See exactly how compounding grows your money.
Enter your starting amount, monthly contribution, expected return, and time horizon to see your projected balance and how much of it is pure compound interest.
Why it matters: It's hard to picture how a steady return snowballs over decades. This shows the split between what you put in and what compounding added — year by year.
Example calculation
You invest $10,000 at 8% for 30 years and add nothing more — it grows to about $100,627.
Add just $200 a month over those same 30 years and the balance reaches roughly $371,000. That's a $271,000 difference from only $72,000 of extra contributions — the rest is compounding doing the work.
How to use this compound interest calculator
Enter your starting investment amount, how much you plan to add each month, your expected annual return rate, and your time horizon. The calculator instantly shows your projected final balance and splits it into what you contributed vs. what the market added through compounding.
For stock market planning, most investors use 7–10% annual return for broad index funds like VOO or VTI, based on historical S&P 500 performance. Use a lower rate (5–6%) for a conservative scenario that accounts for inflation.
How compound interest works on investments
Compound interest means your returns generate their own returns. In year one, you earn interest on your principal. In year two, you earn interest on your principal plus the interest from year one. This cycle accelerates over decades — which is why starting early matters far more than starting big.
Example: $10,000 invested at 8% for 30 years grows to $100,627 without any additional contributions. Add just $200/month and that becomes $371,000 — a $271,000 difference from $72,000 in contributions.
The Rule of 72
Divide 72 by your annual return rate to estimate how many years it takes to double your money. At 8%, your investment doubles roughly every 9 years (72 ÷ 8 = 9). At 10%, every 7.2 years.
Compound interest vs. simple interest
Simple interest calculates returns only on your original principal. Compound interest calculates returns on your growing balance. Over 20+ years, the difference is substantial: $50,000 at 8% simple interest for 30 years returns $120,000 in interest. The same amount at 8% compound interest returns $453,000 — nearly 4× more.
Keep planning
Once you know your target, the investment goal calculator shows whether you're on track and the monthly contribution needed to get there. Pair it with the dollar-cost averaging calculator for steady investing, or the dividend calculator to see how reinvested dividends compound alongside price growth. Every figure here is an educational estimate based on your inputs — not financial advice or a guarantee.
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Frequently asked questions
Educational use only — not financial advice
StockLeo is for educational purposes only and does not provide financial, investment, legal, or tax advice. Calculations are estimates and may not reflect your full tax or financial situation. Consult a qualified professional before making financial decisions.