Understanding Stock Indices

A stock index is a single figure that tracks the combined performance of a defined group of shares, giving a snapshot of how a market or sector is performing. Indices such as the S&P 500, the Nasdaq 100, the FTSE 100, and the DAX are among the most widely followed financial benchmarks in the world, and through CFDs they are among the most actively traded instruments available to retail traders.

What an index measures

An index is a calculation, not a tradable asset. A provider defines a set of rules for which companies are included, how each is weighted, and how the index level is computed from the constituent share prices. The result is a number that rises and falls with the aggregate value of the basket.

Indices serve several purposes. For investors, they are benchmarks against which fund performance is measured. For economists, they are indicators of business confidence and wealth. For traders, they provide a way to take a view on an entire market in a single position rather than selecting individual shares.

How indices are weighted

The weighting method determines how much influence each constituent has on the index level.

Market-capitalisation weighting is by far the most common. Its practical consequence is that a cap-weighted index can rise or fall on the strength of a handful of very large companies even when most constituents move the other way. Traders should know the concentration of the index they are trading.

The major indices

Each index has a distinct sector composition, which determines what moves it. A technology-heavy index responds strongly to interest-rate expectations; a resource-heavy index responds to commodity prices; an index of multinationals responds to the home currency's exchange rate.

What moves stock indices

Corporate earnings

Over the long run, share prices follow earnings. Quarterly reporting seasons, when most constituents publish results within a few weeks, produce concentrated periods of index volatility. Guidance about future earnings often matters more than the reported figures.

Interest rates and central-bank policy

Interest rates affect equities through two channels. Higher rates increase the discount rate applied to future earnings, lowering present valuations, and they make bonds a more attractive alternative to shares. Sectors whose value rests on earnings far in the future, such as technology, are most sensitive. Federal Reserve decisions and communications, described in central banks and currency markets, are therefore closely watched by index traders. The mechanics are covered in interest rates and forex.

Economic data

Growth, employment, and inflation data shape both earnings expectations and rate expectations. The relationship is not always intuitive: strong data can weigh on indices if it raises the prospect of tighter policy, and weak data can support them if it raises the prospect of easing. Scheduled releases appear in the economic calendar.

Risk sentiment

Indices are the clearest expression of global risk appetite. Geopolitical events, financial stress, and shifts in investor confidence move them broadly and quickly. Correlations between major indices rise sharply during sell-offs.

Currency effects

An index of exporters benefits when its home currency weakens, because foreign earnings translate into more domestic currency. The FTSE 100 and the Nikkei 225 are notable examples. A trader in these indices is implicitly taking a view on the currency as well.

Commodity prices

Indices with large energy or mining weightings respond to oil and metals prices. Conversely, higher input costs from rising commodity prices can weigh on indices dominated by consumers of those commodities.

Index composition changes

Providers periodically add and remove constituents. Because index-tracking funds must buy and sell to match, these rebalancing events create predictable flows around the effective date.

Trading indices through CFDs

A CFD on an index tracks the price of the underlying, usually derived from the relevant futures contract, and allows a trader to take a long or short position without owning any shares. Key features:

• Leverage. Index CFDs are traded on margin, so a position controls exposure larger than the margin deposited. The leverage article explains the implications.
• Long and short. A trader can profit from falling as well as rising prices.
• Trading hours. Many index CFDs are quoted beyond the underlying exchange's hours, though liquidity and spreads vary. The trading hours tool shows when each market is most active.
• Overnight financing. Positions held past the daily rollover incur a financing charge or credit.
• Dividend adjustments. When a constituent pays a dividend, price-return index CFDs are typically adjusted so that neither side gains or loses from the ex-dividend price drop.

The available index markets and their contract specifications are listed on the markets page.

Practical considerations

Index volatility varies greatly by session. The most active period for a US index is during US cash-market hours, particularly the open and the close, and around scheduled data. European indices are most active during European hours and again during the US open. Trading outside these windows means wider spreads and thinner liquidity.

Because indices can gap at the cash-market open and move sharply on news, position sizing should reflect the index's typical range rather than a fixed point count. The position size calculator allows the stop distance to be set from volatility, and the risk per trade to be held constant across instruments.

Key takeaways

• A stock index is a calculated figure summarising the performance of a defined basket of shares; it is not itself tradable, so exposure comes through futures, ETFs, or CFDs.
• Most major indices are market-capitalisation weighted, meaning a small number of very large companies can dominate their movement.
• Each index has a distinct sector and currency profile, which determines whether it responds most to rates, commodities, or exchange rates.
• Indices move on earnings, interest-rate expectations, economic data, risk sentiment, and periodic composition changes.
• Index CFDs offer leveraged long and short exposure with extended hours, but liquidity and spreads vary by session and positions should be sized from volatility.