Analysis · Tool
S&P 500 breadth · updated weekly (Saturdays) · latest data loading…
Is this another dot-com? The real question isn't whether prices are "too high" — it's how few names are doing the work. This gauge measures that breadth live across the full S&P 500.
Near the dot-com top the headline index kept climbing while fewer and fewer stocks actually participated. Narrow breadth isn't a precise timing tool — markets can stay narrow for a long time — it's a fragility gauge. The meter scores four signals, each as a percentile of its own history (so it self-calibrates and stays neutral), then averages them into a 0–100 reading:
A high reading means breadth is narrow and fragile; a low reading means the rally is broad and healthier. The reassuring counter-signal is a rally that broadens out — more stocks, sectors, and countries joining — which tends to last longer.
The most important thing about this meter is what it is not: a sell signal. Breadth measures fragility, not timing. The dot-com market stayed narrow for roughly two years before the 2000 top, and anyone who sold at the first narrow reading missed some of the strongest gains of the entire bull market. What history does support is a more modest claim — that when an advance rests on a handful of names, it has fewer legs to stand on if those names stumble. A high reading is a reason to check your concentration and position sizes, not a reason to abandon a long-term plan.
Direction matters more than level. A reading that is high but falling — because more stocks are joining the advance — has historically been a healthier setup than a moderate reading that is climbing as leadership thins. The 2020–21 recovery is a good example of the first pattern: concentration was elevated, but participation broadened dramatically off the COVID low. Late 1999 is the textbook case of the second — the index made new highs while the average stock had already been declining for months. The historical line under the gauge exists precisely so you can see which of those patterns the current reading belongs to.
It's also worth knowing why the meter self-calibrates with percentiles instead of fixed thresholds. There is no magic number where a market becomes a bubble — top-10 concentration near 40% would have seemed unthinkable in 2015 and is simply the operating environment now, driven partly by the real earnings growth of the mega-caps. Scoring each signal against its own history keeps the gauge honest about that drift: it flags when today is extreme relative to the recent regime, which is the only comparison that has any meaning across eras. And because the underlying data uses today's index membership applied backward (the survivorship approximation we disclose across the site), readings from the earliest years of the window are softer evidence than recent ones.
Finally, the meter is one input, not a verdict. Concentration can stay high for years while the market compounds; participation can collapse briefly in every ordinary correction. The dot-com lesson wasn't "narrow markets crash" — it was that narrowing breadth plus extreme leader valuations plus deteriorating participation formed a pattern worth respecting. That's why the gauge blends signals instead of trusting any single one, and why the fourth signal (leader valuation) is included even though its history is short.
Read more: the thinking behind this gauge is in AI Boom or Dot-Com Repeat? The One Warning Sign That Flashed in 2000. Want to see it in individual names? Put a few AI leaders against the broad S&P 500 in Compare mode.