The VOO Investing Experiment: Testing 3 Common Assumptions With Real Numbers

"Experiment" here doesn't mean real money — it means testing a few common investing beliefs against actual math, using the same compound-growth engine that powers every calculator on this site. Each scenario below states its assumptions up front, and you can reproduce or change any of them yourself in the VOO calculator.

The quick answer

Three things move the outcome far more than most people expect: starting a few years earlier, the return assumption you pick, and whether money goes in as a lump sum or gradually over time. None of this is a prediction of VOO's actual future return, which is unknown — these are hypothetical illustrations of how compounding math behaves.

A quick baseline before the experiments: VOO is Vanguard's S&P 500 ETF, launched September 7, 2010, with a 0.03% expense ratio as of this writing. It tracks the S&P 500 index, so its long-term behavior is tied to that index's roughly 500 large U.S. companies rather than to any single stock. That expense ratio is low enough that it barely registers against the swings caused by the return assumption itself — which is exactly what Experiment 2 below shows in dollar terms. (Source: Vanguard's VOO fund profile, checked September 2026.)

Experiment 1: Does starting 5 years earlier really matter?

Assumptions: $300 invested every month, no starting lump sum, a 7% assumed annual return, compounded monthly.

Five extra years of contributions add $18,000 to the amount actually put in — but they add roughly $123,000 to the ending balance. The gap isn't mostly the extra contributions; it's that those five years sit at the end of the period, compounding on top of a balance that's already large. This is the practical case for starting sooner rather than waiting for a "better" time to begin.

Experiment 2: How much does the return assumption change things?

Assumptions: $300 a month, no lump sum, 20 years, compared at three assumed annual returns. Nobody can tell you which of these is correct — they're shown as a range, not a forecast.

Total contributions are identical in all three rows ($72,000) — every dollar of difference comes from the return assumption alone. A 4-percentage-point spread between the low and high case turns into a difference of roughly $89,000 by year 20, which is why the return you assume matters more than almost any other input in a projection like this. For more on why small percentage differences compound into large dollar gaps, see how much $500 a month in VOO could become.

Experiment 3: Lump sum vs. monthly, same total money

Assumptions: $36,000 total, invested either as a single lump sum on day one or as $300 a month over 10 years, both at a 7% assumed annual return.

Same total money in, different result — because the lump sum has 10 full years to compound, while each monthly contribution only compounds for the time remaining after it goes in. This is a mathematical property of compounding, not investing advice: in practice, most people don't have $36,000 sitting in cash ready to deploy, which is the entire reason dollar-cost averaging exists as a practical strategy. See the DCA calculator to model consistent monthly investing on its own, or the VOO investment calculator to combine a starting lump sum with ongoing monthly contributions.

What these experiments don't prove

Every figure above comes from the same compound-growth formula used throughout this site — see the methodology page for the exact math — applied to a stated assumption, not a guarantee. None of these scenarios account for taxes, account type, fees beyond VOO's expense ratio, or the reality that real returns arrive unevenly year to year rather than as a smooth annual rate. VOO's actual future returns are unknown and could be higher, lower, or negative in any given year. Treat every number here as a way to build intuition about how compounding behaves, not as a prediction of what your own investment will become.

Real markets also don't move in a smooth, constant line the way these tables assume — a 7% "average" annual return might actually be a year of +20% followed by a year of -10%, arriving in a different order than any of that math implies. The ending balance can still land close to the projection over long periods, but the path there is far bumpier than a table of round numbers suggests.

If you want to run your own version of any of these experiments — a different monthly amount, a longer or shorter horizon, or your own return assumption — the calculators linked throughout this article use the exact same formula shown here, so the numbers you get back will be just as verifiable.