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The Vanguard Morningstar Total Stock Market ETF and the Schwab U.S. Broad Market ETF both feature a low 0.03% expense ratio. The Vanguard fund holds roughly 1,200 more stocks than the Schwab fund, providing deeper exposure to small-cap companies.

VTI's rock-bottom fee gets all the attention, but there's a second cost buried in the fund's structure that most investors never think to check before they buy.

Both ETFs offer identical, industry-leading expense ratios of 0.03%. The iShares Core S&P Total U.S. Stock Market ETF manages nearly $100 billion in assets and has a longer operational history than its Schwab counterpart.

Three total market index ETFs dominate the do-it-yourself investor's shortlist for whole-market US equity exposure: Vanguard Total Stock Market ETF (NYSEARCA:VTI), iShares Core S&P Total U.S.

Instead of buying the 30 stocks of the Dow Jones Industrial Average, this ETF offers exposure to nearly 2,500 stocks.