Snowballr
Compound InterestDebt PayoffScenariosRetirementSavings GoalSimple InterestGuides
Free · No sign-up required
Investment growth

Compound investment calculator

Free compound investment calculator for index funds, 401(k)s, and monthly contributions. See exactly how your portfolio grows at historical market rates — with visual charts and no sign-up.

How compound investing builds wealth

Compound investing is the engine behind every long-term wealth story. When you invest in broad-market index funds or a diversified portfolio, three things happen simultaneously: your principal earns returns, your returns earn returns, and your regular contributions stack on top of both. Over 20–40 years, this compounds into numbers that look impossible when you start.

The math everyone misses: $500/month at 8% for 30 years = ~$745,000. Your contributions total only $180,000. Compound returns generate the other 76% of the final value. Starting 10 years later with double the contribution still loses.

What return rate should I use?

  • S&P 500 historical average: ~10% annually (nominal, before inflation). Roughly 7% after inflation.
  • 60/40 stock-bond portfolio: ~7-8% nominal historically. Lower volatility than 100% stocks.
  • Aggressive all-stock (US + international): 8-10% nominal over 30+ year periods.
  • Conservative (mostly bonds): 3-5% nominal. Appropriate for near-retirement.

Use 7-8% as a reasonable default for long-term projections. Set aggressive (10%) only if you're young, 100% stocks, and stomach volatility. Always model a conservative scenario too — markets don't deliver smooth averages.

Using this calculator for your 401(k) or IRA

Plug in:

  • Initial amount: your current 401(k) or IRA balance
  • Monthly contribution: your contributions + any employer match (free money — always capture the full match)
  • Annual rate: 7-9% for a balanced target-date fund; 9-10% for all-stock index funds
  • Years: years until you plan to retire

The number this calculator shows is your projected nominal portfolio value. To estimate real purchasing power, subtract expected inflation (2-3%) from the annual rate.

Why starting early beats contributing more

Two investors, same 8% return. Anna invests $5,000/year from age 25-35 (10 years, $50,000 total) then stops forever. Ben invests $5,000/year from 35-65 (30 years, $150,000 total). At 65:

  • Anna: $787,000
  • Ben: $611,000

Anna contributed 1/3 as much and ended with $176,000 more. The extra decade of compounding beat 20 years of extra contributions. Time is the most valuable investment input.

Compound investment calculator FAQ

What is the difference between a compound investment calculator and a compound interest calculator?+
The math is identical — both use the compound growth formula A = P(1 + r)^t plus contributions. The difference is framing: interest calculators assume a fixed rate (savings accounts, CDs), while investment calculators project average market returns that vary year-to-year. Use this calculator when projecting stocks, mutual funds, or index portfolios.
Does this calculator account for fees?+
No — the rate you enter is assumed net of fees. Index funds have expense ratios of 0.03-0.20%, so if your target return is 8%, enter 7.8-8%. Actively managed funds charging 1%+ dramatically reduce compound growth over decades — a 1% fee can consume 25% of your final wealth over 30 years.
Can I use this for dollar-cost averaging?+
Yes — that is exactly what the monthly contribution field models. You invest a fixed amount each month regardless of market price. Research shows lump-sum investing outperforms dollar-cost averaging about 66% of the time historically, but DCA reduces emotional stress and works great with steady income.
What about market crashes?+
This calculator projects the average long-term path. Real markets deliver +22% one year, -8% the next. Crashes like 2008 (-38%) and 2022 (-19%) happen. Over 20+ year periods these average out to ~10% nominal for the S&P 500. If you sell during crashes, you miss the recovery — that is why long-term investors hold through volatility.

Related calculators

Compound interest calculator
For savings accounts, CDs, and fixed-rate products.
Retirement calculator
When can you retire? Find your FI number.
Savings goal calculator
How much to invest monthly to reach any target.
Guide: investing 101
Stocks, bonds, index funds — explained simply.