Free Investment Calculator

How Compound Interest Works

Compound interest is the engine behind long-term wealth building. Unlike simple interest, which only earns on your initial investment, compound interest earns returns on both your principal and your accumulated growth. That creates a snowball effect where your money grows faster the longer it stays invested.

For example, $10,000 invested at 7% annually becomes $19,672 after 10 years, nearly double, without adding a single dollar. Add regular monthly contributions and the effect accelerates sharply.

Two Ways to Forecast

This calculator runs in two modes, switchable at the top.

Standard Forecast projects a single path. You set an expected return and the calculator compounds it month by month. You can optionally vary the return year to year so the projection is not unrealistically smooth. It is the fast way to see roughly where you are headed.

Monte Carlo runs thousands of simulated market histories instead of one. Markets are not predictable, so rather than committing to a single outcome it shows the whole range: the median result, a typical range around it, the odds of reaching a goal you set, and, if you plan withdrawals, the chance of running out of money. It is the honest way to see how much your future depends on the luck of the draw.

Key Features of This Calculator

• Monte Carlo Simulation: Run up to 10,000 simulated market histories and see the full distribution of outcomes, the probability of hitting your goal, and your depletion risk if you plan to draw the portfolio down.
• Forward-Looking Return Presets: Instead of guessing a return, start from a market-based estimate. The presets derive the expected return from current conditions rather than a round number.
• Realistic Variable Returns: Real markets do not return a steady 7% every year. Both modes model genuine market volatility, with calm and turbulent stretches that cluster the way they do in reality.
• Retirement Withdrawal Planning: Schedule monthly withdrawals starting at any year, see when a portfolio might be depleted, and pressure-test sustainable withdrawal rates.
• Contribution Adjustments: Life changes. Raise contributions after a promotion, or pause them during a career transition, and model your real financial journey.
• Inflation and Fee Analysis: See your future portfolio in today's purchasing power, and see exactly how much investment fees quietly cost you over decades.

Smart Investing Tips

About the Math

Both modes compound monthly using the standard future value formula: FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)], where FV is future value, P is principal, r is the annual rate, n is compounding periods per year, t is years, and PMT is the periodic contribution.

In Standard Forecast, the variable returns option does not draw a fresh random number each year. It uses a fat-tailed distribution combined with volatility clustering, so calm stretches and turbulent stretches group together the way they do in real markets. A year down 30% is not followed by a tidy random bounce.

Monte Carlo mode uses a two-regime model, the same family of models used by institutional forecasters. Each month the market sits in one of two states, calm or crisis, each with its own typical return and volatility, and it tends to stay put. That persistence produces believable runs of bull and bear years instead of random whiplash. Returns are drawn from a fat-tailed distribution, so rare extreme months occur at a realistic rate, and the two states are calibrated so the long-run average and volatility match the figures you enter.

An optional mean reversion setting gently pulls returns back toward their long-run trend after extended run-ups and crashes. Equity markets have behaved this way over multi-decade periods, and enabling it narrows the very long-range spread of outcomes.

The return presets are forward looking rather than guesses. They follow the approach of NYU finance professor Aswath Damodaran: the expected return on stocks equals the risk-free rate plus an equity risk premium implied by current market prices. The figures carry a date and are refreshed periodically.

From thousands of paths, Monte Carlo mode reports percentile bands over time as a shaded fan, the distribution of final outcomes as a histogram, the probability of reaching your goal, and the risk of depletion when withdrawals are scheduled. Inflation adjustment converts future values to present-day purchasing power using Real Value = Nominal Value / (1 + inflation)^years, and fee calculations apply an annual expense ratio that compounds over your full time horizon.

Understanding Market Returns

The S&P 500 has delivered an average annual return of roughly 10% before inflation, about 7% after inflation, over the past century. That average hides large year-to-year swings: in any single year, returns can land anywhere from about -40% to +50%.

Different asset classes carry different risk and return profiles. Bonds typically offer lower returns (3% to 5% historically) with less volatility. International and emerging markets may offer higher potential returns with more risk. The 7% figure commonly used in retirement planning is a conservative, inflation-aware estimate; aggressive portfolios might target 8% to 10%, and conservative ones 4% to 6%.

A note on behavioral finance: these projections assume you stay invested and never react emotionally to market swings. In reality, watching a portfolio drop 30% for three years running can trigger panic selling at the worst possible time. Investors who try to time the market consistently underperform those who stay the course. If a 40% drop would keep you up at night, a more conservative allocation you can actually hold may beat an aggressive one you abandon during a crash.

Remember: past performance does not indicate future results. Future returns may differ significantly from historical averages as economic conditions, interest rates, and global factors change.