The two formulas that determine what your bank pays you

Banks calculate savings account interest in one of two ways: simple interest or compound interest. Simple interest pays you a percentage of your starting balance once per year. Compound interest pays you interest on your interest, meaning the amount grows faster because each time the bank calculates interest, it includes the interest you already earned.

Most savings accounts use compound interest, often compounded daily or monthly. This means your balance grows more than it would with simple interest, even at the same stated rate. The difference becomes visible over months and years, especially with larger balances.

To know what you will actually earn, you need to know three things: your account balance, the annual percentage rate (APR) your bank offers, and how often the bank compounds the interest.

Key Takeaways

  • Simple interest pays a percentage of your starting balance once per year, while compound interest pays interest on your interest, making your money grow faster.
  • The annual percentage rate (APR) is the yearly interest rate your bank advertises; the actual amount you earn depends on how often interest is compounded.
  • Daily compounding produces more earnings than monthly or annual compounding at the same APR because interest accrues more frequently.
  • You can calculate interest yourself using the compound interest formula, or use your bank's online calculator or account statements to see what you will earn.

Simple interest: the straightforward calculation

Simple interest is the easiest to calculate by hand. The formula is: Interest = Principal × Rate × Time. Principal is your starting balance, rate is the annual percentage rate (written as a decimal), and time is how many years the money sits in the account.

For example, if you deposit $5,000 in an account with a 4% APR and leave it for one year, the calculation is: $5,000 × 0.04 × 1 = $200. You earn $200 in interest, and your balance becomes $5,200.

If you leave that same $5,000 for two years at 4% simple interest, you earn $5,000 × 0.04 × 2 = $400 total. Simple interest does not accelerate—you earn the same amount each year.

Most savings accounts do not use simple interest, but some certificates of deposit (CDs) and money market accounts do. Your account paperwork or bank website will state which method applies to your account.

Compound interest: how your interest earns interest

Compound interest uses this formula: A = P(1 + r/n)^(nt). Here, A is your final amount, P is your principal (starting balance), r is the annual rate as a decimal, n is how many times per year interest compounds, and t is the number of years.

Using the same $5,000 at 4% APR for one year, but compounded daily (365 times per year): A = $5,000(1 + 0.04/365)^(365×1). This works out to approximately $5,204.04. You earn $204.04 instead of $200—a small but real difference created by daily compounding.

Over longer periods, the difference grows. The same $5,000 at 4% compounded daily for five years becomes approximately $6,104.88. With simple interest, it would be only $6,000. That extra $104.88 comes entirely from earning interest on your interest.

The more frequently interest compounds, the more you earn. Daily compounding beats monthly, which beats quarterly, which beats annual—all at the same APR. This is why high-yield savings accounts advertise their compounding frequency alongside their rate.

Where to find your APR and compounding frequency

Your bank must disclose both the APR and the compounding frequency in your account agreement or on the account details page of your online banking portal. Look for language like "Annual Percentage Rate" or "APR" and "compounded daily" or "compounded monthly."

If you cannot find this information online, call your bank's customer service line or visit a branch with your account number. The representative can tell you the exact rate and compounding method for your specific account.

Rates change frequently, especially for savings accounts. If your bank lowers the rate, you will see the new APR in your account settings or in a notice they send you. Some banks update rates weekly or even daily based on market conditions.

Using your bank's calculator or statements

Most banks provide an online interest calculator on their website. You enter your balance, the APR, and how long you plan to keep the money, and the calculator shows you the projected interest earned. This is faster and more accurate than doing the math yourself, especially for compound interest over multiple years.

Your monthly or quarterly account statement also shows interest earned during that period. If you receive a statement showing $15 in interest for the month, you can multiply that by 12 to estimate your annual earnings—though the actual total will be slightly higher because of compounding.

Some banks also show a running total of year-to-date interest on your online dashboard, updated daily or weekly. This gives you a real-time picture of what your money is earning.

Why APY matters more than APR for savings accounts

Annual Percentage Yield (APY) is different from APR. APY includes the effect of compounding, while APR does not. Banks are required to show you the APY when advertising savings accounts, because APY is what you actually earn.

For example, a bank might offer 4% APR compounded daily. The actual APY—the real rate you earn after compounding—might be 4.08%. The difference comes from earning interest on your interest throughout the year. When comparing accounts, always compare APY to APY, not APR to APY.

Your account statement and online banking portal will show both figures. The APY is the number that matters for calculating what you will actually have at the end of the year.

How inflation affects what your interest is really worth

Interest earnings are real money, but inflation can reduce their purchasing power. If your savings account earns 4% APY but inflation is running at 3%, your money is growing in real terms—you can buy about 1% more with it next year than you can today.

If inflation is higher than your interest rate, your savings lose purchasing power even though the dollar amount grows. This is why some people move money to high-yield savings accounts or other products when inflation is high—to earn a rate that at least keeps pace with rising prices.

Your bank's interest rate is fixed in the account agreement, but inflation changes constantly. You cannot control inflation, but you can control which account you use and whether to move money if rates change.

Frequently Asked Questions

How often does my bank calculate and add interest to my account?

Most banks compound interest daily but credit it to your account monthly. This means interest accrues every day (making the calculation more frequent), but you see the total added to your balance once a month. Some banks credit interest quarterly or annually instead. Check your account agreement or call your bank to confirm the exact schedule.

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

No. Interest accrues daily, so you earn interest on the money for the days it was in the account. If you withdraw on the 15th of the month, you keep the interest earned from the 1st through the 15th. The bank calculates this automatically when you withdraw.

Can I calculate interest if my balance changes during the year?

Yes, but it is more complex because the formula changes each time your balance changes. Your bank handles this automatically by calculating interest on the daily balance. If you want to estimate it yourself, use your bank's online calculator and enter your expected average balance, or add up the interest shown on each monthly statement.

What is the difference between a savings account and a money market account in terms of interest?

The interest calculation method is the same—usually compound interest. The difference is that money market accounts often offer higher rates in exchange for requiring a larger minimum balance or limiting how often you can withdraw. Both use APY to show what you actually earn.

Does my bank owe me interest if the rate drops after I open the account?

No. Banks can change savings account rates at any time, and the new rate applies to future interest calculations. You keep earning whatever rate was in effect when you opened the account until the bank changes it. You will receive notice of any rate change before it takes effect.