# Investment Calculator

Calculate how your investments grow over time with compound interest and monthly contributions. Free investment calculator with future value projections.

## What this calculates

See how your money grows over time with compound interest and regular contributions. Enter your starting amount, monthly contributions, expected return rate, and time horizon to project your investment's future value.

## Inputs

- **Initial Investment** ($) — min 0 — The lump sum you are investing today.
- **Monthly Contribution** ($) — min 0 — The amount you plan to add each month.
- **Annual Return Rate** (%) — min 0, max 50 — Expected annual rate of return. The S&P 500 has historically averaged about 10% before inflation.
- **Investment Period** (years) — min 1, max 60 — How many years you plan to keep investing.

## Outputs

- **Future Value** — formatted as currency — The total value of your investment at the end of the period.
- **Total Contributions** — formatted as currency — The total amount of money you put in (initial + all monthly contributions).
- **Total Interest Earned** — formatted as currency — The total growth from compound interest and returns.
- **Effective Annual Return** — formatted as percentage — Your actual annualized return accounting for the timing of contributions.

## Details

Compound interest is the single most powerful force in long-term investing. When your returns generate their own returns, growth accelerates dramatically over time. A $10,000 initial investment with $500 monthly contributions at 8% annual return grows to about $335,000 after 20 years. Of that total, you only contributed $130,000 out of pocket. The remaining $205,000 came from compound growth.

The formula behind this calculator is FV = P(1+r)^n + PMT x [((1+r)^n - 1) / r], where P is your initial investment, r is the monthly interest rate, n is the total number of months, and PMT is your monthly contribution. This assumes returns compound monthly and contributions happen at the end of each month.

Time in the market matters more than timing the market. Starting five years earlier with the same contributions can mean hundreds of thousands of dollars more at retirement. For example, investing $500 per month starting at age 25 versus age 30 (both at 8% return until age 65) results in roughly $1.75 million versus $1.15 million. That five-year head start adds over $600,000 in extra growth.

Keep in mind that actual market returns vary year to year. The S&P 500 has historically returned about 10% annually before inflation (roughly 7% after inflation). This calculator uses a fixed rate for projection purposes. Real-world results will fluctuate, but over long periods, the math of compounding holds remarkably well.

## Frequently Asked Questions

**Q: What annual return rate should I use?**

A: For a diversified stock portfolio, 8-10% is a reasonable long-term estimate based on historical S&P 500 returns. For a more conservative mix with bonds, try 5-7%. If you want to account for inflation, subtract about 3% from your expected return. For example, use 7% instead of 10% to see growth in today's dollars.

**Q: How does compound interest differ from simple interest?**

A: Simple interest only earns returns on your original principal. Compound interest earns returns on both your principal and previously earned interest. Over 20 years at 8%, $10,000 with simple interest grows to $26,000. With compound interest, it grows to about $46,600. The longer your time horizon, the bigger the difference becomes.

**Q: Does this calculator account for taxes or fees?**

A: No. This calculator shows gross returns before taxes, management fees, or inflation. In a taxable account, capital gains and dividends are taxed each year. In tax-advantaged accounts like a 401(k) or IRA, taxes are deferred or eliminated. To approximate after-fee returns, subtract your fund's expense ratio from the annual return rate (for example, use 7.9% instead of 8% for a fund with a 0.1% expense ratio).

**Q: Why does starting early matter so much?**

A: Compound interest needs time to work. In the early years, most of your balance comes from contributions. But as your balance grows, interest earns interest and growth accelerates. Someone who invests $500 per month from age 25 to 65 at 8% ends up with roughly $1.75 million, while starting at 35 yields about $745,000. Ten fewer years of compounding cuts the final balance by more than half.

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Source: https://vastcalc.com/calculators/finance/investment
Category: Finance
Last updated: 2026-04-08