The basic formula: multiply your balance by the rate, then divide by the time period

To calculate interest on a savings account, you need three numbers: your account balance, the annual percentage yield (APY), and how long the money sits in the account. The simplest version is balance × APY ÷ 365 × number of days. If you have $5,000 in an account earning 4.5% APY and leave it untouched for 30 days, you earn roughly $18.49.

Most banks use daily compounding, which means they calculate interest each day on your current balance—including interest you earned yesterday. This is why the actual interest you receive is usually slightly higher than a simple one-time calculation. The bank's system does this automatically, but understanding the math helps you compare accounts and predict what you'll actually earn.

The formula changes slightly depending on whether interest compounds daily, monthly, or quarterly. Daily compounding (the most common) gives you the most money because you earn interest on your interest more often. Banks are required to disclose their compounding method, usually in the account terms or on the rate page.

Key Takeaways

  • Simple interest is balance × APY ÷ 365 × number of days, but most savings accounts use compound interest instead, which pays you interest on your interest.
  • Daily compounding produces more earnings than monthly or quarterly compounding because interest is calculated and added to your balance every single day.
  • Your bank automatically calculates and deposits interest, so you do not have to do this math yourself—but knowing the formula lets you compare accounts before you open one.
  • The APY shown on a savings account already includes the effect of compounding, so you do not need to adjust it or calculate a separate rate.
  • Your actual earnings depend on your balance staying constant; withdrawals reduce the amount earning interest for the rest of that period.

Simple interest versus compound interest

Simple interest pays you a percentage of your original balance once, at the end of a set period. You earn the same amount each month or year, no matter how much interest has already been added. Compound interest pays you interest on your balance plus all the interest you have already earned. This creates a snowball effect: each time interest is added, the next calculation uses a slightly larger balance.

Savings accounts almost always use compound interest because it benefits the customer. If you earn $10 in interest in month one, month two's interest is calculated on your original balance plus that $10. Over a year or longer, compounding makes a measurable difference. A $10,000 balance at 4.5% APY earns roughly $450 per year with daily compounding, but only $450 if it were simple interest—the difference is small at short timescales but grows as your balance or time horizon increases.

The frequency of compounding matters. Daily compounding means interest is calculated 365 times per year. Monthly compounding happens 12 times. Quarterly happens 4 times. The more often interest is added to your balance, the more you earn, because each addition becomes part of the next calculation. Most online banks and high-yield savings accounts compound daily, which is why they often pay more than traditional banks that compound monthly or quarterly.

How to use the compound interest formula

If you want to predict your balance after a specific time, use the compound interest formula: A = P(1 + r/n)^(nt). Here, A is your final amount, P is your starting balance, r is the annual rate (as a decimal, so 4.5% becomes 0.045), n is how many times per year interest compounds, and t is the number of years.

Example: You deposit $5,000 in an account earning 4.5% APY, compounded daily. After one year, your balance is $5,000 × (1 + 0.045/365)^(365 × 1) = $5,230.11. You earned $230.11 in interest. If the same account compounded monthly instead, you would earn $230.03—slightly less because interest was added fewer times.

For shorter periods, you can simplify: divide the annual rate by 365, multiply by your balance, then multiply by the number of days. This gives you the interest earned during that period. A $5,000 balance at 4.5% for 30 days earns roughly (0.045 ÷ 365) × $5,000 × 30 = $18.49. This is close to the compound formula result for short periods because compounding has not had time to create much of a difference.

Why your actual interest may differ from your calculation

Banks round interest to the nearest cent, so your calculated amount may be off by a few pennies. More importantly, any withdrawal or deposit changes your balance mid-period, which changes how much interest you earn for the rest of that time. If you withdraw $1,000 on day 15 of a 30-day month, you earn interest on the full balance for 15 days and the reduced balance for 15 days. The bank calculates this automatically, but it means your interest will be lower than if you had left the money untouched.

Some banks use the average daily balance method instead of calculating interest on the exact balance each day. This method adds up your balance at the end of each day, divides by the number of days in the period, and applies interest to that average. The result is usually very close to daily compounding but can differ slightly depending on the timing of deposits and withdrawals.

The APY you see advertised is may provide only if your balance and the rate stay the same for the full year. Banks can change rates at any time, though they usually give notice. If your rate drops mid-year, your interest for the rest of the year will be lower. Conversely, if rates rise and your bank increases your APY, you earn more.

Comparing accounts using the APY

The APY already includes the effect of compounding, so you can compare accounts directly without doing any math. An account advertising 4.5% APY will earn you more than one advertising 4.0% APY, assuming your balance and time period are the same. You do not need to recalculate or adjust the APY based on how often interest compounds—the bank has already done that work.

When comparing accounts, look at the APY, not the interest rate alone. Some banks advertise a lower APY but compound more frequently; others advertise a higher rate but compound less often. The APY is the standardized number that accounts for both, so it is the fairest way to compare. Online banks typically offer higher APYs than brick-and-mortar banks because they have lower overhead costs.

Also check whether the APY is may provide or promotional. A promotional rate might be 5.0% for the first three months, then drop to 2.0%. Read the fine print to see when the rate changes and what the long-term rate will be. If you plan to keep money in the account for years, the long-term rate matters more than a temporary bonus.

Tools and calculators for interest math

Most banks provide a savings calculator on their website where you enter your balance, the APY, and the time period, and it shows you the interest earned. These calculators use the compound interest formula behind the scenes and save you from doing the math by hand. They are accurate as long as you enter the correct APY and assume no deposits or withdrawals during the period.

Spreadsheet programs like Excel or Google Sheets can also calculate compound interest. In Google Sheets, the formula is =P*(1+(r/n))^(n*t), where you replace P, r, n, and t with your numbers or cell references. This is useful if you want to model different scenarios—what if you deposit $100 more each month, or what if rates drop by 0.5%.

For quick mental math, remember that a 1% APY on $10,000 earns roughly $100 per year, or about $8.33 per month. A 4% APY on the same balance earns roughly $400 per year. This rough estimate works for most accounts and helps you quickly judge whether an account is worth switching to.

What happens to interest when you make deposits or withdrawals

Every deposit increases your balance, so future interest is calculated on the larger amount. If you deposit $1,000 on day 10 of a month, you earn interest on your original balance for 9 days, then on the new, larger balance for the remaining days. This is why regular deposits—even small ones—add up over time. The interest you earn on your deposits also earns interest, creating a compounding effect on top of compounding.

Withdrawals work the opposite way. Money you withdraw stops earning interest immediately. If you withdraw $2,000 to pay a bill, the interest for the rest of that period is calculated on the reduced balance. Some accounts have withdrawal limits or penalties if you withdraw too often, so check your account terms before moving money around frequently.

Banks calculate interest based on your balance at the end of each day (or sometimes the average balance over the period). If you deposit money late in the day, it may not start earning interest until the next day. Similarly, a withdrawal late in the day may not reduce your interest for that day. The exact timing depends on your bank's processing schedule, which is usually disclosed in the account agreement.

Frequently Asked Questions

Do I have to calculate interest myself, or does the bank do it for me?

The bank calculates and deposits interest automatically. You do not have to do any math. Knowing how to calculate it yourself is useful for comparing accounts before you open one or predicting how much you will earn, but your bank handles all the actual calculations and deposits the interest into your account on a regular schedule.

What is the difference between APY and interest rate?

The interest rate is the percentage the bank pays on your balance. The APY (annual percentage yield) is the interest rate adjusted for how often interest compounds. APY is always equal to or higher than the interest rate because it includes the effect of earning interest on your interest. When comparing accounts, use the APY because it is the true annual return.

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

No. You keep all the interest you earned up to the day you withdraw. Interest is calculated daily, so you earn a small amount each day. Withdrawing money only stops you from earning interest on the amount you withdrew for the rest of that period. Your existing interest stays in the account.

Why do some banks offer higher APY than others?

Online banks typically offer higher APYs because they have lower operating costs than traditional banks with physical branches. They pass some of those savings to customers in the form of higher rates. Economic conditions also matter: when the Federal Reserve raises its benchmark rate, banks often increase savings APYs. When rates fall, APYs fall too.

Can the bank change my APY after I open the account?

Yes. Banks can change rates at any time, though they usually provide notice. If your rate drops, your future interest will be lower. If it rises, you earn more. The rate change applies to new interest going forward, not to interest you have already earned. Check your account statements or bank website to see if your rate has changed.