S&P 500 Heatmap

How to read the map

Start with the size, then look at the colour. The big rectangles are the companies that weigh most in the index: on a day when the five or six largest move together, the whole S&P 500 moves, even if most companies do little. If the map is almost all green but the big rectangles are red, the weighted average can still fall.

Then look at the sectors. Each named block is a sector (technology, health care, financials…) and the figure next to it is its average move, weighted by size. When one sector goes the opposite way to the rest, there is often something that affects only it: earnings, an interest-rate figure or the oil price.

Hover over a company to see its name, price and change. With the market closed, the map shows the last session; the date and time of the snapshot are above the map.

Frequently asked questions

What is a stock market heatmap?

A way to see a whole market at a glance. Each rectangle is a company in the S&P 500, the index of the 500 largest US companies: its size says how much it is worth on the stock market and its colour how much it has moved today. In a few seconds you see whether the day is green or red, which sectors are leading and which big companies are going their own way.

How do I read the colours and the size?

Green means the stock is up against the previous close and red that it is down; the stronger the colour, the bigger the move, up to ±3%, where the colour saturates. Grey is practically flat. The size is the market capitalisation: Apple, Microsoft or Nvidia take up far more room than a 20-billion-dollar company. Companies are grouped by sector and, within each sector, by sub-industry (GICS classification).

What does the figure next to each sector mean?

The move of the whole sector, weighted by the size of each company: a rise in Nvidia counts for more than one in a small company. The average above the map is worked out the same way with every company, and comes close to what the index does that day.

Where does the data come from and how often is it updated?

Prices and market capitalisation come from Yahoo Finance, and the list of stocks in the index from Wikipedia's. It is not a real-time map: it is a snapshot of every price that is renewed periodically, and the date and time it was taken are shown above the map. When the market is closed, it shows the last session.