Compound Interest Calculator
Formula and Calculation Method
Compound interest formula with ongoing regular deposits:
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) ÷ (r/n)]
Where P is principal, PMT is periodic addition, r is annual rate, n is compounding frequency per year, and t is years.
Worked Example and Step-by-Step Calculation
- Initial Deposit (P): $15,000
- Monthly Contribution (PMT): $350
- Interest Rate (r): 8.0%
- Compounding: Monthly (n = 12)
- Tenure (t): 25 years
1. Future value of initial $15,000 = $15,000 × (1 + 0.08/12)^300 = $110,102.26
2. Future value of $350 monthly deposits = $350 × [((1.006667)^300 - 1) ÷ 0.006667] = $332,859.39
3. Total Future Balance = $110,102.26 + $332,859.39 = $442,961.65
4. Total Principal Invested = $15,000 + ($350 × 300) = $120,000
5. Total Compound Interest Earned = $442,961.65 - $120,000 = $322,961.65
How to Use the Compound Interest Calculator
- Enter your initial starting principal investment.
- Enter your planned recurring monthly or annual contribution.
- Specify your expected annual interest rate percentage.
- Choose your compounding frequency (daily, monthly, quarterly, or annually).
- Select your investment horizon in years to inspect the compound curve.
Common Mistakes, Limitations & Practical Guidance
- Assumes a steady constant interest rate; equity investments fluctuate from year to year.
- Taxes on interest, dividends, and capital gains can reduce net compound growth in non-sheltered accounts.
Frequently Asked Questions About Compound Interest Calculator
What is the compound interest formula with monthly contributions?
The formula combines lump-sum compounding with an ordinary annuity: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)], where PMT is the monthly contribution amount.
What is the difference between simple interest and compound interest?
Simple interest is calculated exclusively on your initial principal deposit. Compound interest is calculated on both your original principal and all previously accumulated interest, causing your balance to grow exponentially rather than linearly.
How does compounding frequency impact returns?
More frequent compounding generates higher returns. For example, $10,000 at 8% for 10 years yields $21,589.25 with annual compounding, but yields $22,196.40 with monthly compounding and $22,253.46 with daily compounding.
What is the single most important factor in compound growth?
Time is the single most powerful factor in compound interest. Because growth is exponential, the interest earned in your third and fourth decades of investing will vastly exceed all contributions made in your early years combined.
Can I factor in inflation with this calculator?
Yes. To compute real inflation-adjusted wealth, subtract your estimated annual inflation rate (e.g. 2.5% to 3.0%) from your nominal interest rate before calculating.
How does this tool complement the FIRE Calculator?
This compound interest calculator reveals how quickly your investment capital multiplies, providing the fundamental growth projections you need to test early retirement timelines in our FIRE Calculator.