The two formulas banks use to calculate your interest

Banks calculate savings account interest in one of two ways: simple interest or compound interest. Simple interest pays you a percentage of your starting balance once per year. Compound interest pays you interest on your interest — meaning each time interest is added to your account, the next interest payment is calculated on the larger total. Almost all savings accounts use compound interest, and it grows your money faster.

The difference matters. On $1,000 at 4% annual interest, simple interest gives you $40 per year forever. Compound interest (compounded daily, which is common) gives you about $40.80 in the first year, then slightly more each year after that, because you earn interest on the interest already in your account.

Your bank's disclosure documents will tell you which method they use and how often they compound — usually daily, monthly, or quarterly. The more frequently interest compounds, the more you earn.

Key Takeaways

  • Simple interest pays a fixed percentage of your starting balance once per year; compound interest pays interest on your interest and grows faster.
  • The Annual Percentage Yield (APY) on your account disclosure already accounts for compounding, so you can compare accounts directly without doing math.
  • To estimate your earnings, multiply your balance by the APY and divide by the number of times interest compounds per year.
  • Your actual interest may vary if you deposit or withdraw money during the year, because interest is calculated on your daily or monthly balance.
  • Banks must show you the APY in writing before you open an account, so you can compare rates across institutions.

How to use APY to find your interest without doing calculations

Your bank publishes an Annual Percentage Yield (APY) for each savings account. This is the real rate of return you will earn in one year, and it already includes the effect of compounding. You do not need to do any math — the APY is the number to use when comparing accounts.

If your account has an APY of 4.50% and you keep $10,000 in it for one full year without deposits or withdrawals, you will earn approximately $450 in interest. If the APY is 2.25%, you will earn approximately $225 on the same $10,000.

The APY changes over time because banks adjust their rates based on the Federal Reserve's interest rate decisions. When the Fed raises rates, banks typically raise APYs on savings accounts. When the Fed cuts rates, APYs fall. Your bank will notify you of rate changes, though you may want to check their website periodically to see if your current account is still competitive.

The simple interest formula and when to use it

Simple interest is calculated as: Interest = Principal × Rate × Time. Principal is your starting balance, Rate is the annual interest rate (as a decimal), and Time is the number of years.

Example: You have $5,000 in an account with a 3% simple interest rate. After one year, you earn $5,000 × 0.03 × 1 = $150. After two years, you earn $5,000 × 0.03 × 2 = $300 total (not $300 per year).

Most savings accounts do not use simple interest, so this formula is mainly useful for understanding how much slower simple interest grows compared to compound interest. Some certificates of deposit (CDs) or older savings products may use it, so check your account documents if you want to verify which method applies to you.

How compound interest works and why it accelerates over time

Compound interest is calculated on the principal plus all previously earned interest. The formula is: Final Balance = Principal × (1 + Rate/Compounds)^(Compounds × Time). Compounds is how many times per year interest is added to your account.

Example: $5,000 at 4% APY compounded daily (365 times per year) for one year. Final Balance = $5,000 × (1 + 0.04/365)^(365 × 1) = approximately $5,204.08. You earned $204.08 in interest, which is more than the $200 you would earn with simple interest at the same rate.

The longer your money stays in the account, the more compound interest accelerates your growth. After 10 years at 4% compounded daily, $5,000 grows to approximately $7,459. After 20 years, it grows to approximately $11,127. The compounding effect becomes more visible over decades, which is why starting a savings account early matters.

Why your actual interest may differ from the calculation

The calculations above assume your balance stays the same for the entire year. In reality, most people deposit or withdraw money during the year, which changes how much interest they earn. Banks calculate interest on your daily balance or monthly balance, depending on the account.

If your account uses daily balance compounding, the bank calculates interest on whatever you have in the account each day, then adds that interest daily. If you deposit $2,000 on day 15 of the month, you earn interest on the larger balance starting day 15. If you withdraw $1,000 on day 20, your interest drops for the remaining days of the month.

Your bank statement will show the total interest you earned each month. This is the actual number — it already accounts for all your deposits, withdrawals, and the compounding method your bank uses. You do not need to recalculate it yourself.

How to find your account's APY and compounding frequency

Your bank must provide the APY and compounding frequency in writing before you open an account. This information appears on the Truth in Savings disclosure, which is a standardized document all banks must give you. You can request this document in person, by phone, or download it from the bank's website.

The disclosure also shows the minimum balance required to earn the stated APY, any monthly fees, and what happens if your balance falls below the minimum. Some accounts offer a higher APY only if you maintain a certain balance or set up direct deposit.

If you already have an account, log into your online banking portal or call your bank's customer service line to ask for the current APY. Rates change frequently, so the APY you opened with may not be the APY you earn today. Many banks show the current rate on their website under "Savings Accounts" or "Rates & Fees".

Comparing interest across different banks and account types

Because all banks must disclose APY the same way, you can compare accounts directly. A savings account at Bank A with a 4.75% APY will earn more than a savings account at Bank B with a 3.50% APY, assuming the same balance and time period. You do not need to calculate anything — just compare the APY numbers.

Online banks typically offer higher APYs than traditional brick-and-mortar banks because they have lower overhead costs. As of the time this article was written, online savings accounts ranged from 4% to 5.35% APY, while many traditional banks offered 0.01% to 0.50%. These rates change constantly, so check current rates on bank websites or rate comparison sites before opening an account.

Money market accounts and high-yield savings accounts are different products, but they work the same way for interest calculation — the bank publishes an APY, and you multiply your balance by that rate to estimate your earnings. CDs (certificates of deposit) also use APY, but they lock your money away for a set term (3 months, 1 year, 5 years, etc.) and penalize you if you withdraw early.

Frequently Asked Questions

Does the interest I earn count as income for taxes?

Yes. Interest earned in a savings account is taxable income. If you earn $100 or more in interest during the year, your bank will send you a 1099-INT form by January 31st, and you must report that interest on your tax return. Even if you earn less than $100, you should still report it if you owe taxes.

What if I withdraw money before the year ends?

You still earn interest on the money you had in the account. If you deposit $5,000 on January 1st and withdraw $2,000 on July 1st, you earn interest on $5,000 for six months and $3,000 for the remaining six months. The bank calculates this automatically based on your daily or monthly balance.

Can I lose money if interest rates fall?

No. Interest rates falling means your bank will lower the APY on new deposits or when your current rate period ends, but you will not lose the money you already have. Your balance stays the same; you just earn less interest going forward.

Is there a difference between APR and APY?

Yes. APR (Annual Percentage Rate) does not account for compounding, while APY (Annual Percentage Yield) does. For savings accounts, always use the APY to compare rates, because it shows the real return you will earn. APR is used for loans and credit cards.

How often should I check my interest earnings?

Check your account statement monthly to verify the interest was deposited. If you notice the interest is much lower than expected, your balance may have dropped, the rate may have changed, or there may be a fee reducing your earnings. Contact your bank if something looks wrong.