The basic formula: multiply your balance by the rate, then divide by the number of times interest compounds per year
Interest on a savings account is calculated by taking your account balance, multiplying it by the annual percentage yield (APY), and dividing by how often the bank compounds interest. Most banks compound daily, which means they calculate and add interest to your account every single day. Some compound monthly or quarterly instead.
The simplest version of the calculation is: Balance × APY ÷ 365 = daily interest earned. If you have $10,000 in an account with a 4.5% APY and the bank compounds daily, you earn roughly $1.23 per day ($10,000 × 0.045 ÷ 365). That daily amount gets added to your account, and the next day's calculation uses your new, slightly higher balance.
This matters because daily compounding means you earn interest on the interest you already earned — a small but real advantage over monthly or quarterly compounding. The difference grows larger the longer your money sits in the account.
Key Takeaways
- Daily compounding means the bank calculates interest every day and adds it to your balance, so you earn interest on yesterday's interest.
- The formula is balance multiplied by APY, then divided by 365 (or 360 for some banks), to find your daily earnings.
- Your actual interest depends on your exact balance each day, so deposits and withdrawals change what you earn that month.
- Banks must disclose their compounding frequency in the account terms, usually found online or in your account agreement.
- Over a year, the difference between daily and monthly compounding can add up to several dollars on a $10,000 balance.
Why the compounding frequency matters more than you think
A bank that compounds daily will pay you slightly more than one that compounds monthly, even if both advertise the same APY. This is because compounding daily means your interest gets added to your balance 365 times per year instead of 12 times. Each time interest is added, the next calculation uses a larger balance.
On $10,000 at 4.5% APY, daily compounding earns you about $450 over a year. Monthly compounding on the same balance and rate earns about $448 — a $2 difference. That gap widens as your balance grows. On $100,000, the difference between daily and monthly compounding at 4.5% is roughly $20 per year.
The account terms or the bank's website will state the compounding frequency. Look for the phrase "compounded daily" or "compounded monthly." If you do not see it stated, call the bank and ask — they are required to tell you.
How to calculate interest over a specific time period
If you want to know how much interest you will earn over three months or six months, the calculation changes slightly because you are not working with a full year. The formula becomes: Balance × APY × (number of days ÷ 365) = interest earned.
Say you have $5,000 at 3.5% APY and want to know what you will earn in six months (roughly 182 days). The math is: $5,000 × 0.035 × (182 ÷ 365) = $87.67. That is the interest you would earn if your balance stayed at exactly $5,000 the entire time and the bank compounds daily.
This calculation assumes your balance does not change. In real life, most people deposit or withdraw money during those six months, which changes the total interest earned. If you deposit $2,000 in month three, your interest for months four through six will be higher because your balance is larger.
What happens when your balance changes during the month
Banks track your balance on each day of the month and calculate interest based on that daily balance. If you start the month with $10,000, deposit $5,000 on day 15, and withdraw $3,000 on day 25, the bank calculates three separate daily interest amounts and adds them all together at the end of the month.
This is called the average daily balance method, and it is the most common way banks calculate interest. Your statement will show the total interest earned for the month, but not the day-by-day breakdown. If you want to verify the number, you would need to add up the interest for each day yourself — which is why most people trust the bank's calculation.
The practical takeaway: deposits made early in the month earn more interest than deposits made late in the month, because they sit in the account longer. A $5,000 deposit on day one earns interest for 30 days; the same deposit on day 28 earns interest for only three days.
How to find your account's actual APY and compounding frequency
Your bank must disclose the APY and compounding frequency in writing. For online banks, this information is usually on the account details page or in a document called the "Disclosure" or "Account Terms." For traditional banks, ask for a printed copy or look in your account agreement.
The APY already accounts for compounding, so you do not need to adjust it. If a bank advertises 4.5% APY with daily compounding, that 4.5% is the actual rate you will earn over a year, including the effect of daily compounding. The annual percentage rate (APR) is different — it does not include compounding — but savings accounts use APY, not APR.
If your bank's website does not clearly state the compounding frequency, log into your account online or call customer service. They can tell you whether interest is compounded daily, monthly, or quarterly.
Using a calculator to check your math
Most online savings account providers have a calculator on their website that shows you how much interest you will earn. You enter your balance, the APY, and how long you plan to keep the money, and the calculator does the work. These are free and accurate, though they assume your balance stays the same.
If you want to do the calculation yourself without a calculator, use the simple formula: Balance × APY ÷ 365 = daily interest. Multiply that daily amount by the number of days you plan to keep the money in the account. This gives you a rough estimate that is close enough for planning purposes.
Keep in mind that the interest rate can change. Banks lower rates when the Federal Reserve cuts rates, and raise them when the Fed raises rates. Your current APY is only may provide for as long as the bank keeps that rate in effect. Check your account statement or log in online to see your current rate.
The difference between simple and compound interest
Savings accounts use compound interest, which means you earn interest on the interest you already earned. Simple interest, by contrast, only pays interest on your original balance — the interest does not earn interest. Savings accounts never use simple interest; that term matters mainly for loans and bonds.
With compound interest, your balance grows faster over time because each compounding period adds a slightly larger amount. Over one year, the difference is small. Over five or ten years, compound interest makes a noticeable difference. A $10,000 balance at 4.5% APY grows to $12,462 after five years with daily compounding, compared to $12,250 with simple interest — a $212 difference.
This is why leaving money in a savings account longer pays off. The longer your money sits there, the more time compound interest has to work.
Frequently Asked Questions
Do I need to do this calculation myself, or does the bank do it for me?
The bank does it automatically. Interest is calculated and added to your account according to the compounding schedule, and you can see the total on your monthly statement. You only need to calculate it yourself if you want to estimate future earnings or verify the bank's number.
What if the interest rate changes in the middle of the year?
The bank recalculates using the new rate starting the day it takes effect. Your statement will show interest earned at the old rate for the days before the change, and interest at the new rate for the days after. The total interest for the year will be lower or higher depending on whether rates went down or up.
Is the interest I earn taxable?
Yes. Interest earned in a savings account is taxable income. The 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. Some accounts like Roth IRAs have tax advantages, but a regular savings account does not.
Why do different banks offer different APYs if they all follow the same formula?
Banks set their own rates based on how much they need to attract deposits and what they can earn by lending that money out. Online banks often offer higher rates because they have lower overhead costs. Traditional banks may offer lower rates because they have physical branches and higher expenses.
Can I calculate interest if I make multiple deposits throughout the month?
You can estimate it, but the exact number requires tracking your balance every single day. The formula is the same — balance times APY divided by 365 — but you apply it to each day's balance separately, then add all the daily interest amounts together. Most people let the bank do this and check the statement.