Stock market heatmap

Two variables are on screen at once. Tile size is a company's approximate market weight, taken from a fixed list of large US stocks that is maintained in the page data and refreshed by hand — it does not move with today's action, and it is not updated for buybacks or new issuance. Colour is today's percentage move. Read size as "how much this company matters to the sector group" and colour as "how it is trading this session"; a big red tile is a large company having a bad day, not a large loss of weight.

The colour scale is deliberately capped at ±3%. Beyond that threshold a further move adds no extra saturation, so one stock gapping hard cannot wash the rest of the map into a single flat wash of red or green. That also means two tiles at opposite ends of a wild session can look identical — hover any tile for its exact figure, and click through to the ticker for the full signal. The brightness of the tint still varies inside the cap, so the palest tiles are the quietest movers.

Sector groups are sized by the combined weight of their tiles, which is why the largest groups dominate the layout. When a group looks busy, that tells you how much of the market's value its members represent, not how many companies are up. The count of advancers in the intro line covers only the stocks on this map.

Quotes come from a public Nasdaq endpoint, fetched in batches and cached for five minutes at the edge, so the timestamp under the map is the cache time and a fast market can run slightly ahead of what you see. If too few quotes come back, the page reports the map as unavailable instead of drawing a half-empty grid: a silently incomplete map reads like a crash and would be worse than no map. Coverage is limited to a fixed set of large caps — small caps, ADRs, and listings traded outside the US are not on this page, so a calm map does not mean a calm market.

Use the map to find where attention is concentrated, then do the reading on the ticker page. Today's move alone says nothing about why, and this page carries no history — it is a starting point for research, not a prediction.