The Basic Formula for Savings Account Interest
Savings account interest is calculated using one of two methods: simple interest or compound interest. Most savings accounts use compound interest, which means the bank pays you interest on your original deposit plus any interest you've already earned. The formula is straightforward: multiply your account balance by the annual interest rate, divide by the number of times interest compounds per year, then multiply by the number of compounding periods.
The actual calculation your bank performs looks like this: A = P(1 + r/n)^(nt), where A is your final amount, P is your principal (starting balance), r is the annual interest rate as a decimal, n is how many times per year interest compounds, and t is the number of years. Most banks compound interest daily, which means they calculate and add interest to your account 365 times per year.
You don't need to do this math yourself—your bank publishes the Annual Percentage Yield (APY), which already accounts for compounding. The APY is the real rate you'll earn over a year, and it's always higher than the stated interest rate when compounding happens more than once per year.
Key Takeaways
- Your bank compounds interest daily on most savings accounts, meaning you earn interest on interest, not just on your original deposit.
- The Annual Percentage Yield (APY) is the number you should use to estimate your earnings—it's the rate after compounding is factored in.
- To estimate yearly earnings, multiply your account balance by the APY and divide by 100.
- Interest rates change, so the rate you see today may not be the rate you earn next month.
- You can find your account's APY on your bank's website, your account statements, or by calling customer service.
Using APY to Estimate What You'll Earn
The easiest way to calculate your interest is to use the APY your bank publishes. Find your current balance and your account's APY, then use this simple formula: (Balance × APY) ÷ 100 = Annual Interest Earned. For example, if you have $5,000 in an account with a 4.5% APY, you would earn approximately $225 per year ($5,000 × 4.5 ÷ 100 = $225).
This calculation gives you an estimate because it assumes your balance stays the same all year and the rate doesn't change. In reality, your balance may grow as you deposit more money, and interest rates move up and down. Banks also calculate interest daily and add it to your account, so the actual amount you earn will be slightly different from this estimate.
If you want to know what you'll earn in a specific number of months rather than a full year, divide the annual interest by 12 and multiply by the number of months. For a $5,000 balance at 4.5% APY over six months: ($225 ÷ 12) × 6 = $112.50.
Where to Find Your Account's APY
Your bank is required by law to disclose the APY on your savings account. You can find it in several places: on your monthly or quarterly account statement, on your bank's website in the account details section, or by logging into your online banking portal. Some banks display it prominently on the account summary page; others list it under "Account Terms" or "Rate Information."
If you can't find it online, call your bank's customer service line and ask for the current APY on your specific account. Have your account number ready. The representative will give you the exact rate and can also tell you when it was last changed and whether it's a promotional rate that will expire.
Keep in mind that APY changes over time. Banks adjust rates based on Federal Reserve decisions and market conditions. If you opened your account during a high-rate period, the rate may be lower now. Checking your statement every few months helps you stay aware of what you're actually earning.
How Compounding Frequency Affects Your Earnings
The more often interest compounds, the more you earn—even if the stated interest rate is the same. Daily compounding (365 times per year) earns more than monthly compounding (12 times per year), which earns more than annual compounding (once per year). This is why the APY is always equal to or higher than the stated interest rate.
Most savings accounts compound daily, which is why the APY matters. If a bank advertises a 4.5% interest rate compounded daily, the actual APY will be slightly higher—often around 4.60% or 4.61%. The difference grows larger as the interest rate increases. At lower rates (under 1%), the difference between the stated rate and APY is barely noticeable, but at higher rates it becomes meaningful.
Your account statement or the bank's disclosure document will tell you the compounding frequency. Look for language like "interest compounds daily" or "daily compounding." This information is also part of the account agreement you received when you opened the account.
Comparing Interest Across Different Banks
When you're deciding where to keep your savings, always compare APY, not the stated interest rate. Two banks might advertise similar rates, but one compounds daily and one compounds monthly—the daily-compounding account will earn more. Write down the APY for each account you're considering, then use the formula above to calculate what you'd earn on your expected balance.
Online banks typically offer higher APY than traditional brick-and-mortar banks because they have lower overhead costs. Rates vary widely, so it's worth checking multiple banks before opening an account. A difference of 1% APY on a $10,000 balance means $100 per year in additional earnings.
Remember that promotional rates are temporary. Some banks offer a high APY for the first few months, then drop it to a lower rate. Read the fine print to see when the promotional period ends and what the regular rate will be. If you're comparing accounts, compare the regular rate, not the promotional one, unless you plan to move your money when the promotion expires.
What Happens When Interest Rates Change
Your bank can change your APY at any time, and they're required to notify you before the change takes effect. You'll usually receive notice by email, mail, or through your online banking portal. The notice will tell you the new rate and when it starts. Some banks give you a grace period to close the account if you don't like the new rate.
Interest rates tend to move together across the banking system because they follow Federal Reserve decisions. When the Fed raises rates, most banks raise their savings APY within a few weeks. When the Fed cuts rates, banks typically cut their APY more quickly. This means your earnings can change significantly over time, even if you don't touch your account.
If your bank's rate drops and you find a better rate elsewhere, you can move your money to a different bank. There's no penalty for closing a savings account and opening one somewhere else. Some people move their savings every few months to chase the highest available rate, though the effort may not be worth it for small balances.
Frequently Asked Questions
Do I need to do the interest calculation myself, or does the bank do it for me?
Your bank calculates and deposits interest into your account automatically. You don't need to do anything. The calculation above is just to help you understand how much you're earning or to compare accounts before you open one.
Why is my actual interest different from what I calculated?
The most common reason is that your balance changed during the year. If you deposited money partway through the year, you earned interest on the higher balance for part of the time. Interest is also calculated daily and rounded, so small differences add up. Your bank statement will show the exact amount you earned.
Is the interest I earn on a savings account taxable?
Yes. Interest income is taxable as ordinary income on your federal tax return. Banks send you a 1099-INT form in January if you earned $10 or more in interest during the year. You'll report this on your tax return. State and local taxes may also apply depending on where you live.
What's the difference between APR and APY?
APR (Annual Percentage Rate) is the stated interest rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding. For savings accounts, always use APY to calculate your earnings. APR is used for loans and credit cards.
Can I earn interest on interest if I don't touch my account?
Yes. That's what compounding means. The bank adds interest to your account, and then calculates the next interest payment on the new, higher balance. You don't have to do anything—it happens automatically as long as you keep the money in the account.