Analysis · Tool

AI Bubble Meter

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.

How it's built

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:

Concentration — the combined market-cap weight of the 10 largest names. The higher their share, the more the index rests on a handful of stocks.
Narrow leadership — the trailing-12-month gap between the cap-weighted S&P 500 and the average (equal-weight) stock. A wide gap means the index is rising on a few giants while the typical stock lags.
Weak participation — the share of S&P 500 stocks trading below their own 10-month trend. When fewer stocks are in uptrends, participation is thinning under the surface.
Leader valuation — the earnings yield of the mega-cap leaders (net income ÷ market cap of the top 10); a lower yield means pricier leaders. This signal needs company fundamentals (available from about 2022, applied with a one-year reporting lag to avoid look-ahead), so the scored history begins in 2023 — the meter covers the last few years to today.

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.

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.

Educational content only — not investment advice, a recommendation, or a forecast. Built from historical S&P 500 data using the current index membership and current shares applied back through history (a survivorship/share approximation). It does not predict crashes. See our editorial policy.