Investment Calculator & Portfolio Growth Simulator

How this calculator works

It grows your starting balance month by month at the equivalent monthly rate for your chosen compounding frequency, and adds each contribution at the end of its period (monthly or yearly), optionally raising contributions 3% a year. Tax drag reduces each period's growth by the chosen share, and the real value divides the result by cumulative inflation.

By Suhaib Hassan · Formula verified 27 Sep 2026 · How CalculatePilot verifies formulas →

Understanding investment risk

This calculator assumes a constant annual return for simplicity, but real markets fluctuate year to year. Treat the result as a long-term average projection, not a guarantee — higher expected returns generally come with higher volatility.

What return rate should I actually assume?

The rate you enter drives everything else, so it's worth grounding it in real benchmarks rather than guessing.

• Diversified stock portfolios — historically around 7–10% annually before inflation over multi-decade periods, though any single year can vary widely.
• Bond funds — roughly 4%+ for U.S. government bonds as of recent years, more for higher-risk corporate or high-yield bonds.
• High-yield savings accounts and CDs — typically 3–4%+ depending on the rate environment and term.

A practical approach: run the calculator twice — once with a conservative rate (2–3 points below the historical average) and once with an optimistic rate (1–2 points above) — to see a realistic range rather than a single number that may not happen.

Understanding sequence of returns

This calculator assumes a constant annual return, but real markets rise and fall unevenly year to year. Two investors with the same average return can end up with very different balances depending on when the gains and losses occurred — a concept called sequence of returns risk.

This matters most near the point of withdrawal (like retirement), where a downturn early in that phase has an outsized negative impact compared to the same downturn occurring years later, since there's less time to recover before money is withdrawn.

A worked example

Investing $10,000 upfront plus $300/month at an 8% average annual return (compounded monthly) for 20 years grows to roughly $226,000. Of that, about $82,000 came from contributions and roughly $144,000 came from investment growth — nearly two-thirds of the final balance is growth, not deposits.

This illustrates why starting to invest early tends to matter more than the exact monthly amount — the growth portion compounds regardless of when contributions were made, so earlier dollars have simply had more time to work.

Diversification and risk

This calculator assumes a single constant return rate, but real investments carry risk that varies by asset type — stocks historically offer higher long-term returns than bonds, but with more year-to-year volatility. Diversifying across asset types is a common strategy to manage that volatility while still capturing long-term growth.

Tax treatment also affects real-world returns. Growth inside tax-advantaged accounts like a 401(k) or IRA isn't taxed annually the way a standard brokerage account often is, which can meaningfully improve actual after-tax growth compared to what a simple projection shows. Also worth checking: how much that growth is worth in today's dollars using our Inflation Calculator.

Frequently asked questions

Is 8% a realistic return assumption?expand_more

It's within the historical long-term range for diversified equity portfolios before inflation, but past performance doesn't guarantee future results.

How much difference do monthly contributions make?expand_more

A great deal over long periods — consistent contributions often account for more of the final balance than the initial investment itself, thanks to compounding.

Should I invest a lump sum or spread it out (dollar-cost averaging)?expand_more

Historically, investing a lump sum immediately outperforms spreading it out on average, since markets trend upward over time — but dollar-cost averaging can reduce regret risk and smooth out volatility for some investors.

What's a reasonable fee to pay on investments?expand_more

Low-cost index funds often charge well under 0.5% annually, while some actively managed funds charge 1% or more — fees compound over time just like returns, so lower costs meaningfully help long-term growth.

What is dollar-cost averaging?expand_more

Investing a fixed amount at regular intervals regardless of market price, which smooths out the effect of market timing and is the strategy this calculator's monthly contribution feature effectively models.

Should I use a tax-advantaged account for long-term investing?expand_more

Accounts like a 401(k), IRA, or Roth IRA offer tax benefits that can meaningfully improve long-term after-tax growth compared to a standard taxable brokerage account, making them worth prioritizing for retirement-focused investing.