The basic formula for savings account interest

To find out how much interest your savings account will earn, you need three pieces of information: the amount of money in the account, the annual percentage yield (APY), and how long the money stays there. The simplest version is this: multiply your balance by the APY, then divide by 12 if you want to know what you'll earn in one month.

Here's a concrete example. If you have $1,000 in an account with a 4.5% APY, you multiply $1,000 by 0.045 (which is 4.5% written as a decimal). That gives you $45 — the amount you'd earn in a full year if the balance never changed. Divide $45 by 12, and you get $3.75 per month.

Banks use a more precise method called daily compounding, which means they calculate interest on your balance every single day, then add that interest back into your account. This happens automatically — you don't have to do anything. The daily compounding method earns you slightly more than the straightforward monthly calculation, but the difference is usually small enough that the basic formula gives you a useful estimate.

Key Takeaways

  • The basic calculation is: your balance multiplied by the APY (written as a decimal), then divided by 12 for a monthly estimate.
  • Banks compound interest daily, meaning they calculate and add interest to your account every day, which earns you slightly more than straightforward monthly math.
  • Your actual earnings will be lower if you withdraw money during the month, because interest is calculated on your daily balance.
  • You can compare accounts by looking at the APY, not the interest rate — APY already includes the effect of compounding.
  • Online banks typically offer higher APYs than brick-and-mortar banks, so the same $1,000 will earn more in an online account.

Why the APY matters more than the interest rate

You might see two different numbers on a savings account: the interest rate and the APY. The APY (annual percentage yield) is the one you should use for your calculation, because it already includes the effect of compounding. The interest rate alone doesn't account for the fact that interest gets added back into your account and then earns interest itself.

For example, two banks might both advertise a 4% interest rate. But if one compounds daily and the other compounds monthly, the one that compounds daily will show a slightly higher APY — maybe 4.08% instead of 4.00%. When you calculate your earnings, use that APY number. It's the true picture of what you'll earn.

How daily compounding affects your earnings

Banks calculate interest on your balance every day, not just once a year. This means if you have $1,000 on day one and earn $0.12 in interest, you now have $1,000.12 on day two. The next day's interest is calculated on $1,000.12, not the original $1,000. Over a year, this compounding effect adds up.

The difference between daily compounding and straightforward interest is usually small — on a $1,000 balance at 4.5% APY, you might earn $45.68 with daily compounding versus $45 with straightforward math. But on larger balances or over many years, the difference becomes more noticeable. This is why the APY is always slightly higher than the interest rate: it reflects this compounding benefit.

You don't need to calculate the daily compounding yourself. Your bank does it automatically and shows you the APY so you can compare accounts fairly. When you see the APY on a savings account, that number already includes the compounding effect.

What happens if your balance changes during the month

The calculation changes if you deposit or withdraw money. Banks calculate interest on your daily balance, which means the amount you earn depends on how much money was in the account each day. If you deposit $500 on the 15th of the month, you'll only earn interest on that $500 for the remaining days of the month, not for the whole month.

Here's why this matters: if you have $1,000 for 20 days and $1,500 for 10 days, your average daily balance is not $1,250. Instead, the bank calculates the exact interest earned on each day's balance and adds them together. Most banks show you this calculation in your monthly statement, so you can see how much you earned and on what balance.

For planning purposes, if you know you'll keep a steady balance, use the straightforward formula. If you're making regular deposits or withdrawals, ask your bank to show you how much interest you earned last month — that's the most accurate way to predict what you'll earn this month if your pattern stays the same.

Comparing interest earnings across different accounts

When you're deciding between savings accounts, the APY is your main tool for comparison. A higher APY means more money in your pocket, even if the difference looks small. On a $10,000 balance, the difference between a 4.0% APY and a 4.5% APY is $50 per year — that's real money.

To compare two accounts, use the same balance and time period for both calculations. If Account A offers 4.0% APY and Account B offers 4.5% APY, and you plan to keep $5,000 in the account for one year, Account A will earn $200 and Account B will earn $225. That $25 difference might seem small, but it compounds over time, and it's money you keep without doing any work.

Online banks typically offer higher APYs than traditional brick-and-mortar banks because they have lower overhead costs. The same $10,000 might earn $400 per year at an online bank with a 4.0% APY but only $200 per year at a traditional bank with a 2.0% APY. Over five years, that's a $1,000 difference on the same deposit.

Using a calculator versus doing the math by hand

You can calculate interest earnings with a straightforward calculator, a spreadsheet, or an online savings calculator. The formula is always the same: balance × APY ÷ 12 for a monthly estimate, or balance × APY for a yearly estimate. If you're comparing multiple accounts or planning over several years, a spreadsheet or online tool saves time and reduces the chance of arithmetic errors.

Many banks and financial websites offer free savings calculators where you enter your balance, the APY, and the time period, and the tool shows you how much you'll earn. These calculators use the daily compounding formula, so they're more accurate than the straightforward monthly math. But for a quick estimate while you're shopping for accounts, the basic formula is fast and close enough.

What to watch out for when comparing rates

APY rates change frequently, sometimes weekly. When you see a rate advertised, check whether it's a promotional rate that expires after a certain period, or a regular rate that stays in place. Some banks offer a high APY for the first few months, then drop it significantly. Read the account terms to see when the rate changes.

Also check whether there are any fees that reduce your earnings. A savings account with a 4.5% APY but a $5 monthly maintenance fee will earn you less than an account with a 4.0% APY and no fees, especially if your balance is small. The APY tells you the interest you'll earn, but fees are separate and reduce your actual gain.

Frequently Asked Questions

How often does interest get added to my account?

Banks calculate interest daily, but they usually add it to your account monthly. You'll see the interest deposit once a month, typically on the last day or the first day of the month. Some banks add interest more or less frequently — check your account terms to see the schedule.

If I withdraw money mid-month, do I lose all the interest I earned?

No. Interest is calculated on your daily balance, so you earn interest on the money for the days it was in the account. If you have $1,000 for 15 days and then withdraw it, you earn interest on $1,000 for those 15 days. You don't earn interest on the money after you withdraw it.

Why is the APY different from the interest rate?

The APY includes the effect of daily compounding — interest earning interest. The interest rate is the base rate, and the APY is the true annual return after compounding is factored in. Always use the APY when comparing accounts or calculating what you'll earn.

Can I calculate interest if the APY changes during the year?

Yes, but you'll need to break the calculation into periods. Calculate the interest earned during the time the first rate was in effect, then calculate the interest for the time the second rate was in effect, and add them together. Your bank's statement will show you the actual interest earned, which is the easiest way to verify your math.

Is there a difference between a savings account and a money market account when calculating interest?

The calculation is the same — balance × APY. The difference is that money market accounts sometimes offer higher APYs in exchange for higher minimum balances or limited withdrawals. Use the APY to compare them fairly, just as you would with regular savings accounts.