What Is Interest?
Interest is the cost of borrowing money or the reward for saving it. When you deposit money in a savings account, the bank pays you interest for the use of your funds. When you take out a loan, you pay interest to the lender. The rate is expressed as a percentage of the principal amount per year.
As of July 2026, the Federal Reserve has maintained the federal funds rate at 3.50% to 3.75%. This influences the rates you see on savings accounts, CDs, mortgages, and personal loans. High-yield savings accounts currently offer 4% to 4.5% APY, down from the 5%+ peak in 2024 but still well above the 0.5% rates common before 2022. CD rates range from 3.5% to 4.5% depending on term length. Mortgage rates sit around 6.5% for a 30-year fixed. Personal loan rates typically range from 10% to 36% depending on credit score.
This calculator supports both simple and compound interest calculations, giving you a clear comparison of how each method affects your money over time. For more specialized calculations, try our Compound Interest Calculator, Simple Interest Calculator, or Savings Calculator.
What This Calculator Does
Inputs Required
- Principal Amount: The starting amount of money you are depositing or borrowing
- Annual Interest Rate: The yearly rate at which interest accrues, expressed as a percentage
- Time Period: The number of years the interest will accumulate
- Interest Type: Simple or compound, which affects how interest builds each period
Outputs Provided
- Total Interest Earned: The cumulative interest accrued over the full period
- Final Balance: Your principal plus all interest earned
- Year by Year Breakdown: A chart showing how interest grows annually
How the Calculation Works
There are two primary methods for calculating interest, and the difference between them can be significant over long periods.
Simple Interest
Interest = Principal x Rate x Time
Simple interest is calculated only on the original principal. Each year, you earn the same fixed amount of interest. It does not grow because past interest is not added to the principal for future calculations. Simple interest is commonly used for short-term personal loans, auto loans, and some bonds.
Compound Interest
Balance = Principal x (1 + Rate)^Time
Compound interest is calculated on the principal plus any previously earned interest. Each year, your interest base grows, so the interest earned also grows. This "interest on interest" effect is why compound interest produces significantly higher returns over time. Albert Einstein is often quoted as calling compound interest "the eighth wonder of the world." Whether he actually said this is debated, but the math speaks for itself. Most savings accounts, CDs, and investment accounts use compound interest.
How to Use the Calculator
- Select either Simple Interest or Compound Interest using the tabs
- Enter your starting principal amount using the slider or input field
- Set the annual interest rate you expect to earn or pay. For context, high-yield savings accounts currently offer 4% to 4.5%, while 30-year mortgage rates are around 6.5%
- Choose the number of years for the calculation
- View your total interest and final balance instantly
- Review the chart to see how interest builds year by year
Example Calculations
Example 1: Savings Account Growth
Emily deposits $10,000 in a high-yield savings account offering 4.25% APY. Over 10 years with compound interest:
- Simple interest: $10,000 x 4.25% x 10 = $4,250 interest, final balance of $14,250
- Compound interest: $10,000 x (1.0425)10 = $15,202 final balance, earning $5,202 in interest
- The compound interest option earns $952 more over 10 years
Example 2: Long-Term Investment Growth
Robert invests $25,000 in a diversified portfolio averaging 7% annual returns. Over 30 years with compound interest:
- Simple interest: $25,000 x 7% x 30 = $52,500 interest, final balance of $77,500
- Compound interest: $25,000 x (1.07)30 = $190,127 final balance, earning $165,127 in interest
- The compound interest option earns $112,627 more over 30 years
This is why investment returns compound dramatically over long horizons. The gap between simple and compound interest widens exponentially with time.
Example 3: Evaluating a Personal Loan
James is offered a personal loan with a 12% annual simple interest rate over 3 years on $8,000. The calculator shows he will owe $2,880 in interest, for a total repayment of $10,880. If the same loan used compound interest, the total would be $11,240. He should confirm which method the lender uses before signing. For amortizing loans with monthly payments, use our Loan Calculator instead.
Real World Scenarios
Comparing Savings Products
Lisa has $15,000 to save. Bank A offers a high-yield savings account at 4.25% APY compounded annually. Bank B offers a 12-month CD at 4.5% APY. Bank C offers a traditional savings account at 0.45% APY. Over 5 years, Bank A grows her money to $18,463, Bank B to $18,769 (if renewed at the same rate), and Bank C to $15,341. The difference between Bank A and Bank C is over $3,100, simply from choosing a different savings product. This is why keeping money in a traditional low-yield savings account is so costly over time.
Assessing Real Returns After Inflation
Tom earns 4.25% on his high-yield savings account. Inflation for the 12 months ending June 2026 was 3.5%. His real return (nominal return minus inflation) is only 0.75%. On $50,000 in savings, that is just $375 per year of real wealth growth. To meaningfully grow wealth, he needs investments with higher long-term returns. Use our Inflation Calculator to quantify the erosion of purchasing power over any period.
Credit Card Interest vs. Investment Returns
Maria carries $8,000 in credit card debt at 24% APR. She also has $8,000 in a savings account earning 4.25%. She is earning $340 per year on savings while paying $1,920 per year in credit card interest. Her net position is negative $1,580 per year. This calculator makes the math obvious: paying off the credit card debt is equivalent to a guaranteed 24% return, far better than any investment. For credit card payoff planning, use our Credit Card Calculator.
Common Mistakes to Avoid
- Confusing simple and compound interest: Many people assume all interest works the same way. Always confirm which type applies to your account or loan. Savings accounts use compound interest, most personal loans use simple interest
- Ignoring compounding frequency: Interest can compound daily, monthly, or annually. More frequent compounding means faster growth. A 4.25% rate compounded daily yields an effective annual rate of 4.34%. This calculator uses annual compounding
- Using nominal vs. effective rate: The stated rate (APR) and the effective annual rate (APY) may differ when compounding more than once per year. Always compare APY to APY when evaluating savings products
- Forgetting fees: Some savings accounts charge monthly maintenance fees that reduce your net interest earned. A 4.25% APY with a $10 monthly fee on a $5,000 balance effectively reduces your yield to 4.0%
- Not comparing to inflation: Earning 4.25% when inflation is 3.5% gives a real return of only 0.75%. Always evaluate interest rates against current inflation, not in isolation
Limitations of This Calculator
This calculator uses annual compounding for the compound interest option. In practice, most savings accounts compound daily or monthly, which produces slightly higher returns. It does not account for taxes on interest income, which are due at your marginal federal rate plus state tax. It does not handle recurring contributions or withdrawals. For those scenarios, use our Savings Calculator or Investment Calculator. For loan amortization schedules with monthly payments, use our Loan Calculator.
Authoritative Research & Resources
- Federal Reserve - Selected Interest Rates (H.15) - The Fed's daily report on key interest rates including the federal funds rate, prime rate, and Treasury yields. This is the primary source for tracking the current interest rate environment that influences savings, CD, and loan rates.
- FDIC - Consumer News and Resources - The Federal Deposit Insurance Corporation provides guidance on savings accounts, CD ladders, and understanding APY vs. APR. Useful for evaluating which savings products offer the best real returns.
- Consumer Financial Protection Bureau - The CFPB offers tools and resources for comparing loan offers, understanding interest rates, and avoiding predatory lending. Particularly useful for evaluating personal loan and credit card interest rates.