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Kalcufy

Investment Calculator

What Is an Investment Calculator?

An investment calculator projects how your money will grow over time using compound interest and regular contributions. Unlike a simple savings calculator, it accounts for real-world factors like inflation erosion, management fees, capital gains taxes, and increasing contributions over time. The power of compound interest means your money earns returns on both your original investment and your accumulated returns — creating exponential growth over long periods. Albert Einstein allegedly called compound interest the eighth wonder of the world, and whether or not the attribution is accurate, the math certainly is remarkable.

How Compound Interest Works

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. With monthly compounding, your annual return is divided into 12 parts and applied each month, with each month's calculation including the interest earned in prior months. This creates a snowball effect: a $10,000 investment at 8% compounded monthly grows to $22,196 in 10 years, compared to $18,000 with simple interest — that's $4,196 more from compounding alone. The three key factors are: rate of return (higher = faster growth), time horizon (longer = dramatically more growth due to exponential curve), and contribution frequency (more frequent = more compounding periods).

Frequently Asked Questions

What rate of return should I use?

For a diversified stock portfolio, use 7-10% (nominal) or 4-7% (inflation-adjusted). The S&P 500 has returned ~10% annually since 1926. For conservative estimates use 6-7%. For bonds or savings, use 3-5%. Always plan with conservative estimates to avoid disappointment.

How does compounding frequency affect returns?

More frequent compounding yields slightly higher returns. $10,000 at 8% for 10 years: annually = $21,589, monthly = $22,196, daily = $22,253. The difference between monthly and daily is minimal (~$57), so monthly compounding is a reasonable assumption for most investments.

Should I invest a lump sum or contribute regularly?

Historically, lump sum investing outperforms dollar-cost averaging about 2/3 of the time because markets tend to go up. However, dollar-cost averaging through regular contributions reduces risk and is more practical for most people who invest from paychecks.

How much do fees really matter?

Enormously over time. On a $500/month investment over 30 years at 8% return: with 0.1% fees you'd have $691K, with 1% fees you'd have $569K — a $122K difference, or 18% less wealth. Choose low-cost index funds with expense ratios under 0.2%.

What is the Rule of 72?

Divide 72 by your annual return to estimate how many years it takes to double your money. At 8% return: 72 ÷ 8 = 9 years to double. At 10%: 7.2 years. At 6%: 12 years. This quick mental math helps evaluate investment opportunities.

How does inflation affect my investment?

At 3% inflation, your money loses about half its purchasing power every 24 years. A $1M portfolio in 2026 buys the equivalent of ~$475K in 2050 dollars. This calculator's inflation adjustment shows your real purchasing power so you can plan accordingly.