How your bank calculates the interest you earn

Banks calculate interest using one of two methods: simple interest or compound interest. Simple interest is rare now. Compound interest is what most savings accounts, money market accounts, and certificates of deposit (CDs) use, and it means you earn interest on your interest.

The calculation starts with your annual percentage yield (APY) — the rate your bank publishes. The bank divides that rate by the number of times per year it compounds (daily, monthly, or quarterly), then applies that smaller rate to your balance. Each time it compounds, the new interest gets added to your balance, and the next calculation includes that addition. Over a year, this compounds into more money than simple interest would give you.

Your actual earnings depend on three things: how much money sits in the account, what APY the bank offers, and how often the bank compounds. A $10,000 balance at 4.5% APY compounded daily will earn more than the same balance at 4.5% compounded monthly, because daily compounding adds interest to your balance more often.

Key Takeaways

  • Compound interest means you earn interest on the interest already added to your account, which is why the same APY produces different earnings depending on how often the bank compounds.
  • APY already includes the effect of compounding, so you can compare rates directly between banks without doing math yourself.
  • Daily compounding produces slightly more earnings than monthly or quarterly compounding on the same APY, but the difference shrinks as rates fall.
  • Your bank must disclose the APY and compounding frequency in the account terms or on the rate sheet before you open the account.

The simple interest formula and when it applies

Simple interest uses this formula: Interest = Principal × Rate × Time. If you deposit $5,000 at 3% simple interest for one year, you earn $150 and end with $5,150. The next year, you earn $150 again on the original $5,000 — not on the $5,150.

Almost no retail savings accounts use simple interest anymore. You might see it on some money market accounts or very old CDs, but even those are rare. The reason banks moved away from it is that compound interest earns them more money on their side of the ledger, so they can afford to offer it to you.

If your account terms say "simple interest," the bank will tell you the rate and the compounding period. Read the disclosure document before opening the account — it will say "simple" or "compound" explicitly.

How compound interest actually grows your money

Compound interest works by adding earned interest back into your balance, then calculating the next interest payment on the larger number. If you start with $10,000 at 4% APY compounded annually, after one year you have $10,400. In year two, the bank calculates 4% on $10,400, not $10,000, so you earn $416 instead of $400.

The more often the bank compounds, the more you earn. Daily compounding divides the annual rate by 365, then applies that tiny rate to your balance every day. Monthly compounding divides by 12 and applies monthly. Over a full year, daily compounding produces slightly more money because interest gets added to your balance more often, and each new calculation includes the previous interest.

The difference between daily and monthly compounding is small at current rates. On $10,000 at 4.5% APY, daily compounding earns about $6 more per year than monthly compounding. The gap widens at higher rates and with larger balances, but it narrows when rates fall below 2%.

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

Annual Percentage Yield (APY) is the rate your bank publishes, and it already includes the effect of compounding. This is why you should always compare APYs between banks, not the "stated rate" or "nominal rate" — APY does the compounding math for you.

If Bank A offers 4.5% APY and Bank B offers 4.5% APY, you will earn the same amount in one year, regardless of how often each bank compounds. The APY accounts for that difference. The bank calculates APY using this formula: APY = (1 + periodic rate)^number of periods − 1. You do not need to do this math yourself — the bank publishes the APY in all account disclosures.

Banks must disclose the APY in the Truth in Savings Act disclosure, which they provide before you open the account. The disclosure also lists the compounding frequency, the minimum balance required to earn the stated rate, and any fees that reduce your earnings.

How to estimate your earnings without a calculator

For a rough estimate, use the Rule of 72: divide 72 by the APY to find how many years it takes your money to double. At 4% APY, your balance doubles in about 18 years (72 ÷ 4 = 18). This is not exact, but it gives you a sense of how compound interest works over time.

For a more precise estimate, most banks provide an interest calculator on their website. You enter your starting balance, the APY, and how long you plan to keep the money, and the calculator shows your ending balance. This accounts for the exact compounding frequency and any deposits or withdrawals you plan to make.

If you want to calculate by hand, the formula is: Final Balance = Principal × (1 + APY)^years. For $10,000 at 4.5% for 5 years: $10,000 × (1.045)^5 = $12,462. This assumes you make no deposits or withdrawals and the rate stays the same — most savings accounts have variable rates that change with the market.

Why your actual earnings might differ from the stated APY

Your earnings match the stated APY only if your balance stays the same all year and the rate does not change. Most savings accounts have variable rates, which means the bank can lower the APY whenever it wants. When the Federal Reserve cuts interest rates, banks typically lower their APYs within days or weeks.

If you deposit money partway through the year, you earn interest only on the balance you actually held. If you withdraw money, the bank stops earning interest on that amount. Some banks use the "average daily balance" method, which calculates interest based on your average balance over the month or quarter, not your ending balance.

Fees also reduce your earnings. If your account charges a monthly maintenance fee, that fee comes out of your interest. A $5 monthly fee on a $10,000 balance earning 4.5% APY costs you $60 per year — about 27% of your annual interest. Always check whether the account has fees and whether they apply to your balance size.

Comparing interest rates across different account types

Different account types earn different rates. High-yield savings accounts currently offer the highest rates for everyday savings, typically 4% to 5% APY. Traditional savings accounts at large banks offer much less, often under 0.5% APY. Money market accounts fall in the middle, usually 3% to 4.5% APY. Certificates of deposit (CDs) lock your money away for a set term — 3 months, 1 year, 5 years — and pay a fixed rate that does not change.

The trade-off is liquidity. High-yield savings accounts let you withdraw money anytime without penalty. CDs charge a penalty if you withdraw before the term ends, typically equal to several months of interest. Money market accounts usually allow a few withdrawals per month before penalties kick in.

When comparing rates, look at the APY, not the stated rate. Check whether the rate is fixed (locked in for the account's life) or variable (can change). Read the fee schedule to see whether maintenance fees, withdrawal penalties, or minimum balance requirements apply to you.

Frequently Asked Questions

Does compound interest work the same way at every bank?

The compounding frequency varies — some banks compound daily, others monthly or quarterly. But because banks publish the APY, which already includes compounding, you earn the same amount at the same APY regardless of compounding frequency. Compare APYs directly between banks, not the stated rates.

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

You earn interest only on the balance you held. If you deposit $10,000 on January 1 and withdraw $5,000 on July 1, the bank calculates interest on the average of those balances for the year, or on the daily balance depending on the account terms. The disclosure document explains which method your bank uses.

Can a bank lower my APY without warning?

Yes, if your account has a variable rate. Banks can lower the rate anytime, though most notify you before the change takes effect. Fixed-rate CDs lock in the rate for the full term — the bank cannot lower it. Savings and money market accounts are almost always variable.

Is the interest I earn taxed?

Yes. Interest income is taxable as ordinary income. Banks send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. This is separate from the interest calculation — the bank pays you the full APY, and you handle taxes when you file.

Why do high-yield savings accounts pay so much more than regular savings accounts?

High-yield accounts are usually offered by online banks with lower overhead costs, so they pass the savings to customers through higher rates. Traditional banks at physical branches have higher costs and offer lower rates to offset them. The money is equally safe — both types of banks are FDIC-insured up to $250,000.