the bank sends you a Form 1099-INT. This form reports the interest to the IRS and to you. You must include this amount as income on your tax return. If you earned less than $10, the bank does not send a form, but you still owe tax on the interest if you file a return.

You can also log into your online banking portal and check your interest earnings in real time. Most banks show year-to-date interest earned in the account summary or in a dedicated interest tracker. This is useful if you want to monitor how your savings are growing before the formal statement arrives.

Why different banks pay different amounts of interest

Two banks offering the same APY will pay you the same interest on the same balance over the same time period. The difference comes from the APY itself. Online banks typically offer higher APYs than brick-and-mortar banks because they have lower overhead costs. A bank offering 4.5% APY pays significantly more than one offering 0.01% APY, even on the same $5,000 balance.

Compounding frequency also creates small differences. A bank that compounds daily pays slightly more than one that compounds monthly, all else equal. However, the difference is usually less than a dollar per year on typical savings account balances. The APY already reflects the compounding method, so you do not need to adjust for it when comparing banks.

Some banks offer promotional rates for new accounts or for maintaining a minimum balance. These rates are temporary and revert to a lower standard rate after the promotion ends. Always check the fine print to see when a promotional rate expires and what the regular APY will be after that.

Tools and methods to track your interest over time

A simple spreadsheet can track your interest earnings month by month. Create columns for the date, your balance, the APY, and the interest earned that month. Multiply balance by APY divided by 12 to estimate monthly interest. Update it each time you receive a statement. Over a year, you can see how your interest grows as your balance increases.

Many online banks provide a calculator on their website where you enter your balance, the APY, and the time period, and it shows you the projected interest. These calculators assume you make no additional deposits or withdrawals, so they work best for short-term estimates. For ongoing tracking, your monthly statements are more accurate because they account for your actual deposits and withdrawals.

If you are saving toward a specific goal and want to know how long it will take, you can rearrange the formula to solve for time. If you want to earn $500 in interest at 4.5% APY on a $10,000 balance, divide $500 by ($10,000 × 0.045) to get 1.11 years, or about 13 months. This helps you set realistic expectations for how much your savings will grow.

Frequently Asked Questions

Does interest compound on weekends and holidays?

Yes. Banks calculate and compound interest every day, including weekends and holidays. The calendar day count does not stop. However, the interest is not posted to your account until the next business day, so you will not see it reflected in your balance until then.

What happens to my interest if I withdraw money mid-month?

It depends on whether your bank uses average daily balance or ending balance. With average daily balance, you earn interest on the money for the days it was in the account. With ending balance, you earn nothing on money you withdrew. Check your account agreement or call the bank to confirm which method applies to you.

Can I lose money if interest rates drop?

No. Interest rates dropping means you will earn less interest going forward, but you will not lose the interest you already earned or your principal balance. If you locked in a rate with a certificate of deposit (CD), that rate stays the same for the full term, even if the bank's rates drop.

Is the interest I earn taxable?

Yes. Interest earned on a savings account is taxable income. If you earn $10 or more in a calendar year, the bank sends you a Form 1099-INT, and you report that amount on your tax return. Even if you earn less than $10, you still owe tax on it if you file a return.

How do I know if my bank is calculating interest correctly?

Check your statement against the formula: balance × APY × time. Your bank's calculation should be close to this, with small differences due to daily compounding and the exact number of days in each month. If the interest seems significantly lower than expected, contact the bank and ask them to explain their calculation.