Banks use one of two methods to calculate what you earn: straightforward interest or compound interest

straightforward interest calculates earnings only on the money you put in. Compound interest calculates earnings on your original deposit plus all the interest you've already earned. Almost all savings accounts use compound interest, which means your money grows faster because you earn "interest on interest."

The actual dollar amount you earn depends on three things: how much money sits in the account, what interest rate the bank offers, and how often the bank compounds (adds interest to your balance). A bank might compound daily, monthly, or quarterly. Daily compounding means you earn slightly more than monthly compounding, because interest gets added to your balance more frequently.

The interest rate itself is set by the bank and changes based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise the rates they offer on savings accounts. When the Fed cuts rates, savings account rates usually fall within weeks or months.

Key Takeaways

  • Compound interest earns you money on your original deposit plus all previously earned interest, while straightforward interest only earns on your initial deposit.
  • The three factors that determine your earnings are the account balance, the annual interest rate, and how often the bank compounds interest.
  • Banks compound interest daily, monthly, or quarterly—daily compounding produces slightly higher earnings because interest is added more frequently.
  • The interest rate your bank offers changes when the Federal Reserve adjusts its benchmark rate, usually within weeks or months.
  • You can compare accounts using the APY (annual percentage yield), which shows the total return you'll earn in one year including all compounding.

How the math works with compound interest

The formula banks use is: A = P(1 + r/n)^(nt), where P is your starting balance, r is the annual interest rate, n is how many times per year the bank compounds, and t is the number of years. You don't need to calculate this yourself—your bank does it automatically—but understanding the pieces helps you see why daily compounding beats monthly compounding.

Here's a concrete example. Say you deposit $5,000 in an account that pays 4.5% annual interest, compounded daily. After one year, you'll have earned about $231 in interest. If that same account compounded monthly instead, you'd earn about $230. The difference is small in year one, but it compounds over time. After five years, daily compounding would give you roughly $1,196 in total interest, while monthly compounding would give you about $1,191.

The reason daily compounding wins is that interest gets added to your balance 365 times per year instead of 12 times. Each time interest is added, the next interest calculation includes that new amount. Over months and years, those small daily additions stack up.

What APY means and why it matters more than the interest rate

Banks list two numbers: the interest rate (also called APR) and the APY (annual percentage yield). The interest rate is the percentage the bank pays on your balance. The APY is what you actually earn in one year after all compounding is included.

If a bank offers 4.5% interest compounded daily, the APY will be slightly higher—maybe 4.60%—because compounding adds extra earnings throughout the year. When you compare savings accounts, always look at the APY, not the interest rate. Two banks might offer the same interest rate, but if one compounds daily and the other compounds monthly, the APY will be different.

The APY is the number that tells you the real return on your money. It's the only fair way to compare accounts, because it accounts for how often the bank compounds interest.

How often banks add interest to your account

Banks compound interest on different schedules. Some compound daily, some monthly, and some quarterly. After the interest is calculated, it gets added to your account balance, and that new balance is what earns interest next time.

Daily compounding is the most common at online banks and high-yield savings accounts. Traditional banks often compound monthly or quarterly. The difference in your earnings grows over time, especially if you keep money in the account for years. A $10,000 deposit earning 4.5% APY will grow to about $12,462 after five years. That same deposit at 4.5% compounded only quarterly might grow to about $12,450—a difference of $12 over five years, which is small but real.

You can find the compounding schedule in the account's disclosure document, usually labeled "Truth in Savings" or "Account Terms and Conditions." It will say something like "interest is compounded daily and credited monthly" or "interest is compounded and credited quarterly."

Why your interest rate changes and how it affects your earnings

The interest rate your bank offers is not fixed forever. It moves when the Federal Reserve changes its benchmark rate, called the federal funds rate. When the Fed raises rates, banks raise the rates they offer on savings accounts to attract deposits. When the Fed cuts rates, banks cut savings account rates to reduce what they pay out.

The lag between a Fed change and a bank rate change is usually a few weeks to a few months. Some banks move faster than others. If you're in a high-yield savings account, you might see your rate change within days of a Fed announcement. Traditional banks sometimes take longer.

This means the interest you earn in year two might be different from year one. If rates fall, your earnings will be lower. If rates rise, your earnings will be higher. Over a five-year period, you might see your rate change three or four times as the Fed adjusts policy.

How to calculate what you'll actually earn

You don't have to use the compound interest formula yourself. Most banks and financial websites have calculators that show you the projected balance after a certain number of years. You enter your starting balance, the APY, and how long you plan to keep the money there, and the calculator shows you the total interest earned.

These calculators assume the interest rate stays the same for the entire period, which is not realistic. Interest rates change, so your actual earnings will be different. But the calculator gives you a reasonable estimate for comparing accounts right now.

If you want to do a rough calculation by hand, multiply your balance by the APY and divide by 100. That gives you the approximate interest you'll earn in one year. For example, $5,000 at 4.5% APY is roughly $225 in year one. In year two, you'd multiply $5,225 by 4.5% to get the next year's earnings, and so on. This is simpler than the full formula and close enough for planning purposes.

The difference between savings accounts and money market accounts

Money market accounts and savings accounts both use compound interest, but money market accounts often offer higher interest rates in exchange for higher minimum balances and limited withdrawals. The calculation method is the same—the bank compounds interest on your balance at whatever frequency they use—but the rate is usually better.

Certificates of deposit (CDs) also use compound interest, but the rate is locked in for a set period (three months, one year, five years, etc.). You cannot withdraw the money early without paying a penalty. Because your money is locked in, banks offer higher rates on CDs than on regular savings accounts.

All three account types—savings, money market, and CD—are FDIC insured up to $250,000 per account holder per bank, so the safety is the same. The difference is in the rate, the minimum balance, and how easily you can access your money.

Frequently Asked Questions

Does the interest rate and APY mean the same thing?

No. The interest rate is the percentage the bank pays on your balance. The APY includes the effect of compounding, so it's always equal to or higher than the interest rate. When comparing accounts, use the APY because it shows your actual earnings.

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

It depends on the bank's rules. Most banks calculate interest daily, so you earn interest on the money you had in the account each day. If you withdraw on the 15th, you keep the interest earned from the 1st through the 14th. Some banks have different rules, so check your account terms.

Why do online banks offer higher interest rates than traditional banks?

Online banks have lower overhead costs because they don't operate physical branches. They pass those savings to customers by offering higher interest rates on savings accounts. The money is still FDIC insured the same way, so the safety is identical.

Can I move my money to a different account if my bank lowers the interest rate?

Yes. Banks can change interest rates at any time, and you have no obligation to stay. If your rate drops and another bank offers a better rate, you can open a new account and transfer your money. There's no penalty for moving your savings to a different bank.

What happens to my interest if I don't touch my account for years?

The interest keeps compounding and adding to your balance automatically. Your money grows even if you never make another deposit or withdrawal. However, the interest rate itself may change multiple times over those years as the Federal Reserve adjusts policy.