An investment growth calculator projects what a starting balance plus ongoing contributions could become, assuming a steady average return. It's the clearest way to picture the long game: small, regular investing turning into a meaningful sum.
The default above — $10,000 to start, $500 a month, 7% a year for 20 years — reflects a broadly diversified portfolio over a long horizon. Change any input to match your own plan.
How investment growth compounds
Your returns get reinvested and start earning returns of their own. Early on, your contributions are the bulk of your balance. But after enough years, growth on past growth takes over — eventually you can earn more in a single year from the market than you contribute, a tipping point the calculator highlights for you.
This is why the growth curve bends upward rather than rising in a straight line. The back half of a long investing horizon does far more work than the front half.
Why regular contributions beat timing
Trying to time the market is famously hard. Contributing a fixed amount on a schedule — dollar-cost averaging — sidesteps the problem: you buy more when prices are low and less when they're high, and you stay invested through the ups and downs that actually generate the long-run return.
Use the “Add $100/month” and “Double your contribution” what-if chips to see how sensitive your final number is to how much you put in. For most people, contribution size is the lever they control most directly.
Returns, inflation, and being realistic
A 7% long-run return is a reasonable planning assumption for a stock-heavy portfolio, but real markets are bumpy — some years are up 20%, others down. Turn on the “likely range” toggle to bracket your projection, and remember that no calculator predicts the future; it models an average.
Inflation quietly erodes purchasing power, so a six-figure balance decades from now buys less than it sounds. Switch on “in today's money” to see the inflation-adjusted value — the number that actually reflects what your portfolio could buy.
Frequently asked questions
What is a realistic investment growth rate?
For a diversified stock portfolio, 6–8% a year is a common long-run planning figure after accounting for a typical mix. Individual years vary wildly. Use a conservative rate and the likely-range toggle to avoid over-promising yourself.
How much will my investments grow in 20 years?
It depends on your starting amount, contributions, and return. As an example, $10,000 plus $500 a month at 7% for 20 years grows to roughly $300,000 — and most of that is growth, not the money you put in. Enter your own numbers above.
Should I invest a lump sum or monthly?
Both work. A lump sum invested earlier has more time to compound, while monthly investing spreads out your risk and matches how most people earn. The calculator lets you model a starting amount and a monthly contribution together.
Does this account for inflation?
Yes — toggle “in today's money” to see the inflation-adjusted value of your projected balance, so you know what it would actually be worth in current purchasing power.
Worked examples
Each example below shows inputs fed directly into the compound interest engine. Every figure in the stat grid is computed at build time from those inputs, never hand-typed.
30-year investment portfolio
$10,000 starting investment, $200 added each month, at 8% annualized return compounded monthly for 30 years.
Total deposits of $82,000 grow to roughly $407,429 — the $325,429 gap is compounded growth. Compounding accounts for 80% of the ending balance, illustrating that consistent investing matters more than the starting amount.
Lump sum investment, 20 years
$20,000 one-time investment at 8% compounded monthly for 20 years, no additional deposits.
A $20,000 lump sum at 8% for 20 years grows to about $98,536 — 4.9× the starting amount, entirely from compounding. This is the power of time in long-term investing.
More questions answered
What is a realistic return rate for long-term investing?
The S&P 500 has historically returned about 10% nominally and 7% after inflation over long periods. A diversified portfolio including bonds typically returns 6–8% nominally. For planning purposes, most financial planners use 6–7% for conservative assumptions and 8–10% for moderate ones. Use the actual expected return for your specific portfolio allocation, not a generic average.
How much do I need to invest to become a millionaire?
At 7% compounded monthly, you need about $524/month for 30 years to reach $1,000,000 — a total deposit of $188,640. Starting with a $10,000 lump sum reduces the required monthly contribution to about $470. At 10%, you need only about $263/month from a standing start for 30 years. The biggest variable is time: starting 10 years earlier cuts the required monthly contribution roughly in half.
Method and assumptions
This calculator projects a balance from the inputs you give it. It is an arithmetic model, not a forecast, and not financial advice — a real account’s return varies year to year while this projection holds your rate constant.
- Period-by-period, not a formula
- The balance is stepped forward one compounding period at a time rather than evaluated with a closed-form equation. That is what makes the year-by-year breakdown real numbers from the same run you see at the top, rather than a separate approximation.
- How the periodic rate is derived
- Your annual rate is divided by the number of compounding periods in a year — a 6% rate compounded monthly applies 0.5% each month. This is the standard convention, and it is why the nominal rate and the APY are not the same number: compounding those twelve 0.5% steps produces slightly more than 6% over the year.
- When contributions land
- Contributions are spread evenly across every compounding period and deposited at the end of each one, after that period’s interest has accrued — the ordinary-annuity convention most calculators use. Depositing at the start of each period instead would produce a slightly higher balance, so treat this projection as the conservative side of that choice.
- Inflation adjustment
- The “in today’s money” figures deflate the balance by your inflation rate compounded annually. Contributions are shown in the dollars you actually deposit, so the inflation-adjusted interest figure is the real terminal value minus what you put in — not a separately deflated interest total. See nominal versus real figures for the distinction.
- Time to double
- The doubling figure describes your rate alone — how long a balance takes to double with no further deposits. It deliberately ignores contributions, because mixing them in would measure your savings habit rather than the return; the Rule of 72 is the mental shortcut for the same number.
- What is not modelled
- Taxes, account fees, fund expense ratios, and any penalty for early withdrawal are all excluded. Returns are assumed constant rather than variable, so no sequence-of-returns risk is captured. A taxable account will trail these figures; a tax-advantaged one will track them more closely.