Banks multiply your balance by an interest rate, then divide by the number of days in a year

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest to your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360 on some accounts), equals one day's interest. That daily amount gets added to your balance, and tomorrow the bank calculates interest on the new, slightly larger balance.

The result is that your money grows faster than it looks. If you have $1,000 in an account earning 4.50% annual percentage yield (APY), you don't earn exactly $45 in the first year. You earn a bit more, because the interest itself starts earning interest partway through the year. That compounding effect is why the bank publishes an APY rather than just an interest rate—the APY already accounts for how often interest compounds.

The exact timing of when interest posts to your account varies by bank. Some post daily, some weekly, some monthly. The frequency doesn't change how much you earn over a year (because the APY already reflects that), but it does affect when you see the money appear in your balance.

Key Takeaways

  • Daily compounding means the bank calculates interest on your balance every day and adds it back, so interest earns interest throughout the year.
  • The annual percentage yield (APY) already includes the effect of compounding, so you can compare accounts directly by APY without doing extra math.
  • The actual interest rate (called the annual percentage rate or APR) is lower than the APY because APY accounts for compounding.
  • How often interest posts to your account (daily, weekly, or monthly) does not change your total earnings over a year, only when you see the money appear.
  • Your bank must disclose both the APY and the APR in writing before you open the account, usually in a document called a Truth in Savings disclosure.

The difference between APY and the interest rate

Banks publish two numbers: the interest rate (also called the annual percentage rate or APR) and the annual percentage yield (APY). The interest rate is what the bank actually pays on your money each year. The APY is what you actually earn, after compounding is factored in.

If a bank offers 4.50% APR with daily compounding, the APY will be slightly higher—often around 4.60% or 4.61%, depending on the exact compounding schedule. That difference is small on smaller balances but grows as your balance grows. Over ten years, that compounding difference can add up to hundreds of dollars on a large balance.

When you are comparing savings accounts, always compare the APY, not the interest rate. The APY is the true number that tells you how much you will earn.

How compounding frequency affects your earnings

Compounding can happen daily, weekly, monthly, quarterly, or annually. The more often interest compounds, the more you earn, because interest starts earning interest sooner. Daily compounding is the most common for savings accounts and produces the highest earnings.

The difference between daily and monthly compounding is real but small. On a $10,000 balance at 4.50% APY, daily compounding might earn you about $450 in a year, while monthly compounding might earn you about $448. The gap widens with larger balances and higher rates, but for most people with typical savings account balances, the difference is a few dollars per year.

Your bank's disclosure document will tell you the compounding frequency. If it does not say, call the bank and ask—it is a required disclosure, and they must provide it in writing.

What happens when interest rates change

Banks adjust interest rates on savings accounts based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks usually raise savings account rates within days or weeks. When the Fed cuts rates, banks cut savings account rates just as quickly, sometimes faster.

Your APY is not locked in. It can go up or down at any time, and the bank must notify you before the change takes effect. Most banks send an email or mail a notice a few days before the new rate begins. Some banks post the notice on their website instead. Check your account settings to see how your bank sends notices, and make sure your contact information is current.

If your bank cuts rates and you want to keep earning more, you can move your money to a different bank. There is no penalty for moving savings between banks, and you can do it online in most cases. High-yield savings accounts at online banks often pay more than traditional banks, especially when rates are falling.

How to calculate interest yourself

You can estimate your earnings using this formula: (Balance × APY) ÷ 365 = daily interest. Multiply that daily amount by the number of days you hold the money, and you get a rough total. This is an estimate because your balance changes as you add or withdraw money, but it gives you a sense of how much you are earning.

For example: a $5,000 balance at 4.50% APY earns roughly ($5,000 × 0.045) ÷ 365 = $0.62 per day. Over 30 days, that is about $18.60. Over a full year, it is about $225. Your actual earnings will be slightly higher because of compounding, but this calculation is close enough for planning purposes.

Most banks also show you a running total of interest earned in your online account. You can see how much you have earned so far this month or this year without doing any math. This number updates as interest posts to your account.

Interest on money market accounts and certificates of deposit

Money market accounts and certificates of deposit (CDs) use the same compounding formula as savings accounts, but the rates are often different. Money market accounts usually pay slightly more than savings accounts because you agree to keep a higher minimum balance. CDs pay more because you agree to leave your money untouched for a set period—three months, six months, one year, five years, or longer.

CDs lock in a fixed rate for the entire term. If you withdraw the money early, you pay a penalty, usually a few months' worth of interest. Savings accounts and money market accounts let you withdraw anytime without penalty, so their rates are lower to compensate.

The compounding frequency on CDs varies. Some compound daily, some monthly, some at maturity (meaning you get all the interest in one lump sum when the CD matures). The disclosure document will specify. For CDs, the APY already includes the compounding effect, so you can compare CDs directly by APY just like savings accounts.

Why your bank's interest rate might differ from the national average

Banks set their own rates based on their costs and competition. Online banks typically pay higher rates than brick-and-mortar banks because they have lower overhead—no branches, fewer employees, lower rent. Credit unions sometimes pay higher rates than banks because they are member-owned and return profits to members rather than shareholders.

Large national banks often pay lower rates because they have less incentive to compete for deposits. Smaller banks and online banks pay higher rates to attract customers. The difference can be significant: a large bank might pay 0.01% APY while an online bank pays 4.50% APY on the same type of account.

Your bank's rate also depends on the current interest rate environment. When the Fed is raising rates, banks raise savings rates to stay competitive. When the Fed is cutting rates, banks cut savings rates, sometimes faster than the Fed cuts. Shopping around every few months is worth your time if you have a large balance.

Frequently Asked Questions

Do I pay taxes on savings account interest?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form in January showing how much interest you earned the previous year. You report this on your tax return. The amount is usually small unless you have a very large balance or a high interest rate.

What is the difference between simple interest and compound interest?

Simple interest is calculated only on your original balance and never changes. Compound interest is calculated on your balance plus all the interest that has been added so far. Savings accounts use compound interest, which is why you earn more over time. The difference grows larger the longer you leave the money in the account.

Can a bank change my interest rate without telling me?

No. Banks must notify you before changing your rate, usually by email or mail. The notification must come before the change takes effect. You have the right to close your account if you disagree with the new rate, and you can move your money to another bank without penalty.

Why does my bank show interest earned but I do not see the money in my balance?

Interest posts to your account on a schedule set by your bank—daily, weekly, or monthly. Until it posts, it shows as earned but not yet added to your spendable balance. Once it posts, it becomes part of your balance and you can withdraw it. Check your account settings or call your bank to find out when interest posts.

Does keeping my money in a savings account longer earn me more interest?

Yes, as long as you do not withdraw it. The longer your money sits in the account, the more days it earns interest, and the more compounding happens. Even small daily interest amounts add up over months and years. Moving your money to a different account or bank resets the clock, but you do not lose any interest you have already earned.