Interest / Contributions Ratio
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Contributions
Interest Earned
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How to Use This Calculator
Using this compound interest calculator is simple. Enter four values and get instant results:
Initial Investment — the lump sum you're starting with. This could be an existing savings balance, an inheritance, or any amount you're ready to invest today. If you're starting from zero, enter $0.
Monthly Contribution — the amount you plan to add every month. Consistency is key: even small regular contributions make a huge difference over time thanks to dollar-cost averaging and compound growth.
Annual Interest Rate — your expected annual rate of return. For stock market investments, 7% (after inflation) is a commonly used benchmark based on historical S&P 500 returns. For savings accounts or bonds, use a lower rate (2-4%).
Time Period — how many years you plan to keep your money invested. This is the most powerful variable: doubling your time horizon more than doubles your final balance because of compounding.
Click Calculate and you'll see your projected future value, a breakdown of contributions vs. interest earned, an interactive growth chart, and a year-by-year table you can export to CSV.
The Formula Behind Compound Interest
This calculator uses the standard compound interest formula with regular contributions:
Future Value = P(1 + r)n + PMT × [((1 + r)n − 1) / r]
Where:
- P = initial principal (your starting investment)
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of compounding periods (years × 12)
- PMT = monthly contribution amount
The first part of the formula, P(1 + r)n, calculates how your initial investment grows. The second part, PMT × [((1 + r)n − 1) / r], is the future value of an annuity — it calculates the combined growth of all your monthly contributions.
The key insight is that compounding is exponential, not linear. The longer your money compounds, the faster it grows. In the early years, most of your balance comes from contributions. Over time, interest overtakes contributions and becomes the primary driver of growth.
Compound Interest Examples
Here are three real-world scenarios to illustrate how different variables affect your outcome:
| Scenario | Initial | Monthly | Rate | Years | Final Value | Interest Earned |
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| Conservative Saver | $5,000 | $100 | 4% | 20 | $47,480 | $18,480 |
| Steady Investor | $10,000 | $300 | 7% | 25 | $284,520 | $184,520 |
| Aggressive Growth | $25,000 | $500 | 10% | 30 | $1,581,200 | $1,376,200 |
Notice how the Aggressive Growth scenario earns over $1.3M in interest alone — nearly 7x the total contributions. That's the power of higher returns combined with a longer time horizon.
Frequently Asked Questions
What is compound interest and how does it work?
Compound interest is interest earned on both your initial investment and on previously accumulated interest. For example, if you invest $1,000 at 5% annual interest, after year one you have $1,050. In year two, you earn 5% on $1,050 (not just $1,000), giving you $1,102.50. This compounding effect accelerates over time, making your money grow exponentially.
What's the difference between compound interest and simple interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Over time, compound interest produces significantly higher returns. For example, $10,000 at 5% over 20 years: simple interest yields $20,000, while compound interest yields $26,533.
How much will $10,000 grow in 20 years?
It depends on the rate of return. At 5% annual return compounded monthly, $10,000 grows to about $27,126. At 7%, it grows to $40,387. At 10%, it grows to $67,275. Adding monthly contributions dramatically increases these numbers — $10,000 plus $200/month at 7% over 20 years grows to about $144,600.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes for an investment to double. Simply divide 72 by the annual interest rate. At 6% interest, your money doubles in approximately 12 years (72 ÷ 6 = 12). At 8%, it doubles in about 9 years. At 10%, about 7.2 years.
Does this calculator account for inflation?
This calculator shows nominal returns (before adjusting for inflation). To account for inflation, subtract the expected inflation rate (historically about 2-3%) from your expected return. For example, if you expect 10% returns, use 7% for an inflation-adjusted estimate. This gives you purchasing-power-equivalent results.
How often should interest be compounded?
This calculator compounds monthly, which is the most common frequency for investment accounts and savings products. More frequent compounding (daily vs. monthly) has a small positive effect, but the difference between monthly and daily compounding is minimal — typically less than 0.1% per year.
Can I use this calculator for retirement planning?
Yes, this calculator is excellent for basic retirement projections. Enter your current savings as the initial investment, your planned monthly contribution, an expected return rate (7% is a common long-term estimate for a balanced portfolio), and the number of years until retirement. For more precise planning, consult a financial advisor who can account for taxes, Social Security, and withdrawal strategies.