Compound Interest Calculator – Investment Growth
Calculate compound interest and investment growth with our free calculator. Enter your starting balance, monthly contributions, interest rate, and time period to see how your investments grow. Includes yearly breakdown and comparison of compound vs. simple interest.
Compound Interest Calculator
How the Calculation Works
Formula: Compound interest grows exponentially based on principal, interest rate, compounding frequency, and time. Regular contributions accelerate growth.
This calculator uses standard compound interest mathematics to project investment growth with monthly contributions at your selected rate and compounding frequency.
Compounding Frequency: This calculator compounds interest monthly. Monthly contributions are assumed to occur at the beginning of each period.
Worked Example
Example: Invest $10,000 with $200 monthly contributions at 7% annual interest for 20 years (240 months), with monthly compounding.
- Starting Balance: $10,000
- Monthly Contributions: $200 (at beginning of period)
- Annual Interest Rate: 7% (0.583% monthly)
- Compounding Frequency: Monthly (12 times per year)
- Time Period: 20 years (240 months)
- Final Balance: Approximately $146,329
- Total Contributions: $10,000 + ($200 × 240) = $58,000
- Total Interest Earned: $146,329 - $58,000 = $88,329
The final balance of approximately $146,329 represents your total invested money plus the compound interest earned. This demonstrates how compound interest and regular contributions work together to build significant wealth over time. The $88,329 in interest earned is nearly 1.5 times your initial principal—the power of compound growth over two decades.
How to Use This Calculator
Key Factors: Compounding frequency, interest rate, time invested, and regular contributions all significantly impact final results. Higher rates and longer timeframes create exponential growth.
Choosing an Interest Rate: When using this calculator, enter an interest rate appropriate for your investment type. Actual rates and investment returns vary and are not guaranteed. Conservative investors might model lower rates; aggressive investors might model higher rates based on their investment strategy. Use realistic estimates for your situation rather than best-case scenarios.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and the accumulated interest from previous periods. Unlike simple interest (which only earns on the principal), compound interest creates exponential growth over time—"interest earning interest."
How often can interest compound?
Interest can compound on various schedules: daily (365 times/year), monthly (12 times/year), quarterly (4 times/year), or annually (once/year). More frequent compounding results in slightly higher returns, though the difference depends on the interest rate.
What's the difference between compound and simple interest?
Simple interest only earns on your principal amount. Compound interest earns on principal plus accumulated interest. Over time, compound interest creates exponentially higher growth, especially over decades.
How do monthly contributions affect investment growth?
Monthly contributions significantly boost your final balance. Even small regular contributions ($100-200/month) combined with compound interest can build substantial wealth over 20-30 years. The calculator shows how each contribution earns returns.
What does APY mean?
APY (Annual Percentage Yield) shows the total interest you'll earn in one year, accounting for compounding frequency. It's more accurate than APR (Annual Percentage Rate) for savings accounts because it reflects how often interest compounds.
What interest rate should I use?
Enter an interest rate appropriate for your investment type and situation. Higher-risk investments may provide higher returns but with greater volatility. Lower-risk investments offer stability but lower returns. Use realistic estimates based on your investment strategy, not best-case scenarios. Actual results and returns vary and are not guaranteed.