VTI vs VOO

VOO tracks the S&P 500 — roughly the 500 biggest US companies. VTI tracks the entire investable US stock market, around 3,500 to 4,000 companies including mid- and small-caps. Both are Vanguard funds with the same rock-bottom expense ratio.

Because the S&P 500 makes up about 85% of total US market capitalization, VTI and VOO overlap heavily. The top holdings and sector weights are almost identical; the difference is the smaller-company tail VTI adds.

Mid- and small-cap stocks historically carry higher volatility and, over very long horizons, a modest return premium — though that premium has been inconsistent and has lagged in recent large-cap-dominated years. When smaller companies outperform, VTI edges ahead; when mega-caps lead, VOO does.

In practice the two funds' long-run returns have been very close. VTI gives you the more academically complete 'own everything' position, which is why it anchors the classic Bogleheads Three-Fund Portfolio.

If you want a single fund representing all of US equities, VTI is the cleaner choice. If you specifically want the S&P 500 — for example to match a benchmark or pair with a separate small-cap holding — VOO is the right tool.

There is no meaningful cost penalty either way. Investors deciding between the two rarely go wrong; the decision matters far less than staying invested and controlling contributions.