How your bank calculates what you earn

Your bank calculates savings account interest by multiplying your balance by the annual interest rate, then dividing by the number of days in a year. The result is what you earn each day. Most banks compound this interest daily or monthly, meaning they add yesterday's interest to your balance before calculating today's interest — so you earn interest on your interest.

The actual formula banks use is: Daily Interest = (Account Balance × Annual Interest Rate) ÷ 365. If you have $10,000 in an account earning 4.5% annually, you earn roughly $1.23 per day. That daily amount gets added to your balance, and the next day's calculation includes it.

The timing matters because interest accrues from the day your deposit clears, not the day you make it. A deposit that clears on a Friday starts earning interest that day. A withdrawal stops earning interest when ready — you lose interest on that money from the moment it leaves your account.

Key Takeaways

  • Daily interest is calculated by multiplying your balance by the annual rate and dividing by 365, so higher balances and higher rates both increase what you earn each day.
  • Compounding means your bank adds accrued interest to your balance regularly (usually daily or monthly), so you earn interest on that interest in future periods.
  • Interest starts accruing the day a deposit clears and stops the moment a withdrawal leaves your account, so timing of deposits and withdrawals directly affects your total earnings.
  • The stated annual percentage yield (APY) already accounts for compounding, so you do not need to calculate the compounding effect yourself — that is what APY shows.

The difference between APR and APY

APR (annual percentage rate) is the straightforward interest rate without compounding. APY (annual percentage yield) is what you actually earn after compounding is factored in. Banks are required to show you the APY, because that is the real number.

For example, a savings account might advertise 4.5% APR compounded daily. The actual APY would be slightly higher — around 4.60% — because of compounding. The difference grows larger with higher rates and more frequent compounding. When you see an interest rate advertised for a savings account, it is almost always the APY, which is the number you should use to compare accounts.

How compounding frequency changes what you earn

Banks can compound interest daily, monthly, quarterly, or annually. Daily compounding is most common for savings accounts and produces the highest earnings because interest gets added to your balance more often, and each addition becomes part of the next calculation.

The difference between daily and monthly compounding is small on most balances — roughly 0.04% to 0.08% in additional earnings per year — but it adds up over time. A $50,000 balance earning 4.5% APY compounded daily versus monthly would earn roughly $20 to $40 more per year with daily compounding. Banks that compound daily are slightly better, but the rate itself matters far more than the compounding frequency.

What happens to interest when you withdraw money

Interest stops accruing the moment your withdrawal clears. If you withdraw $5,000 on the 15th of the month, you lose interest on that $5,000 from the 15th forward, even if you deposit it back on the 20th. The bank calculates interest only on the money that was actually in the account each day.

Some accounts have minimum balance requirements tied to interest rates. If your balance drops below the minimum, the bank may lower your rate or stop paying interest entirely. Read your account agreement to see whether your rate changes if your balance falls below a certain threshold.

How to estimate your annual interest earnings

To estimate what you will earn in a year, multiply your average balance by the APY. If you keep $25,000 in an account earning 4.5% APY, you earn roughly $1,125 per year. This is an estimate because your actual balance probably fluctuates — deposits and withdrawals change what you earn each day.

For a more precise calculation, add up your balance at the end of each day for a month, divide by the number of days, then multiply by the APY and by 12. This gives you a realistic picture of what a typical month earns, which you can scale to a full year. Most banks show you the interest earned in your monthly statement, so you can also look at recent months and multiply by 12 to see the pattern.

Why rates change and how it affects your earnings

Banks adjust savings account rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks typically raise savings rates within days or weeks. When the Fed cuts rates, banks cut savings rates more slowly — sometimes weeks or months later. Your earnings will change whenever your bank changes its rate.

If your account rate drops from 4.5% to 4.0%, a $25,000 balance now earns $125 less per year. You do not have to accept the new rate — you can move your money to another bank offering a higher rate. Banks do not penalize you for moving savings to a competitor, though some require you to keep a minimum balance to earn the advertised rate.

Interest on money market accounts and CDs

Money market accounts work the same way as savings accounts — interest accrues daily and compounds, usually monthly. The main difference is that money market accounts often have higher rates in exchange for higher minimum balances or limits on how often you can withdraw.

Certificates of deposit (CDs) lock in a fixed rate for a set term — 3 months, 6 months, 1 year, 5 years, and so on. Interest accrues the same way, but you cannot withdraw the money before the term ends without paying a penalty. The rate does not change during the term, so you know exactly what you will earn. A $10,000 CD earning 5.0% APY for 1 year earns $500, regardless of what happens to rates during that year.

Frequently Asked Questions

Do I earn interest on interest in a savings account?

Yes. When your bank compounds interest, it adds accrued interest to your balance, and the next interest calculation includes that added amount. This is compounding, and it means you earn interest on your interest. The APY already accounts for this effect, so you do not need to calculate it separately.

What is the difference between daily and monthly compounding?

Daily compounding adds interest to your balance every day; monthly compounding adds it once a month. Daily compounding produces slightly higher earnings because interest gets added more often and starts earning interest sooner. The difference is usually less than 0.1% per year on most balances, so the interest rate itself matters more than compounding frequency.

Can I lose interest if I withdraw money early?

You do not lose interest you have already earned, but you stop earning interest on the withdrawn amount from the moment it leaves your account. If you withdraw $5,000 on the 15th, you earn interest only on the remaining balance from that day forward. Certificates of deposit are different — early withdrawal triggers a penalty that can erase months of interest earnings.

Why does my bank show a different interest rate than what I calculated?

Your balance probably changed during the month, so your actual earnings depend on what was in the account each day, not just the ending balance. Banks calculate interest daily and add it to your balance, so the amount you earn compounds. Your statement shows the exact interest earned; if it does not match your estimate, your balance fluctuated more than you expected.

Does the interest rate stay the same forever?

No. Banks change savings rates regularly based on Federal Reserve decisions and competition. Your rate can go up or down, and you will see the change reflected in your next statement. You can move your money to a different bank if your rate drops and you find a better offer elsewhere.