How to Use the Investment Calculator
Enter your starting balance (the amount you have saved today — enter 0 if you are starting from nothing), your monthly contribution, your expected annual return rate, and your investment time horizon in years. The calculator instantly shows your projected final balance, total amount contributed out of pocket, and total interest earned through compound growth.
To compare scenarios, adjust the annual return rate between conservative (5-6%), moderate (7-8%), and aggressive (9-10%) estimates and observe how the final balance changes. A $500/month contribution at 8% over 30 years yields roughly $745,000 — despite only contributing $180,000 directly. The remaining $565,000 is pure compound growth. For a detailed year-by-year breakdown, review the growth table showing the balance milestone at each year.
Why Use This Investment Calculator?
- Models compound interest with monthly contributions — the most realistic projection method
- Shows total contributions vs. total interest so you can see exactly how much comes from growth
- Helps visualize why starting early dramatically outperforms investing more later
- Free, private, and runs entirely in your browser — no financial data leaves your device
- No account required — enter numbers and get results instantly
Frequently Asked Questions
What return rate should I use in the investment calculator?
For a diversified stock portfolio (S&P 500 index fund), use 7-8% for inflation-adjusted (real) returns or 10% for nominal returns. For bonds, use 2-3% real. For a conservative estimate, use 6%. Always model multiple scenarios — best case, base case, and worst case — rather than relying on a single number. The S&P 500 has averaged approximately 10% nominal and 7% real annually over the past 90+ years.
How much should I invest per month to become a millionaire?
At an 8% average annual return, investing approximately $670 per month for 30 years reaches $1 million. Starting at 25 instead of 35 reduces the required monthly amount by nearly 50% thanks to the extra decade of compounding. Even $200 per month at 8% grows to roughly $298,000 over 30 years — despite only $72,000 contributed directly.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest, creating exponential growth. $10,000 at 8% simple interest for 30 years yields $34,000. At 8% compound interest, it yields $100,627 — nearly three times more. This difference grows dramatically with time, which is why compound interest is called the eighth wonder of the world.
Is investing $100 a month worth it?
Absolutely. At 8% annual return, $100 per month grows to approximately $149,000 over 30 years, despite only $36,000 contributed. The key insight is that consistency and time matter far more than the amount. Starting with $100 per month today produces better long-term results than waiting until you can afford $500 per month.
Should I pay off debt before investing?
Pay off high-interest debt (credit cards, personal loans above 7-8% interest) before investing, since eliminating that debt provides a guaranteed risk-free return equal to the interest rate. However, always contribute enough to capture your employer's full 401(k) match first — that is a guaranteed 100% return. For low-interest debt like mortgages at 3-5%, investing simultaneously usually makes mathematical sense.
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