Investment Calculator - Growth, Goal Planning, Monte Carlo

Compound interest means you earn returns on earlier returns, so growth speeds up over time. This calculator treats your return as a yearly figure: money already invested grows by exactly that rate each year, in steps at your contribution frequency (weekly, bi-weekly, monthly or annually). The frequency changes when contributions arrive, not the yearly rate.

There is no single right figure. US returns from 1928 to 2025 (Damodaran, NYU Stern) compound to about 6.5% a year for 30% stocks, 60% bonds and 10% cash. The figure is 8.5% for 60% stocks, 35% bonds and 5% cash, and 9.5% for 90% stocks, 8% bonds and 2% cash. Each mix is rebalanced yearly, and the figures are rounded to the nearest half percent. That is history, not a forecast. Start with Simple mode, then try a lower return to see a cautious case.

The Rule of 72 estimates how long it takes to double your money: divide 72 by your annual return rate. At 8% returns, your money doubles in about 9 years (72 ÷ 8 = 9).

Paying off a debt earns its interest rate with no market risk, while an investment's return is uncertain. Compare the debt's rate with the return you assume here. The Credit Card Payoff Calculator shows what a card balance costs in interest.

Growth depends on your starting amount, monthly contributions, expected annual return, and how long you stay invested. Enter those values and the calculator projects the future balance, showing how much comes from contributions versus compound growth. For example, $10,000 plus $300/month at a 7% annual return grows to roughly $191,000 over 20 years. For a one-time amount, such as $100,000, enter it as the initial investment and set the contribution to 0; to see regular investing alone, set the initial investment to 0.

Switch to Goal Planning mode, choose Find Contribution Needed, and enter the Investment Goal, the starting amount, the expected return and the years. The result is the Required Monthly Contribution, or Goal Already Met when the starting amount grows past the target by itself. The Million Dollar Goal example, $15,000 today at 8.5% a year for 25 years, needs about $903 a month. That is about $285,800 of your own money in total, including the $15,000, with growth supplying the other $714,200. Contributions are monthly unless you change the frequency in Advanced mode, and the result follows it. The goal is in the dollars of the year you reach it. To aim at what $1 million buys today, raise it for inflation first: at 3% a year for 25 years, that is about $2.09 million.

In a taxable brokerage account, this calculator's Taxed at sale option assumes funds pay no distributions before sale and taxes the gains once, at sale. Funds that do pay dividends or capital gain distributions are taxed on them in the year paid (IRS Publication 550). Interest-paying assets are taxed every year, which lowers the effective compounding rate. Tax-advantaged accounts (401(k), IRA, Roth IRA) defer or eliminate that tax, letting the full pre-tax return compound. Turn on tax-adjusted values in Advanced mode to see the after-tax comparison.

Use the Portfolio Comparison and Investment Strategies sections to evaluate different contribution amounts and return assumptions side by side, including each strategy's asset allocation. The Monte Carlo section runs many randomized return sequences to show volatility and the 5th to 95th and 25th to 75th percentile ranges of the simulated outcomes, a spread rather than one number.

The Monte Carlo section replays your plan 500 to 10,000 times, drawing a return for each year at random around your expected return. The volatility is the standard deviation: 12.0% a year for the Moderate mix, measured on US returns from 1928 to 2025. The 5th percentile is the ending only 5% of simulated scenarios fall below, the median is the middle one, and the 95th percentile is the ending only 5% rise above. The range from the 5th to the 95th percentile holds 90% of the simulated endings, and the range from the 25th to the 75th holds the middle 50%. With a goal entered, it also shows the share of simulated scenarios that reach it. These are shares of simulated runs under the model's assumptions, not the chance of a real outcome. The draws average your expected return, but a loss costs more than an equal gain adds back, so the median run grows more slowly than the steady projection. A contribution solved exactly for a goal therefore reaches it in only about 40% of runs. In the Million Dollar Goal example the median lands near $910,000, the 5th percentile near $475,000 and the 95th near $1.8 million. A larger contribution raises the share of runs that reach the goal.