Investment Calculator

Q: What is a realistic investment return to assume?

The U.S. stock market has averaged about 10% annually over the past century — roughly 7% after inflation. Planners commonly model 6%-8% nominal for diversified stock portfolios and less for balanced portfolios. Assuming double-digit returns for planning purposes is risky.

Q: How much will $500 a month be worth in 20 years?

At a 7% average annual return, $500 monthly grows to roughly $260,000 in 20 years — of which $120,000 is contributions and about $140,000 is growth.

Q: Why show inflation-adjusted results?

A dollar in 20 years buys less than a dollar today. At 3% inflation, $260,000 in 2046 has the purchasing power of about $144,000 today. Real (inflation-adjusted) figures tell you what your future balance will actually feel like.

Project what regular investing becomes over time — in tomorrow's dollars and, more honestly, in today's purchasing power.

Projected portfolio value

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Total contributed

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Investment growth

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In today's dollars

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Year	Contributed	Growth	Balance

The math (and its honest limits)

The projection compounds your balance monthly at your assumed annual return:

Balance_next = Balance × (1 + r_monthly) + Contribution
Real value = Nominal ÷ (1 + inflation)^years

Real markets don't return the same number every year — they lurch. A smooth 7% line is a planning average, not a promise; actual sequences of returns can land meaningfully higher or lower, especially over short periods.

Worked example

$5,000 start + $500/mo at 7% for 20 years:

Projected value ≈ $280,000 ($125,000 contributed, ~$155,000 growth)

In today's purchasing power at 3% inflation: ≈ $155,000.

What actually moves the outcome

In order: time in the market (each extra decade roughly doubles-plus the result — the compounding curve is steepest at the end), contribution rate (the only variable fully in your control), costs (a 1% annual fee compounds against you exactly like a 1% lower return — six figures over a career), and only then asset selection. Historical context: the S&P 500's long-run average is ~10% nominal, ~7% real, but individual decades have ranged from negative to +18%/yr. Model conservatively, contribute aggressively. For the payout phase, see the withdrawal calculator; for the tax wrapper decision, the 401(k) and Roth IRA calculators.

Frequently asked questions

What is a realistic investment return to assume?

Stocks have averaged ~10% nominal (~7% real) over the past century; 6–8% is a common planning range for diversified portfolios.

How much will $500 a month be worth in 20 years?

At 7%, roughly $260,000 — $120,000 contributed, ~$140,000 growth.

Why show inflation-adjusted results?

Because future dollars buy less. $260,000 in 20 years ≈ $144,000 of today's purchasing power at 3% inflation.

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Note: Projections assume constant returns, which markets do not deliver; actual results will differ. Educational only — not investment advice or a recommendation of any security or strategy. Last reviewed: July 2026.