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Investment Calculator — Compound Growth & Return on Investment

Calculate how your investment grows with compound interest and monthly contributions. See your final value, total interest earned, and how long to double your money.

What is this calculator?

This investment calculator shows how your portfolio grows over time through compound interest on an initial investment plus regular monthly contributions. It also uses the Rule of 72 to estimate how long it takes to double your money at the given return rate.

Formula

r = annualReturn / 100 / 12 (monthly rate) n = years × 12 (total months) Final Value = initialAmount × (1+r)^n + monthlyContribution × ((1+r)^n − 1) / r Total Contributions = initialAmount + monthlyContribution × n Total Returns = Final Value − Total Contributions Doubling Time ≈ 72 / annualReturn

Example

$10,000 initial, $200/month, 7% annual return, 20 years: r = 0.5833%/month, n = 240 months. Final ≈ $144,959. Total contributed: $58,000. Total returns: $86,959. Doubling time at 7%: ≈ 10.3 years.

How to use

  1. 1Enter your initial investment amount and optional monthly contribution.
  2. 2Set the expected annual return rate (default 7%) and investment period in years (default 10).
  3. 3Click Calculate to see your final portfolio value, total amount invested, total returns earned, and your investment's doubling time via the Rule of 72.

Frequently Asked Questions

How is compound interest calculated with monthly contributions?
The total future value combines two parts: (1) the lump-sum growth of your initial investment — FV = P × (1+r)^n — and (2) the future value of your monthly contributions as an annuity — FV = C × ((1+r)^n − 1) / r. Here r is the monthly rate (annual rate / 12) and n is total months (years × 12). The two are added together for the final portfolio value.
What is the Rule of 72?
The Rule of 72 is a shortcut to estimate how long it takes to double your investment: Doubling Time ≈ 72 / Annual Return Rate. At 7% return, your investment doubles in about 72/7 ≈ 10.3 years. At 10%, it doubles in 7.2 years. This rule works because ln(2) / ln(1+r) ≈ 0.693 / r ≈ 72 / (100r) for typical interest rates.
What annual return rate should I assume?
Historical averages for common investment types: S&P 500 index fund ≈ 10% nominal, 7% real (inflation-adjusted); balanced stock/bond portfolio ≈ 6–8%; bond funds ≈ 3–5%; savings account ≈ 1–5%. This calculator defaults to 7%, which is a conservative estimate for a diversified portfolio over the long term.
Does this calculator account for taxes or inflation?
No. The calculator shows nominal returns before taxes or inflation. To estimate real (inflation-adjusted) returns, subtract the inflation rate from your annual return before entering it. For example, if you expect 7% nominal and 3% inflation, enter 4% to see real growth. Tax treatment depends on account type (taxable vs. 401k vs. IRA).
What is the difference between total contributions and total returns?
Total contributions is simply the money you actually put in: initial investment plus all monthly contributions. Total returns (interest) is the growth generated by compounding — it is the difference between final value and total contributions. For long periods at moderate rates, total returns typically dwarf total contributions due to the exponential nature of compound growth.

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