Stock Market Investment Calculator

Here is the single most-asked version of the question: one deposit, nothing added, left alone. At the S&P 500's long-run average of 10.5%, $10,000 roughly doubles every seven years — so a decade nearly triples it, and three decades multiply it twentyfold.

Two things are worth noticing. First, the gap between the columns is not a rounding difference — over 30 years, inflation eats more than half of the headline number. The left column is what your statement will say; the right column is what it will buy. Second, almost none of the growth happens early. In the first decade the 10.5% column adds $17,141; in the third decade alone it adds $126,264. That back-loading is why the most valuable thing you can give a portfolio is time, not money.

Set monthly contributions to $0 in the calculator above to reproduce this table, or leave them in to see the far more common scenario — a starting balance that you keep feeding.

The calculator defaults to 10.5%because that is roughly the S&P 500's long-run average annual total return with dividends reinvested, before inflation. It is the right number for projecting a future account balance. But inflation quietly takes about three points a year, which is why 7%— the real, after-inflation return — is the honest number for answering “what will this buy me?”

Neither choice is wrong; they answer different questions. What matters is that a few points of assumed return compound into enormous differences. Same $10,000 start, same $500 a month, same 30 years:

Four and a half percentage points separate the top and bottom rows, and they separate $545,000 from $1.34 million on identical contributions of $190,000. This is also the argument for cheap index funds: a fund charging 1% a year isn't taking 1% of your money, it is taking a full point off the rate that drives that table. Subtract your expense ratio from the expected return before you run the projection and you'll see the real cost.

If you want to hold the return fixed and vary the account type instead, our HYSA vs CD vs index fund comparison runs the same money through a savings account, a CD, and the market.

The chart's two colors tell the story of every long-term portfolio: blue is the money you put in, green is the money the market paid you. Watch where green overtakes blue. With the default inputs — $10,000 to start, $500 a month, 10.5% — here's the sequence:

• Year 1: the market pays you about $1,334 while you contribute $6,000. Your effort is doing nearly all the work.
• Year 6: returns hit about $6,082 — the first year the market contributes more than you do. It never trails again.
• Year 12: cumulative returns pass your cumulative contributions. More than half your balance is now money you never earned.
• Year 30: the market pays you about $126,684 in that single year — more than 21 times your $6,000 of contributions, and two-thirds of everything you contributed across three decades.

After 20 years you'd hold $454,639 having deposited $130,000. The other $324,639 — 71% of the balance — is compound growth. Nothing about that requires skill or timing; it requires a low-cost fund and the patience to leave it alone. The same dynamic, viewed as a starting-age problem rather than a rate problem, is what makes starting at 25 instead of 35 worth several hundred thousand dollars.

Every number on this page assumes a smooth 10.5% a year. The market has never once delivered that. It fell roughly 37% in 2008 and rose roughly 32% in 2013; declines of 20% or more have arrived repeatedly and without warning. The average is a destination that a long horizon eventually reaches, not a path you ride.

Three practical consequences:

• Short horizons break the projection. Over one year, the range of outcomes swamps the average. Money you may need within about five years does not belong in stocks — keep it in cash or a high-yield savings account, where a bad month can't force you to sell at the bottom.
• The bad years are the price of the average. Selling during a 30% drawdown converts a temporary decline into a permanent loss and forfeits the recovery that the historical average is built on. The investors who earn 10.5% are the ones who sat through the years that earned −37%.
• Monthly contributions turn volatility into an advantage. A fixed $500 buys more shares when prices are down. You cannot time the bottom, but contributing on schedule means you automatically buy some of it.

Treat the final balance as the center of a wide distribution, not a promise. Run it again at 7% and at 6% — if the plan still works at the low end, volatility is a discomfort rather than a threat.