Compound Interest Calculator
Enter your principal, monthly contribution, annual rate, and time period to visualize compound growth with an interactive chart.
Final Balance
29,435,503
- Total Principal
- 11,800,000
- Total Interest (Compound Effect)
- 17,635,503
- Interest ratio: 59.9%
- 59.9%
- Rule of 72: ~14.4 years to double
Growth Chart
Year-by-Year Breakdown
| Year | Balance | Principal | Interest |
|---|---|---|---|
| 1 | 1,419,527 | 1,360,000 | 59,527 |
| 2 | 1,860,518 | 1,720,000 | 140,518 |
| 3 | 2,324,072 | 2,080,000 | 244,072 |
| 4 | 2,811,341 | 2,440,000 | 371,341 |
| 5 | 3,323,541 | 2,800,000 | 523,541 |
| 6 | 3,861,945 | 3,160,000 | 701,945 |
| 7 | 4,427,895 | 3,520,000 | 907,895 |
| 8 | 5,022,800 | 3,880,000 | 1,142,800 |
| 9 | 5,648,142 | 4,240,000 | 1,408,142 |
| 10 | 6,305,477 | 4,600,000 | 1,705,477 |
| 11 | 6,996,443 | 4,960,000 | 2,036,443 |
| 12 | 7,722,760 | 5,320,000 | 2,402,760 |
| 13 | 8,486,237 | 5,680,000 | 2,806,237 |
| 14 | 9,288,775 | 6,040,000 | 3,248,775 |
| 15 | 10,132,372 | 6,400,000 | 3,732,372 |
| 16 | 11,019,129 | 6,760,000 | 4,259,129 |
| 17 | 11,951,254 | 7,120,000 | 4,831,254 |
| 18 | 12,931,069 | 7,480,000 | 5,451,069 |
| 19 | 13,961,012 | 7,840,000 | 6,121,012 |
| 20 | 15,043,650 | 8,200,000 | 6,843,650 |
| 21 | 16,181,677 | 8,560,000 | 7,621,677 |
| 22 | 17,377,928 | 8,920,000 | 8,457,928 |
| 23 | 18,635,382 | 9,280,000 | 9,355,382 |
| 24 | 19,957,169 | 9,640,000 | 10,317,169 |
| 25 | 21,346,581 | 10,000,000 | 11,346,581 |
| 26 | 22,807,079 | 10,360,000 | 12,447,079 |
| 27 | 24,342,298 | 10,720,000 | 13,622,298 |
| 28 | 25,956,061 | 11,080,000 | 14,876,061 |
| 29 | 27,652,389 | 11,440,000 | 16,212,389 |
| 30 | 29,435,503 | 11,800,000 | 17,635,503 |
This calculator provides estimates for informational purposes only and does not guarantee actual investment returns. Real-world results may differ due to taxes, fees, market fluctuations, and other factors. Invest at your own risk and consult a qualified financial advisor for personalized advice.
How to Use
Enter up to five values — initial principal, monthly contribution, annual rate, time period, and compounding frequency — and the results update instantly.
The chart shows "Principal (blue)" and "Interest earned through compounding (green)" as a stacked bar chart. The longer the time period, the more the interest bar grows, making the power of compounding easy to see.
The year-by-year table shows the end-of-year balance, cumulative principal, and cumulative interest for each year. It's useful for modeling a savings or investment plan, or back-calculating how much you need to save monthly to hit a target.
Use the Copy Result button to paste the summary into a notes app or share it with others.
How the Calculation Works
Monthly compounding: monthly rate r_m = annual rate ÷ 12 ÷ 100; total months n_m = years × 12. Future value of lump sum = P0 × (1 + r_m)^n_m. Future value of monthly contributions (ordinary annuity) = PMT × ((1 + r_m)^n_m − 1) / r_m. Final balance = sum of both. (Source: Investopedia, "Future Value of an Annuity")
Rule of 72 — an intuitive shortcut: The number of years for your money to roughly double is approximately 72 ÷ annual rate (%). At 3% it takes about 24 years, at 5% about 14.4 years, and at 7% about 10.3 years. The theoretical exact value is ln(2) ÷ r ≈ 0.693 ÷ r, but 72 is used in practice because it's easy to divide mentally and gives a close approximation. Set your monthly contribution to $0 in this tool and watch the chart to see when the balance crosses the 2× mark.
Lump-sum vs. monthly contributions — same rate, very different outcomes: At 5% annual rate, monthly compounding, over 30 years: Case A ($10,000 initial + $300/month) → approximately $294,000; Case B ($0 initial + $300/month only) → approximately $249,000; Case C ($10,000 initial only, no contributions) → approximately $44,700. The gap between Case A and Case B (~$44,700) matches Case C almost exactly — that is precisely what the early $10,000 grows to over 30 years of compounding. Conversely, spreading contributions over time (Case B) averages your purchase price (dollar-cost averaging), smoothing out market volatility. Neither approach is universally better; the right mix depends on your goals and risk tolerance.
FAQ
- What is the difference between compound and simple interest?
- Simple interest applies only to the original principal. Compound interest applies to the principal plus accumulated interest, so your balance grows faster over time. For example, $10,000 at 5% for 30 years earns $15,000 in simple interest (total $25,000) but grows to roughly $44,700 with annual compounding — a difference of nearly $20,000.
- Monthly vs. annual compounding — which should I choose?
- Match the setting to how your financial product actually works. Many savings accounts and investment funds compound daily or monthly; annual compounding is common in bonds or some savings plans. Given the same annual rate, more frequent compounding always yields a slightly higher final balance. Check the product's terms to pick the right setting.
- Is a 5% annual return realistic?
- This tool does not recommend any specific return. Broad stock market index funds have historically averaged around 4–7% annually over long periods, but past performance does not guarantee future results. Use this tool as a scenario planner — try 3%, 5%, and 7% to see how sensitive the outcome is to your assumed rate.
- Can I verify the Rule of 72 in this calculator?
- Yes. Set monthly contribution to $0 and try an annual rate of 5%. The year-by-year table will show the balance crossing 2× the principal around year 14–15 (72 ÷ 5 = 14.4 years). Adjust the rate to see how the doubling time changes.
- Are taxes and fees included in the calculation?
- No. Investment gains outside of tax-advantaged accounts (such as Roth IRA or 401(k)) are subject to capital gains tax. Fund expense ratios (typically 0.03–0.50% per year) also reduce effective returns. For a more accurate picture, subtract estimated fees from the annual rate before running the simulation.