What Is Tracking Error and Should You Care?
Tracking error is the gap between an ETF and the index it promises to copy. For a fund like VOO it's a rounding error; for a leveraged or exotic fund it can be the whole story.
Don't have time? Here's what you need to know:
- 1Tracking error is how far a fund's return drifts from its index; the main causes are fees, cash drag, sampling, and trading costs.
- 2Broad U.S. index ETFs like VOO and VTI track within a few hundredths of a percent a year, roughly their expense ratio.
- 3Securities-lending income can shrink the gap, occasionally letting a fund trail its index by less than its fee alone.
- 4It matters most for leveraged, inverse, and niche funds, and least for the broad core ETFs most investors actually hold.
Tracking Error in One Sentence
Tracking error is how much an index fund's return deviates from the return of the index it is built to copy. If the S&P 500 returns 12.0% in a year and an S&P 500 ETF returns 11.95%, the fund has trailed its index by 0.05 percentage points. The two most common ways to express this are the simple performance gap (sometimes called tracking difference) and the standard deviation of that gap over many periods, which is the textbook definition of tracking error.
The reason it exists at all is that an ETF is a real, operating fund that pays fees, holds some cash, and trades in the real world, while the index it follows is a frictionless math formula on paper. The index never pays a commission or a salary; the fund does. That difference, however small, is what tracking error captures.
The Four Things That Cause It
Almost all tracking error comes from a short list of mechanical frictions. None of them are scandals; they are the ordinary cost of turning an index into an investable fund.
Securities lending is the one factor that can push the other way. Many funds lend out their shares to short sellers and collect a fee, and that income can partially offset the expense ratio. In some years a well-run fund's lending revenue is large enough that its tracking difference is smaller than its fee alone would predict.
- Fees: the expense ratio is deducted from the fund daily, so a 0.03% fund starts each year about 0.03% behind the index before anything else happens.
- Cash drag: a fund holds a sliver of cash to handle redemptions and dividends, and cash earns less than the index in a rising market.
- Sampling: bond and broad-market funds often hold a representative sample rather than every single security, so their basket drifts slightly from the index.
- Trading and rebalancing costs: when the index changes its constituents, the fund must buy and sell, paying spreads and commissions the index ignores.
How Much Is Normal? A Reality Check
For a large, plain-vanilla index ETF, tracking error is tiny. Funds tracking the S&P 500 or a total U.S. market index, such as VOO or VTI, typically track their indexes to within a few hundredths of a percent a year, roughly in line with their expense ratios. That is about as good as it gets, and it is the main reason these funds are recommended as core holdings.
Tracking error climbs as the underlying market gets harder to replicate. International and emerging-market funds face time-zone gaps, foreign taxes, and less liquid stocks, so a fund like VWO tends to run a wider gap than a domestic large-cap fund. Bond funds that sample a giant index, such as BND or AGG, also show a bit more drift than an S&P 500 fund.
| Fund type | Typical annual tracking error | Main driver |
|---|---|---|
| S&P 500 / total U.S. market ETF | ~0.01-0.10% | Expense ratio |
| Developed international ETF | ~0.1-0.5% | Foreign taxes, time zones |
| Emerging-market ETF | ~0.3-1.0%+ | Liquidity, sampling |
| Broad bond ETF (sampled index) | ~0.1-0.5% | Sampling, bid-ask spreads |
| Leveraged / inverse ETF | Large and compounding | Daily reset, not designed to track long-term |
Tip: Compare a fund's actual multi-year return against its index's return on the fact sheet. A persistent gap much larger than the expense ratio is a yellow flag worth investigating.
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When You Should Actually Care
For a buy-and-hold investor in a broad index ETF, tracking error is rarely the deciding factor. A gap of 0.05% a year is dwarfed by your contribution rate, your asset allocation, and whether you stay invested through a downturn. Two S&P 500 funds with near-identical tracking error should be chosen on expense ratio and tax efficiency, not on a hundredth of a percent of drift.
Where it genuinely matters is at the edges. Leveraged and inverse ETFs reset daily, so over weeks or months their returns can diverge wildly from a simple multiple of the index, that is by design, not a defect to fix. Niche, thinly traded, or brand-new funds can also track poorly. And if you are choosing between two funds on the same index, the one with the consistently smaller tracking difference is quietly handing you free return.
Important: Do not assume a leveraged ETF that is '3x' the S&P 500 will deliver 3x the index over a year. Daily compounding means its long-run return can be far higher or far lower, and in choppy markets it usually decays.
How to Check a Fund's Tracking Before You Buy
You do not need a Bloomberg terminal. Every ETF publishes a fact sheet that lists the fund's returns next to its benchmark's returns over 1-, 5-, and 10-year periods. Subtract one from the other and you have the tracking difference for that window. If the fund has consistently matched its index minus roughly its fee, it is doing its job.
Three quick checks separate a reliable fund from a sloppy one: a long track record (newer funds have noisier tracking), a large asset base (bigger funds trade more efficiently), and a tracking difference that stays close to the expense ratio year after year. A fund that clears all three is tracking its index about as well as it realistically can.
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Frequently Asked Questions
Is a lower tracking error always better?
For an index fund, yes, lower is better, because the fund's entire job is to mirror its benchmark as closely as possible. A consistently small tracking difference means you are getting the index return minus only the unavoidable costs. The exception is leveraged and inverse funds, which are not designed to track an index over the long run at all, so the concept applies differently to them.
What's the difference between tracking error and tracking difference?
Tracking difference is the simple gap between the fund's return and the index's return over a period, for example the fund returned 0.05% less than its index last year. Tracking error, strictly defined, is the standard deviation of those gaps over many periods, a measure of how consistent the gap is. In casual use people often say tracking error to mean the plain return gap.
Why does my ETF return slightly less than the index it tracks?
Mostly the expense ratio, which is deducted daily, plus small frictions like cash drag, sampling, and trading costs. A 0.03% S&P 500 fund should trail its index by roughly 0.03% a year, sometimes less if securities-lending income offsets part of the fee. A gap far larger than the expense ratio is the only kind worth worrying about.
Do bond ETFs have higher tracking error than stock ETFs?
Often, yes. Large bond indexes can contain thousands of issues, many of them illiquid, so funds hold a representative sample rather than every bond. That sampling, plus wider bid-ask spreads in fixed income, tends to produce a slightly larger tracking difference than a liquid S&P 500 fund, though it is still small for major funds like BND and AGG.
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Alex Harrington
CFA Level II Candidate, Finance & Economics
Alex Harrington is an independent ETF researcher and personal finance writer with over 8 years of experience analyzing exchange-traded funds. A CFA Level II candidate with a background in economics, Alex has reviewed 800+ ETFs and helped thousands of beginners build their first investment portfolios through clear, jargon-free education.
This content is for educational purposes only and does not constitute financial advice. Past performance does not guarantee future results. Consult a licensed financial advisor before making investment decisions.