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

Interest earned on a savings account comes from a simple multiplication: your account balance times the annual percentage yield (APY) times how long your money sits in the account. If you have $1,000 in an account with a 4.5% APY and leave it there for one full year, you earn $45. The math is $1,000 × 0.045 × 1 = $45.

Most savings accounts use daily compounding, which means the bank calculates interest every single day and adds it to your balance. That accrued interest then earns interest itself the next day — a small advantage that grows over months and years. The daily compounding formula is more complex than simple multiplication, but your bank does this calculation automatically. What matters is understanding what the APY number actually represents: it already accounts for daily compounding, so you do not have to adjust it yourself.

If you want to see what you will earn before opening an account, use the bank's online calculator or ask them directly. Most banks publish their current APY on their website, and that rate is what you use in your calculation. The rate can change, so if you are planning for several years ahead, remember that future interest may be higher or lower than today's rate.

Key Takeaways

  • The basic calculation is balance × APY × time in years, so $5,000 at 4% APY for one year earns $200.
  • Daily compounding means interest is calculated every day and added to your balance, so you earn a small amount of interest on your interest.
  • The APY your bank publishes already includes the effect of daily compounding, so you do not need to adjust the rate yourself.
  • Interest earned changes if you deposit or withdraw money during the year, because the bank uses your average daily balance or ending balance depending on their method.
  • Your bank sends a 1099-INT form at tax time if you earned $10 or more in interest during the year, and you must report that income to the IRS.

How daily compounding affects your total earnings

With daily compounding, the bank adds a tiny fraction of the annual interest to your account each day. On day two, you earn interest not just on your original deposit but also on the interest from day one. This creates a snowball effect, though the difference is small in the first few months.

Here is a concrete example: $10,000 at 4% APY compounded daily earns roughly $408 over one year, not exactly $400. The extra $8 comes from compounding. Over five years, the same $10,000 grows to about $12,214 instead of $12,000 — the compounding adds $214. The longer your money stays in the account, the more noticeable the compounding effect becomes.

You do not have to calculate this yourself. When a bank advertises a 4% APY, that rate already reflects daily compounding. If you use an online savings calculator and enter the APY, the tool automatically accounts for compounding and shows you the correct final balance.

What happens when you deposit or withdraw money mid-year

Most banks calculate interest based on your average daily balance — they add up what you had in the account each day of the month, then divide by the number of days. If you deposit $5,000 on the 15th of a 30-day month and had $0 before that, your average daily balance for that month is roughly $2,500. Interest is then calculated on that $2,500, not the full $5,000.

Some banks use the ending balance method instead, where they only look at what you have on the last day of the month. This method rewards you for deposits made late in the month and penalizes withdrawals made late in the month. Ask your bank which method they use — it is usually in the account agreement or on their website.

If you make multiple deposits and withdrawals throughout the year, your total interest will be lower than if you had deposited the full amount on day one and left it untouched. The timing of each transaction matters. A $1,000 deposit made on January 1st earns interest for the full 365 days, while a $1,000 deposit made on December 1st earns interest for only 31 days.

Comparing interest across different time periods

Banks sometimes advertise promotional rates that last only a few months. If an account offers 5% APY but only for the first three months, you need to know what rate applies after that period ends. Calculate the interest for the promotional period separately, then calculate the interest for the remaining nine months at the regular rate.

Example: You deposit $10,000 in an account with 5% APY for three months, then 0.5% APY for the remaining nine months. Interest for the first three months is roughly $125 (one-quarter of $500). Interest for the remaining nine months is roughly $38 (three-quarters of $500 at the lower rate). Your total interest for the year is about $163, not $500.

When comparing accounts, always look at the APY that applies to your money for the full time you plan to keep it there. A high promotional rate that expires quickly may not beat an account with a steady, slightly lower rate.

How to report interest earnings on your taxes

If you earn $10 or more in interest during a calendar year, your bank sends you a Form 1099-INT by January 31st of the following year. This form shows the total interest you earned and goes to the IRS as well as to you. You must report this amount on your tax return, even if the bank did not send you a form (though they should if you earned $10 or more).

Interest income is taxed as ordinary income at your regular tax rate. If you earned $500 in interest and your tax bracket is 22%, you owe roughly $110 in federal tax on that interest. Some states also tax interest income. The interest is added to your other income for the year when calculating what you owe.

Keep your 1099-INT form with your tax records. If you earned interest from multiple banks, you will receive multiple 1099-INT forms, and you must report the total from all of them. If you earned less than $10 total, you still report the interest, but the bank does not send you a form.

Using online calculators to estimate your earnings

Most banks provide a savings calculator on their website. You enter your starting balance, the APY, how long you plan to keep the money there, and whether you plan to make regular deposits. The calculator shows you the projected ending balance and total interest earned.

These calculators assume the APY stays the same for the entire period. In reality, rates change, sometimes monthly. A calculator is useful for comparing accounts at a single moment in time, but do not treat the result as a may provide of what you will actually earn. If rates drop, you will earn less; if rates rise, you will earn more.

Some calculators let you enter monthly deposits. If you plan to add $100 every month to your savings account, the calculator shows you how much that habit will grow over one, five, or ten years. This is helpful for visualizing the long-term effect of regular saving, even at modest amounts.

The difference between APY and APR in savings accounts

APY (annual percentage yield) includes the effect of compounding and is what you use to calculate interest earned on savings. APR (annual percentage rate) does not include compounding and is used for loans and credit cards. Banks advertising savings accounts always show APY, not APR, so you do not have to convert between them.

If you see an APR advertised for a savings product, that is a red flag — it means the bank is not showing you the full picture of what you will earn. Stick with accounts that clearly state the APY.

Frequently Asked Questions

Do I earn interest on interest in a savings account?

Yes, through daily compounding. Each day, the bank adds a small amount of interest to your balance, and the next day you earn interest on that new, slightly higher balance. The APY your bank advertises already includes this effect, so you do not have to calculate it separately.

What if my bank changes the interest rate during the year?

You earn the old rate on your balance until the change takes effect, then the new rate applies going forward. Calculate interest for each period separately and add them together. Your bank will notify you before changing the rate, usually with at least a few days' notice.

Can I lose money if the interest rate drops?

No. Interest rates can only go down to zero; they cannot go negative in a standard savings account. Your principal (the money you deposited) is always safe. You simply earn less interest if rates fall, but you do not lose what you already earned.

Is the interest I earn the same every month?

No. Because of daily compounding, the amount of interest added each month grows slightly as your balance grows. The first month you earn less than the second month, which earns less than the third month, and so on. This is why the total interest over a year is slightly higher than one-twelfth of the annual interest multiplied by twelve.

What happens to my interest if I withdraw money before the year ends?

You keep all the interest you have already earned. If you withdraw $5,000 after six months, you receive your $5,000 plus the interest earned during those six months. You simply stop earning interest on the $5,000 once it is withdrawn.