How your bank calculates the interest you earn
Your bank calculates interest using three pieces of information: your account balance, the annual percentage rate (APR) your account earns, and how often the bank compounds that interest. Most savings accounts compound interest daily or monthly, which means the bank calculates what you owe, adds it to your balance, and then calculates interest on that larger amount the next period. The more often interest compounds, the more you earn.
The actual formula banks use is: Interest = Principal × Rate × Time. Your principal is the money in the account, the rate is the APR divided by the number of compounding periods per year, and time is how many periods have passed. If your account compounds daily, the bank runs this calculation 365 times per year. If it compounds monthly, the calculation happens 12 times.
You can see this in action with a concrete example. Say you have $10,000 in an account earning 4.5% APR, compounded daily. The bank divides 4.5% by 365 to get a daily rate of about 0.0123%. On day one, you earn roughly $1.23. On day two, the bank calculates interest on $10,001.23, earning you slightly more. By the end of a year, daily compounding will have earned you about $460 instead of the $450 you would get with simple interest.
Key Takeaways
- Banks calculate interest by multiplying your balance by the annual rate, divided by how many times per year interest compounds, then repeating that calculation each compounding period.
- Daily compounding earns more than monthly compounding because interest gets calculated on your interest, not just your original deposit.
- The APR shown on your account disclosure is the annual rate; the bank divides it by 365 (for daily) or 12 (for monthly) to find what you earn each period.
- You can estimate your earnings by multiplying your balance by the APR and dividing by the number of compounding periods, though the exact amount depends on how your balance changes throughout the year.
Understanding APR versus the actual rate per compounding period
The annual percentage rate is the yearly rate your account earns, but your bank does not apply that entire rate all at once. Instead, it divides the APR by the number of compounding periods. If your account earns 4.5% APR and compounds daily, the bank uses a daily rate of 4.5% ÷ 365, or about 0.0123% per day.
This matters because the difference between daily and monthly compounding is real money. With $10,000 at 4.5% APR compounded daily, you earn about $460 per year. With the same balance and rate compounded monthly, you earn about $459. The difference is small at this rate, but it grows as your balance or the APR increases. At higher rates or with larger balances, choosing an account that compounds daily instead of monthly can mean dozens of dollars more per year.
Your account disclosure statement will tell you both the APR and how often interest compounds. If it does not, call your bank or check online. The compounding frequency is not always obvious from the account name alone.
How to calculate what you will earn over a specific time period
If you want to know roughly how much interest you will earn in a month or a year, you can use a simplified calculation. Multiply your current balance by the APR, then divide by 12 for monthly earnings or by 365 for daily earnings. This gives you an estimate, not the exact amount, because it does not account for compounding or changes to your balance.
For example: $10,000 × 4.5% ÷ 365 = $1.23 per day. Over 30 days, that is roughly $37. Over a full year, it is roughly $460. The actual amount will be slightly higher because of compounding, but this calculation gets you close enough to understand what your money is earning.
If your balance changes during the month — because you deposit or withdraw money — the calculation becomes less accurate. Banks track your daily balance and use that to calculate interest, so a deposit mid-month will earn interest for only part of the month. If you want to know the exact amount, use your bank's online calculator or contact them directly.
The difference between simple and compound interest
Simple interest means the bank calculates interest only on your original deposit. Compound interest means the bank calculates interest on your deposit plus any interest you have already earned. Savings accounts use compound interest, which is why your money grows faster over time.
With simple interest on $10,000 at 4.5% APR, you would earn $450 per year, every year, for a total of $10,450 after one year. With compound interest compounded daily, you earn about $460 in year one because the bank is calculating interest on the interest. In year two, you earn interest on $10,460, not just $10,000, so you earn slightly more. Over decades, this difference becomes substantial.
Most savings accounts, money market accounts, and certificates of deposit use compound interest. Some older accounts or specialty products may use simple interest, but this is rare. Your account disclosure will state which method your bank uses.
What happens to your interest if you withdraw money mid-period
Banks calculate interest based on your daily balance, so if you withdraw money before the interest is credited to your account, you earn less. If you withdraw $5,000 on the 15th of a month, the bank calculates interest on your full balance for the first 15 days, then on the reduced balance for the remaining days.
Some banks have a minimum balance requirement to earn interest at all. If your balance drops below that threshold, you earn no interest for that period, even if you had the minimum for most of the month. Check your account disclosure to see whether your account has this rule.
Interest is usually credited to your account on a specific day each month, often the last day or the first day of the next month. Until that day, the interest you have earned sits in the bank's calculation system but is not yet part of your balance. If you withdraw money before interest is credited, you lose the interest you earned up to that point.
How to find your account's APR and compounding frequency
Your bank sends you a disclosure document when you open the account and updates it when the rate changes. This document lists the APR and the compounding frequency. If you did not keep the original, you can find it online by logging into your account and looking for "account details," "disclosures," or "terms and conditions."
You can also call your bank's customer service line or visit a branch. Have your account number ready. The representative can tell you the current APR, how often interest compounds, and whether there is a minimum balance requirement. Rates change frequently, so if your account is more than a few months old, the rate may have changed since you opened it.
If you are comparing savings accounts at different banks, make sure you are comparing the APR, not just the interest rate. Some banks advertise a "rate" that is not the same as the APR. The APR is the standardized figure that lets you compare accounts fairly.
Why your actual earnings might differ from your calculation
If you calculate your expected interest and the actual amount credited to your account is different, the most common reason is a change in your balance during the month. Banks use your daily balance to calculate interest, so every deposit and withdrawal changes how much interest you earn that day. If you made several deposits or withdrawals, the calculation becomes complex.
Another reason is a change in the APR. Banks can change rates at any time for savings accounts, though they usually give you notice. If your rate changed mid-month, the bank calculates interest at the old rate for the days before the change and the new rate for the days after.
A third reason is the way the bank rounds. Banks calculate interest to many decimal places but round the final amount to the nearest cent. This rounding is usually in your favor, but it can make your actual earnings slightly different from your estimate.
Frequently Asked Questions
Does interest compound on weekends and holidays?
Yes. Banks calculate daily interest on every calendar day, including weekends and holidays. The interest is credited to your account on the scheduled date, which may be a business day, but the calculation includes all days.
What is the difference between APR and APY?
APR is the annual percentage rate without accounting for compounding. APY is the annual percentage yield, which includes the effect of compounding. APY is always equal to or higher than APR. Some banks show both; others show only APY. Either one tells you what you will earn, but APY is more accurate because it reflects compounding.
Can I calculate interest if my balance changes every month?
You can estimate it by calculating interest on your average balance, but the exact amount requires knowing your daily balance for each day of the month. Your bank tracks this automatically, so the easiest approach is to check your statement or use your bank's online calculator.
Do I pay taxes on savings account interest?
Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return.
Is there a maximum amount of interest I can earn?
No. The more money you have in the account and the higher the APR, the more interest you earn. There is no cap on how much interest a savings account can generate.