Interest Calculator
Calculate simple and compound interest for savings accounts, CDs, credit cards, or loans. See how compounding frequency affects your wealth.
Key Features
- •Simple interest calculator (linear growth)
- •Compound interest calculator (exponential growth)
- •Flexible compounding periods: Daily, Weekly, Monthly, Quarterly, Semi-Annually, Annually
- •Future value projection with interactive line chart
- •Regular contribution addition (monthly or yearly)
- •Inflation-adjusted real return calculation
- •Credit card payoff simulator (minimum payment analysis)
- •CD (Certificate of Deposit) yield calculator
- •Savings goal planner (how much to save monthly)
- •Printable interest summary
About Interest Calculator
Interest is the price of money. Whether you are earning it on savings or paying it on debt, understanding the mechanics of interest is the single most important factor in building wealth or avoiding financial ruin. Our dual-mode Interest Calculator separates the two fundamental types: Simple Interest (linear growth) and Compound Interest (exponential growth). We also support 5 different compounding frequencies: Daily, Weekly, Monthly, Quarterly, and Annually.
Why Compounding Frequency Matters
The difference between annual and daily compounding on a $10,000 savings account over 20 years at 5% interest is $2,605. That is free money left on the table simply because the bank calculates interest daily versus yearly. Our tool visualizes this with an interactive chart, allowing you to slide the compounding frequency and watch the future value change in real-time.
Simple Interest Explained
Simple interest is calculated only on the principal amount. Formula: I = P × r × t. If you lend a friend $1,000 at 5% simple interest for 3 years, they owe you $150 in interest. It is straightforward and used for short-term loans, bonds, and some car loans. Use our Simple mode to calculate interest on a promissory note or to verify bank CD returns for terms under 1 year.
Compound Interest Explained
Compound interest is interest on interest. It is the engine behind wealth creation. Albert Einstein reportedly called it the "eighth wonder of the world." Formula: A = P (1 + r/n)^(nt). Our Compound mode includes a "Regular Contributions" field—the secret to millionaire status. Contributing just $200/month to an account earning 8% compounded monthly will grow to over $600,000 in 30 years, even if you start with zero.
Credit Card Interest: The Dangerous Side of Compounding
Credit cards use daily compounding interest. If you carry a $5,000 balance at 22% APR, making only the minimum payment (often 2% of balance) will take you over 20 years to pay off and cost over $8,000 in interest. Use our calculator to input your credit card balance, rate, and monthly payment. The tool shows your "payoff date" and "total interest cost." This reality check motivates faster repayment.
FAQ: Interest Calculator
What is APY vs APR?
APY (Annual Percentage Yield) includes compound interest and is used for savings accounts. APR (Annual Percentage Rate) is the simple interest rate used for loans. The same rate will have a higher APY due to compounding. For example, 5% APR compounded monthly equals a 5.12% APY.
How do I calculate the interest on a late payment?
Input the overdue amount as principal, the penalty interest rate, and the number of days late. For a 60-day late payment on a $500 invoice at 10% annual simple interest: Interest = 500 × 0.10 × (60/365) = $8.22.
Interest Calculator is optimized for fast browser-based use, so you can test multiple scenarios in seconds.
Formula & Logic
- 01Simple interest uses I = P × r × t and future value FV = P + I where t is in years.
- 02Compound interest uses FV = P × (1 + r / m)^(m × t) with selectable compounding frequency m.
- 03Daily compounding uses m=365, Weekly uses m=52, Monthly uses m=12, Quarterly uses m=4, Annually uses m=1.
- 04Credit card minimum payment simulation: iterates balance reduction with interest accrual daily until balance < minimum payment threshold.
- 05Inflation-adjusted FV = Nominal FV / (1 + inflation rate)^years.
Practical Examples
- 01Baseline check: Use realistic inputs in Interest Calculator to generate a first-pass estimate.
- 02Sensitivity check: Change one key input at a time to compare how the output shifts.
- 03Decision check: Save two or more scenarios and use the differences to choose the better option.
Important Limitations
- •Results depend on the accuracy of your inputs.
- •Displayed values may be rounded for readability.
- •Financial outputs are planning estimates and do not include institution-specific fees unless provided.
Frequently Asked Questions
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