Stock Index

Tracking hundreds or thousands of individual share prices at once is impractical, so index compilers — organizations like S&P, MSCI, or FTSE — select a basket of stocks and collapse them into one figure. That figure rises and falls as the underlying stocks move. The index number itself has no unit like dollars or shares; it is a dimensionless score whose meaning comes from how it changes over time, not from its absolute level. Our guide on reading percentage moves explains why the change column matters more than the level.

Different indexes cover different slices of financial markets: some track an entire country's large companies, others focus on a single industry like technology or energy. The stocks included, and how much influence each stock gets, are determined by the index's methodology — the rule set the compiler publishes. Two indexes covering the same country can show different returns on the same day simply because they chose different stocks or weighting rules. See index weighting for why that happens.

Indexes also serve as the basis for financial products. ETFs (exchange-traded funds) and index futures are designed to replicate a specific index's return. When market data tables show a stock index, the quoted number is usually the index's closing level or its latest calculated value during trading hours, alongside a point change and a percentage change. The percentage change is what most economists and analysts compare across indexes, since raw index levels are not comparable to each other. Live index data is available on the stocks and shares pages.