The two ways banks calculate savings interest

Banks use either simple interest or compound interest to calculate what you earn. 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 that larger total.

Most savings accounts today use compound interest, usually compounded daily or monthly. This means your money grows faster than with simple interest, because you earn returns on returns. The difference is small in the first month but becomes noticeable over years.

Your bank's disclosure documents will state which method they use and how often they compound. You can also call and ask directly — the answer is always one of: daily, monthly, quarterly, or annually.

Key Takeaways

  • Simple interest multiplies your starting balance by the annual rate; compound interest multiplies a growing balance, so you earn more over time.
  • The formula for simple interest is: (starting balance × annual rate ÷ 100) × number of years.
  • The formula for compound interest is: starting balance × (1 + rate ÷ 100 ÷ compounds per year) raised to the power of (compounds per year × number of years).
  • Your bank compounds interest daily, monthly, quarterly, or annually — check your account agreement or call to find out which.
  • Online calculators can do the math for you, but understanding the formula helps you compare accounts and spot errors.

Calculating simple interest by hand

Simple interest is the easiest to calculate because the rate stays the same every year. The formula is: starting balance × annual interest rate ÷ 100 × number of years.

Example: You deposit $5,000 in an account that pays 4% simple interest per year. After one year, you earn $5,000 × 4 ÷ 100 × 1 = $200. After three years, you earn $5,000 × 4 ÷ 100 × 3 = $600. Your account balance would be $5,600.

Simple interest is rare in savings accounts now, but some money market accounts and certificates of deposit (CDs) use it. If your account statement shows interest added once per year and the amount is always the same, you have simple interest.

Calculating compound interest by hand

Compound interest uses a more complex formula because the balance grows each time interest is added. The formula is: starting balance × (1 + annual rate ÷ 100 ÷ compounds per year) raised to the power of (compounds per year × number of years).

Example: You deposit $5,000 in an account that pays 4% interest compounded monthly. The calculation is: $5,000 × (1 + 4 ÷ 100 ÷ 12) to the power of (12 × 1). That breaks down to: $5,000 × (1.00333) to the power of 12 = $5,204.04. After one year, you earn $204.04 instead of $200 — a small difference, but it grows larger over time.

If you compound daily instead of monthly, the number changes slightly: $5,000 × (1 + 4 ÷ 100 ÷ 365) to the power of (365 × 1) = $5,204.08. Daily compounding earns you four cents more in this case.

Most people use a calculator or spreadsheet for compound interest because the exponent (the "to the power of" part) is tedious to do by hand. Many online banks show your projected balance on the account page itself.

Using a spreadsheet or calculator

If you use Excel or Google Sheets, you can build a simple compound interest calculator. In Google Sheets, the formula looks like: =starting_cell * (1 + rate/100/compounds_per_year)^(compounds_per_year*years). Replace the cell references with your numbers and it calculates instantly.

Online calculators exist specifically for this purpose — search "compound interest calculator" and you will find dozens. Enter your starting balance, annual rate, how often interest compounds, and how many years you plan to keep the money. The calculator shows your final balance and total interest earned.

Your bank's website may also have a calculator built in. Log into your account and look for a "savings calculator" or "interest calculator" link, usually under Tools or Resources.

Why the annual percentage yield (APY) matters more than the rate

Banks advertise two numbers: the annual percentage rate (APR) and the annual percentage yield (APY). The APR is the raw interest rate. The APY is what you actually earn after compounding is factored in.

If a bank offers 4% APR compounded daily, the APY will be slightly higher — usually around 4.08% — because compounding adds extra earnings. When you compare two accounts, always compare the APY, not the APR. A 4% APY at one bank beats a 4% APR at another bank, even if the numbers look the same.

Your account statement and the bank's disclosure documents will show both numbers. The APY is what you should use when calculating how much you will actually earn.

What happens if interest rates change

Most savings accounts have variable rates, meaning the bank can change the rate whenever it wants. If your bank lowers the rate, your interest earnings drop immediately — the next compounding period uses the new, lower rate.

If you calculated your earnings based on today's rate and the rate drops in three months, your actual earnings will be less than you predicted. This is why long-term projections are estimates, not guarantees.

Certificates of deposit (CDs) lock in a fixed rate for a set term, so your calculation will be accurate. Savings accounts and money market accounts do not lock in rates, so recalculate every few months if rates are changing.

Checking your bank's math

Your monthly or quarterly statement should show the interest added to your account. You can verify this is correct by calculating what you expect to earn and comparing it to what the bank shows.

If your account shows $5,000 and the bank pays 4% APY compounded monthly, you should earn roughly $16.67 in the first month ($5,000 × 4 ÷ 100 ÷ 12). If the statement shows $5.00 or $50.00, something is wrong — call the bank and ask them to explain the calculation.

Banks rarely make errors, but it happens. Checking the math yourself is the only way to catch them.

Frequently Asked Questions

Does compound interest really make a big difference?

Over one or two years, the difference between simple and compound interest is small — usually a few dollars on a $5,000 balance. Over 10 or 20 years, compound interest adds hundreds or thousands of dollars. The longer your money sits, the more compounding matters.

What if my bank compounds interest quarterly instead of daily?

Quarterly compounding (four times per year) earns you less than daily compounding (365 times per year), but the difference is usually less than $10 per year on a typical savings account. Compare the APY between accounts to see the real difference — that number already includes the compounding frequency.

Can I calculate interest if the rate changed during the year?

Yes, but you have to break the year into periods. Calculate interest for the months the rate was 4%, then calculate interest for the months it was 3.5%, using the new balance as your starting point for the second calculation. Your bank does this automatically, so your statement will show the correct total.

Is there a difference between APY and APR I should know about?

APY includes the effect of compounding; APR does not. When comparing accounts, always use APY. When calculating how much you will earn, use APY. APR is mainly useful for loans, not savings accounts.

What if I add money to my account during the year?

Your interest calculation becomes more complex because each deposit earns interest for a different length of time. The easiest approach is to let your bank calculate it — your statement shows exactly what you earned. If you want to estimate, calculate interest on each deposit separately based on how many months it sat in the account.