Stock Indexes: The Complete Guide

What Is a Stock Index?

A stock index is a single number that summarises the collective value — or price movement — of a defined group of stocks. Think of it as a speedometer for a market: instead of watching hundreds of individual share prices tick up and down, you watch one number that captures the overall direction. The companies included are called the index's constituents, and the rules governing which companies qualify are set by the index provider.

Indexes don't represent every company that exists. They are curated lists, built around specific criteria such as company size, the country where shares are listed, or the industry sector a business operates in. That curation is why different indexes can tell very different stories about the same trading day — one might be rising while another falls, depending on which companies each tracks.

You can explore live readings for major indexes on the stocks overview page at any time.

Famous Indexes and What They Cover

A handful of indexes dominate financial headlines worldwide. Each covers a distinct universe of companies, which is why understanding what an index measures matters as much as knowing the number itself.

United States

The S&P 500 (sometimes quoted as US500 on data platforms) tracks 500 large companies listed on US exchanges. It is widely regarded as the broadest measure of the American large-cap stock market, meaning it focuses on the biggest publicly traded companies. Because it covers so many sectors — from technology to healthcare to energy — economists and fund managers treat it as a proxy for the US economy's corporate health.

The Dow Jones Industrial Average (DJIA, or simply "the Dow") contains just 30 large US companies. It is one of the oldest and most quoted indexes in the world, but its narrow roster means it can diverge significantly from the broader market. Notably, it is price-weighted rather than weighted by company size — more on that mechanic in a moment.

The Nasdaq Composite covers thousands of companies listed on the Nasdaq exchange, with a heavy concentration in technology firms. A related, narrower measure — the Nasdaq-100 — tracks the 100 largest non-financial companies on that exchange and is especially sensitive to moves in mega-cap tech stocks.

Europe and the UK

The FTSE 100 ("Footsie") lists the 100 largest companies traded on the London Stock Exchange, measured by market capitalisation — the total market value of a company's outstanding shares. Because many FTSE 100 members earn revenues globally and report in dollars, the index sometimes moves in the opposite direction to the pound: a weaker pound can actually flatter the index's reading in sterling terms.

Germany's DAX tracks 40 major companies listed in Frankfurt. One important quirk: the DAX's headline figure is a total return index, meaning it assumes dividends are reinvested. That makes it look higher over time than a simple price index of the same stocks would — a distinction explained in the next section.

Asia-Pacific

Japan's Nikkei 225 covers 225 companies listed on the Tokyo Stock Exchange. Like the Dow, it is price-weighted, which gives higher-priced shares a larger influence regardless of how big the underlying company actually is. The Topix is a broader Japanese measure that covers all domestic companies on the Tokyo exchange's Prime Market.

Other widely followed regional indexes include Hong Kong's Hang Seng, South Korea's Kospi, and Australia's ASX 200. For a deeper tour of how these markets differ, see Global Stock Markets: A Tour.

Points vs Percent: Reading the Number

When you see a headline say "the S&P 500 rose 45 points today," that raw number can be misleading without context. A 45-point move means something very different depending on where the index stands. If the index were at 900 (a purely hypothetical example for illustration), 45 points would be a dramatic 5% swing. If it stood at 5,000, the same 45 points would be less than 1%. This is why percentage change is almost always more meaningful than a raw point count.

Data platforms typically display percentage moves across several time windows — day, week, month, year-to-date, and year-over-year. Understanding how to read those columns is the fastest way to put any index move in context. The guide Day, Week, YTD, YoY: Reading Percentage Moves walks through exactly what each column means.

Index points themselves are not prices in the way a stock price is a price. They are calculated numbers derived from the constituent stocks according to the index's specific formula. Two indexes with the same point level are not necessarily "equally valued" — they are simply two different formulas applied to two different groups of companies.

Price Return vs Total Return: The Invisible Difference

A price return index tracks only the change in share prices of its constituents. A total return index assumes that every dividend paid out by those companies is immediately reinvested back into the index. Over long periods, that distinction compounds dramatically: dividends have historically contributed a substantial portion of equity market returns, so a total return version of any index will show a much higher cumulative gain than its price-only counterpart.

Most US indexes you see quoted — including the S&P 500 as commonly reported — are price return figures. The DAX, as noted above, is an exception and is quoted as total return by default. When comparing index performance across borders or over long time horizons, it pays to check which version you are looking at. A fund tracking an index will usually specify whether it targets price return or total return in its documentation.

The concept of dividend yield — the annual dividend payment expressed as a percentage of the share price — matters here too. An index with high-yielding constituents may look modest on a price-return basis but deliver considerably more when dividends are counted.

How Indexes Are Built: Weighting Mechanics

Not every company inside an index influences the number equally. The method used to assign that influence is called index weighting, and it has a large effect on how the index behaves.

In a market-capitalisation-weighted index (the most common type), each company's influence is proportional to its total market value. A company worth (hypothetically) $2 trillion moves the needle far more than a company worth $10 billion. This means the biggest companies dominate the index's daily moves — which is why traders watch the handful of mega-cap stocks in the S&P 500 or Nasdaq-100 so closely.

In a price-weighted index like the Dow or the Nikkei 225, a company with a high share price has more influence, regardless of how large or small the company actually is. A stock trading at $400 per share pulls the index more than one trading at $40, even if the $40 company is worth far more in total. This is an artifact of historical convention rather than a deliberate economic logic.

A third approach is equal weighting, where every constituent has the same influence. Equal-weight versions of major indexes exist and can behave quite differently from their cap-weighted siblings, particularly when the market's largest companies are driving — or dragging — the headline number.

For a full breakdown of how these mechanics work and why they matter, see Price-Weighted vs Cap-Weighted Indexes.

Trading Hours: Cash Markets and Nearly 24-Hour Futures

Stock exchanges have set hours. The New York Stock Exchange and Nasdaq run a core session from 9:30 a.m. to 4:00 p.m. Eastern Time on weekdays, excluding public holidays. The London Stock Exchange trades roughly 8:00 a.m. to 4:30 p.m. UK time. Tokyo's exchange operates in two sessions around a midday break. When those windows close, the cash market — the actual buying and selling of shares — stops.

Index futures contracts, however, trade on separate futures exchanges that run nearly around the clock on business days. Futures on the S&P 500, for example, keep trading through Asian and European hours, giving global markets a way to express views on US stocks even when Wall Street is dark. This is why you often see a futures price quoted overnight that differs from the previous day's closing cash level — it reflects new information arriving after the bell. The guide What Is a Futures Contract? explains how futures work in detail.

Pre-market and after-hours sessions also exist for individual stocks on some US exchanges, but liquidity — the ease of trading without moving the price — is typically much thinner outside core hours, so moves can be exaggerated. Traders typically treat overnight futures levels as a directional signal rather than a precise forecast of where the cash index will open.

Why Indexes Matter: Benchmarks, Sentiment, and Investable Products

Indexes serve three broad purposes that extend well beyond simply giving traders something to watch.

Benchmark

A benchmark is a reference point against which performance is measured. A fund manager running a US equity portfolio is almost always measured against the S&P 500: did they beat it, match it, or underperform it? Without a benchmark, there is no way to know whether an 8% annual return is impressive or disappointing. Indexes provide that common yardstick, making them central to the entire asset-management industry.

Sentiment Gauge

Because they aggregate the buying and selling decisions of millions of market participants, indexes function as real-time sentiment readings. Economists read sharp index declines as signals of deteriorating business confidence or tightening financial conditions. The relationship between index volatility — how wildly the numbers swing — and broader economic anxiety is tracked closely; the VIX, for example, measures the implied volatility of S&P 500 options and is often called Wall Street's "fear gauge." Index behaviour around events like earnings season or central bank meetings tells economists a great deal about where expectations stand.

Indexes also capture the risk-on / risk-off dynamic that ripples across global markets: when investors feel confident, equity indexes typically climb; when fear rises, money tends to rotate out of stocks and into perceived safe havens.

Investable Products

Indexes are not just abstractions — they are the foundation of a vast range of investable products. Index funds and exchange-traded funds (ETFs) are designed to replicate an index's performance, giving anyone with a brokerage account exposure to hundreds of companies through a single purchase. Index futures and options allow more sophisticated market participants to hedge or speculate on the direction of an entire market, rather than picking individual stocks. This investability is what turns a statistical measure into something with real-world financial consequences.

Understanding how stock markets work provides essential context for why these products exist and how they interact with the underlying index constituents. And because indexes are ultimately collections of companies, understanding how sectors are classified helps explain why an index heavy in energy stocks behaves differently from one heavy in technology.