Investment Growth Calculator Formula

Understand the math behind the investment growth calculator. Each variable explained with a worked example.

Use the Investment Growth Calculator

Formulas Used

Future Value

future_value = monthly_rate > 0 ? initial_investment * pow(1 + monthly_rate, num_months) + monthly_contribution * (pow(1 + monthly_rate, num_months) - 1) / monthly_rate : initial_investment + monthly_contribution * num_months

Total Invested

total_invested = initial_investment + monthly_contribution * num_months

Investment Gains

total_returns = future_value - total_invested

Variables

VariableDescriptionDefault
initial_investmentInitial Investment(USD)25000
monthly_contributionMonthly Contribution(USD)500
annual_returnExpected Annual Return(%)8
yearsInvestment Period(years)25
monthly_rateDerived value= annual_return / 12 / 100calculated
num_monthsDerived value= years * 12calculated

How It Works

How Investment Growth Works

Investment growth uses compound returns with regular contributions to project your portfolio value.

Formula

FV = PV(1+r)^n + PMT * [(1+r)^n - 1] / r

Where:

  • FV = Future value
  • PV = Present value (initial investment)
  • PMT = Monthly contribution
  • r = Monthly rate of return
  • n = Number of months

    • Worked Example

    You invest $25,000 and add $500/month at 8% return for 25 years.

    initial_investment = 25000monthly_contribution = 500annual_return = 8years = 25
    1. 01Monthly rate: 8% / 12 = 0.6667% (0.006667)
    2. 02Total months: 25 * 12 = 300
    3. 03Initial investment grows: $25,000 * (1.006667)^300 = $171,212.89
    4. 04Monthly contributions grow: $500 * [(1.006667)^300 - 1] / 0.006667 = $475,513.14
    5. 05Future Value = $171,212.89 + $475,513.14 = $646,726.03
    6. 06Total invested: $25,000 + ($500 * 300) = $175,000
    7. 07Investment gains: $646,726.03 - $175,000 = $471,726.03

    Frequently Asked Questions

    What is a realistic rate of return?

    The S&P 500 has historically returned about 10% annually before inflation. A conservative estimate is 7-8% for long-term stock investments after accounting for inflation.

    How does compounding frequency affect growth?

    More frequent compounding leads to slightly higher returns. Monthly compounding grows faster than annual compounding at the same rate.

    Should I invest a lump sum or contribute monthly?

    Historically, lump sum investing tends to outperform dollar-cost averaging about two-thirds of the time. However, regular contributions help build discipline and reduce timing risk.

    Learn More

    Guide

    Understanding Compound Interest

    Learn how compound interest works and why it is the most powerful force in personal finance. Covers the compound interest formula, compounding frequency, the Rule of 72, and real-world applications.

    Ready to run the numbers?

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