Compound Interest Calculator
Visualize your investment growth with compound interest, contributions, and inflation adjustments.
Rule of 72 — How Long to Double?
Enter interest rate to calculate
With vs Without Monthly Contributions
Growth Over Time
Year-by-Year Breakdown
| Year | Starting Balance | Contributions | Interest Earned | Ending Balance |
|---|
Calculation History
What is Compound Interest?
Compound interest is the interest earned on both the initial principal and the accumulated interest from previous periods. It's often described as "interest on interest" and is one of the most powerful forces in investing and wealth building. The more frequently interest compounds (daily, monthly, quarterly, or annually), the faster your money grows.
How Does Compound Interest Work?
Compound interest is calculated using the formula: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)], where A is the final amount, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, t is the time in years, and PMT is the regular payment amount.
For example, if you invest $10,000 at 7% annual interest compounded monthly for 30 years with $500 monthly contributions, your money doesn't just grow by 7% each year. Each month, interest is calculated on the growing balance, and that interest itself starts earning interest. Over decades, this exponential growth can turn modest investments into substantial wealth.
The Rule of 72
The Rule of 72 is a quick way to estimate how long it takes for your money to double at a given interest rate. Simply divide 72 by your annual interest rate. For example, at 7% interest, your money doubles approximately every 72 ÷ 7 ≈ 10.3 years. This simple rule works remarkably well for rates between 1% and 10%.
Impact of Regular Contributions
While compound interest on your initial investment is powerful, adding regular contributions dramatically accelerates wealth building. Monthly contributions combined with compound interest create exponential growth. Over 30 years at 7% interest, $500/month in contributions can grow to over $600,000 — far more than the $180,000 you actually contributed.
Inflation and Real Returns
Your nominal return (the stated interest rate) differs from your real return (after inflation). If your investment grows at 7% but inflation is 2.5%, your real purchasing power only grows at about 4.4% annually. Over long periods, inflation significantly impacts the real value of your returns. This calculator's inflation adjustment helps you understand the true growth of your wealth.
How to Use This Compound Interest Calculator
Enter your initial investment, monthly contribution amount, annual interest rate, and time period. Select your compounding frequency (most savings accounts and bonds compound daily or monthly). The calculator instantly shows your future value, total interest earned, and provides a visual chart of your growth over time. Enable inflation adjustment to see your real purchasing power. Export your year-by-year breakdown to CSV for detailed record-keeping.
Frequently Asked Questions
What's the difference between nominal and real returns?
Nominal return is the stated interest rate or investment return. Real return accounts for inflation and represents the true increase in purchasing power. A 7% nominal return with 2.5% inflation equals approximately 4.4% real return. Over long periods, this difference compounds significantly.
Does compounding frequency matter?
Yes, more frequent compounding results in slightly higher returns. Daily compounding earns slightly more than monthly, which earns more than quarterly or annual. However, the difference becomes significant only with large sums and high interest rates. Most savings accounts use daily compounding.
How often should I make contributions?
More frequent contributions allow more money to benefit from compounding. Monthly contributions are common and convenient. However, the total amount contributed over time matters most — contributing $6,000/year monthly ($500/month) grows faster than contributing $6,000 annually.
What if I want to withdraw money during the investment period?
Withdrawals reduce the principal amount earning interest, significantly impacting long-term growth. Early withdrawals from retirement accounts may also trigger penalties and taxes. This calculator assumes no withdrawals — plan accordingly for actual needs.
Is 7% a realistic interest rate?
Historically, stock market returns average around 10% annually (before inflation). Bonds typically yield 3-5%. Savings accounts currently offer 4-5%. Your actual rate depends on your investment vehicle. This calculator uses 7% as a moderate example for long-term planning.
Can I use this for retirement planning?
Yes, this is a useful retirement planning tool. Enter your target retirement age to estimate your portfolio value. Adjust the interest rate based on your expected investment mix (stocks/bonds). Remember to account for withdrawals, taxes, and inflation when planning actual retirement needs.
Love using BreezyTools?
Go Pro for an ad-free experience, priority features, and support independent development — for less than a coffee a month.