The basic formula for savings account interest
To calculate how much interest your savings account will earn, you need three numbers: the amount you have saved (called the principal), the interest rate your bank pays (the APY, or annual percentage yield), and how long the money sits in the account. The simplest calculation assumes the money stays untouched for one full year.
The formula is: Interest earned = Principal × APY ÷ 100. If you have $5,000 in an account earning 4.5% APY, you multiply $5,000 by 4.5, then divide by 100. That gives you $225 in interest over one year. Your account balance would grow to $5,225.
This works because APY is already expressed as an annual rate. You do not need to convert it further. The bank has already factored in how often interest compounds (daily, monthly, or quarterly) when it quotes the APY to you.
Key Takeaways
- Interest earned equals your account balance multiplied by the APY, divided by 100, for one full year.
- APY already includes the effect of compounding, so you do not need to calculate compound interest separately unless you want to know earnings for a partial year.
- For partial years, divide the APY by 12 (for months) or 365 (for days) before multiplying by your principal.
- Your actual interest may be slightly lower if you withdraw money during the year, because interest is calculated on your daily balance.
Calculating interest for less than one year
If you want to know how much interest you will earn in three months, or six months, or any period shorter than a year, you adjust the formula. Divide the APY by the number of periods in a year, then multiply by your principal and the number of periods you are calculating for.
For example, to find interest earned in three months at 4.5% APY on $5,000: divide 4.5 by 4 (because there are four quarters in a year), which gives 1.125%. Then multiply $5,000 by 1.125 and divide by 100. That equals $56.25 in three months.
For daily calculations, divide the APY by 365. If you want to know how much interest accrues in 30 days at 4.5% APY on $5,000: divide 4.5 by 365 (equals 0.0123%), multiply by $5,000, then multiply by 30 days. That gives roughly $18.49.
How compounding affects your actual earnings
Most savings accounts compound interest daily or monthly. This means the bank calculates interest on your balance, adds it to your account, and then calculates the next period's interest on the new, larger balance. Over time, you earn interest on your interest.
The APY your bank advertises already reflects this compounding effect. When you use the simple formula (Principal × APY ÷ 100), you are already getting the compounded result. You do not need to do a separate compound interest calculation.
However, if you want to see how your balance grows month by month with daily compounding, you would need to recalculate the balance after each compounding period. Most people do not need this level of detail—your bank statement shows the exact amount earned each month.
Why your actual interest might differ from your calculation
If you withdraw money during the year, your interest earnings will be lower than the formula predicts, because interest is calculated on your daily balance. A bank does not pay you interest on money you have already taken out. If you deposit $5,000 and earn $225 in interest over a year, but withdraw $2,000 after six months, you will earn less than $225 total.
Some accounts also have minimum balance requirements. If your balance drops below the minimum, the bank may reduce your APY or charge a fee, which lowers your net earnings. Check your account agreement to see whether your rate changes if your balance falls.
Interest rates also change. If your bank lowers the APY mid-year, your earnings for the full year will be lower than a calculation based on the original rate. Conversely, if rates rise, you earn more. Your bank will notify you of rate changes before they take effect.
Using an online calculator versus doing it by hand
Most banks and financial websites offer free savings calculators that do this math for you. You enter your principal, the APY, and how long you plan to keep the money, and the calculator shows your projected balance. These are accurate and save time, especially if you want to compare earnings across multiple accounts or time periods.
Doing the calculation by hand is useful if you want to understand how interest works or if you need a quick estimate without opening another browser tab. The formula is simple enough that most people can do it on a phone calculator in under a minute.
If you are comparing accounts, calculate the interest the same way for each one so your comparison is fair. Use the same principal amount, the same time period, and the same APY for each account.
What to do if your bank quotes interest differently
Banks must disclose the APY in writing before you open an account. This is the number you should use in your calculation. Some banks also quote an APR (annual percentage rate), which does not include compounding and will give you a lower number. Always use APY, not APR, for savings accounts.
If your bank statement shows interest earned that does not match your calculation, the most common reason is that your balance changed during the month. Your bank calculates interest on your daily balance, not your opening balance. If you deposited money mid-month or withdrew money, the interest will be lower than if you had kept the full amount the entire time.
If the difference is large or you cannot explain it, contact your bank and ask them to show you how they calculated the interest. They can walk you through the daily balance method they use.
Frequently Asked Questions
Do I need to pay taxes on savings account interest?
Yes. Interest earned on a savings account is taxable income. Your bank will send you a Form 1099-INT if you earn $10 or more in interest during the year. You report this on your tax return. The amount varies by state and your tax bracket, so consult a tax professional about your specific situation.
Why is my APY different from other banks?
Banks set their own rates based on their costs and competition. Online banks often offer higher APYs than brick-and-mortar banks because they have lower overhead. Rates change frequently, sometimes weekly. If you want the highest rate, check comparison sites regularly or set up alerts when rates change.
Does the formula change if I make regular deposits?
Yes. If you deposit money every month, you earn interest on each deposit for a different length of time. The easiest approach is to use a savings calculator that accounts for regular deposits, or ask your bank to show you the projected balance. Calculating by hand becomes tedious with multiple deposits.
What if the interest rate drops after I open the account?
Your earnings for the remainder of the year will be lower. Banks can change rates at any time, though they must notify you first. If rates drop significantly, you can move your money to a different bank offering a higher rate, though you may lose interest if you close the account early.
Can I lose money if interest rates go negative?
No. In the United States, savings account interest rates do not go negative. The worst case is a 0% APY, meaning you earn no interest but do not lose money. Your principal stays the same.