Investment Calculator
Editorial Review
Reviewed for formula accuracy, plain-language explanations, and calculator limitations by DP Tech Studio.
Reference sources
Important: This calculator assumes a constant annual return rate. Real investment returns fluctuate and past performance does not guarantee future results. This tool is for educational planning purposes only.
What Is an Investment Calculator?
An investment calculator shows you how money grows when it earns compound interest over time. You enter a starting amount, any regular contributions you plan to add, an expected annual return, and a time horizon — and the calculator projects what your portfolio could be worth at the end.
The most powerful insight this tool provides is not the final number itself, but the breakdown: how much of your ending balance came from your own contributions versus the returns the market generated for you. That gap — investment growth above and beyond what you put in — is the tangible benefit of compounding.
How Compound Interest Is Calculated
The calculator combines two future value formulas and adds them:
FV₁ = Initial Investment × (1 + r)ⁿ
Future Value of Regular Contributions:
FV₂ = Monthly Contribution × ((1 + r)ⁿ − 1) / r
Total Final Value = FV₁ + FV₂
Where:
r = Monthly rate (Annual rate ÷ 12 ÷ 100)
n = Total months (Years × 12)
Interest is compounded monthly in this calculator. Most brokerage accounts and investment products also compound or apply returns on a monthly or more frequent basis.
Example: $10,000 Invested for 20 Years
Monthly Contribution: $200
Annual Return: 8%
Period: 20 years
FV of $10,000: ≈ $49,268
FV of $200/month: ≈ $117,804
Total Final Value: ≈ $167,072
Total Contributed: $10,000 + ($200 × 240) = $58,000
Return on Investment: $167,072 − $58,000 = $109,072
You contributed $58,000. The market contributed nearly twice that. This is compound interest doing the work.
How Much Does Time Actually Matter?
More than almost any other variable. Consider two investors both putting in $200 per month at 8% annual return:
- Investor A starts at age 25 and stops at 65 (40 years): ~$702,000
- Investor B starts at age 35 and stops at 65 (30 years): ~$299,000
Both contribute at the same monthly rate, but Investor A ends up with more than double Investor B's balance — purely because of the extra 10 years of compounding. This is why financial planners consistently stress that starting early matters more than starting big.
What Return Rate Should You Use?
The right assumption depends on where your money is invested:
- S&P 500 index fund (long-term avg.): ~10% nominal, ~7% inflation-adjusted
- Diversified global equity fund: ~7–9% nominal
- Balanced fund (60/40): ~5–7% nominal
- Bonds / fixed income: ~3–5% nominal
- High-yield savings account: ~4–5% (variable, short-term)
For multi-decade investment planning, most advisors use 6–8% as a reasonable working assumption for diversified equity-heavy portfolios. Always run projections at both a lower rate (pessimistic) and higher rate (optimistic) to see the full range.