How savings account interest actually works

Interest in a savings account is money the bank pays you for letting them hold your money. The bank lends your deposits to other customers and keeps the difference between what they pay you and what borrowers pay them. The amount you earn depends on three things: how much money you have in the account, the interest rate the bank offers, and how often the bank adds interest to your balance.

Most banks use daily compounding, which means they calculate interest on your balance every single day and add it back into your account. This matters because once interest is added, the next day's calculation includes that interest too—so you earn interest on your interest. The longer money sits in the account, the more this compounds and grows.

Key Takeaways

  • Interest rate is shown as an annual percentage yield (APY), which already accounts for compounding over a full year.
  • Daily compounding means the bank calculates interest each day on your current balance, including any interest already added.
  • You can estimate monthly earnings by dividing the APY by 12, though the actual amount will be slightly higher because of compounding.
  • Moving money between accounts or making withdrawals changes your balance and reduces the interest you earn that month.
  • Banks are required to disclose the APY before you open an account, so you can compare rates across different institutions.

Understanding APY versus interest rate

The number banks advertise is called the annual percentage yield (APY), not the interest rate. APY includes the effect of compounding over a full year, so it is always slightly higher than the base interest rate. When you see "4.50% APY," that means if you leave $10,000 untouched for one year, you will have roughly $10,450 at the end—though the exact amount depends on how the bank compounds.

The base interest rate (sometimes called the nominal rate) is lower than the APY because it does not account for compounding. Banks must show you both numbers before you open an account, usually in a document called the Deposit Account Agreement or Truth in Savings disclosure. The APY is what matters for comparing accounts across different banks, because it shows the real return you will get.

The formula for calculating interest yourself

If you want to calculate what you will earn, use this formula:

Interest earned = Principal × (APY ÷ 365) × Number of days

This gives you a rough estimate for a specific period. For example: if you have $5,000 in an account with 4.50% APY and you want to know what you earn in 30 days, the math is $5,000 × (0.045 ÷ 365) × 30 = $18.49. This is an approximation because banks use slightly different methods, but it is close enough to understand what you are earning.

For a full year with daily compounding, the formula is more complex, which is why banks use the APY number instead. The APY already does the heavy lifting and tells you the true annual return. If you want the exact amount for a year, multiply your principal by (1 + APY) and subtract the principal—so $5,000 × (1 + 0.045) − $5,000 = $225.

How deposits and withdrawals affect your earnings

Banks calculate interest on your average daily balance during the month, not on the balance at the end of the month. This means every deposit increases what you earn that day forward, and every withdrawal reduces it. If you deposit $10,000 on the first day of the month and withdraw it on the last day, you earn interest on the full amount for the entire month. If you withdraw it on day 15, you earn interest on the full amount for only 15 days, then on zero for the remaining days.

Some banks use a different method called the daily balance method, where they calculate interest each day on whatever balance you have that day, then add all those daily amounts together at the end of the month. The result is nearly identical to the average daily balance method, but the daily method is slightly more precise if your balance changes frequently.

Why your actual earnings may differ from estimates

When you calculate interest yourself, you will often get a number slightly different from what the bank shows. This happens for several reasons. Banks may use 360 days instead of 365 in their formula. They may round interest to the nearest cent. They may calculate interest on the balance at the end of each day rather than the average, or they may use a different compounding schedule than you assumed.

The difference is usually small—a few cents on a typical balance—but it adds up over time. The bank's Deposit Account Agreement explains exactly which method they use. If you want to verify the bank's calculation, ask them to show you the formula they applied. Most banks will provide this information if you request it.

Comparing interest rates across banks

Because APY accounts for compounding, you can compare APYs directly across different banks without doing any math yourself. A 4.50% APY at one bank will earn you the same amount as a 4.50% APY at another bank, assuming your balance and the time period are the same. The only reason to choose one bank over another based on interest is if one offers a higher APY.

Interest rates change frequently—sometimes weekly—so the rate you see today may not be the rate you get when you open an account. Banks are required to honor the APY shown at the time you open the account for at least 30 days. After that, they can change the rate with notice, usually 30 days. If rates drop, your earnings drop with them. If rates rise, you benefit from the increase.

How compounding builds wealth over time

Compounding is powerful over long periods because you earn interest on interest. In year one, you earn interest on your principal. In year two, you earn interest on your principal plus the interest from year one. By year five or ten, the effect becomes noticeable. A $10,000 deposit at 4.50% APY grows to $10,450 after one year, $10,920 after two years, and $12,462 after five years—even with no additional deposits.

The longer you leave money untouched, the more compounding works in your favor. This is why savings accounts are better for money you will not need for several years. For money you need soon, the interest earned is small, but the safety and liquidity of a savings account still matter more than the return.

Frequently Asked Questions

Is the APY the same as the interest rate?

No. The interest rate is the base percentage, while APY includes the effect of compounding over a year. APY is always higher and is what you should use to compare accounts across banks.

How often does the bank add interest to my account?

Most banks calculate interest daily but add it to your account monthly. Some add it quarterly or annually. Check your Deposit Account Agreement to see the exact schedule for your account.

Can I lose money if interest rates drop?

No. Your principal is protected. If rates drop, you straightforward earn less interest going forward, but the money you already have stays in the account. Banks cannot take away what you have already earned.

What happens to my interest if I withdraw money mid-month?

You earn interest only on the balance you held for each day of the month. If you withdraw money on day 15, you earn interest on the full balance for 15 days and a lower balance for the remaining days.

Why do different banks offer different APYs?

Banks set their own rates based on how much they need deposits and what they can earn by lending that money out. Online banks often offer higher rates because they have lower overhead costs than brick-and-mortar branches.