Commodities & indices · Beginner

What Are Stock Indices? An Overview

An index is a weighted calculation, not an average, and the top handful of constituents often decide where it closes.

Last reviewed 15 September 2026 by the MarketPro research desk · Editorial policy

In short

A stock index tracks the combined value of a defined basket of shares in one number. Most headline indices are weighted by market capitalisation, so the largest companies dominate the result. Retail traders access indices through CFDs or futures priced off that calculation, which means no share ownership, no dividends, and a point value that determines position size.

What to remember

A cap-weighted index is not an average. If the top seven names hold 32% of the weight, a 5% fall in them drags the index down 1.6% with everything else flat.
An index removes single-company risk and leaves market risk untouched. In a broad selloff every constituent falls together.
Index positions pay no dividends. Brokers apply a dividend adjustment instead, credited to longs and debited from shorts.
Know whether your quote is the cash index or the front-month future. It decides your trading hours, your gaps and your roll dates.

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A stock index is a single number that tracks the combined value of a defined basket of shares, so one figure stands in for the direction of a whole market or sector. When you trade an index you are not buying the companies. You are trading a derivative whose price references a calculation, and understanding what goes into that calculation is the difference between a considered position and a guess.

Why 500 companies can move like seven

Most headline indices are weighted by market capitalisation, which means influence is proportional to company size. In a broad US index where the seven largest constituents carry roughly 32% of the total weight, the arithmetic is unforgiving.

Scenario	Top 7 (32% weight)	Other 493 (68% weight)	Index result
Tech selloff	-5.0%	0.0%	-1.60%
Broad rally, megacaps flat	0.0%	+1.5%	+1.02%
Both directions	+3.0%	-1.0%	+0.28%

Row three is the one that catches people out. The index closes up a quarter of a percent on a day when 493 of its 500 members fell. Anyone reading that close as a sign of broad strength has read it wrong, and any strategy that assumed the index was a proxy for the average company has just been given a false signal.

What you actually hold in an index position

Retail exposure usually comes through a CFD or a spread bet priced off either the cash index or the front-month index future. The distinction matters more than it sounds. A cash index is only calculated while its underlying exchange is open, so a broker quoting it around the clock is deriving the out-of-hours price from the future. A futures-based quote runs nearly 24 hours but carries its own expiry and roll.

Neither gives you a dividend. When a constituent goes ex-dividend the cash index drops mechanically, and a broker offsets this with a dividend adjustment credited to longs and debited from shorts. A broad US index yielding around 1.3% distributes that in lumps, and if you are short across a heavy ex-dividend day the debit arrives whether the position is working or not.

Point value, and the number that decides your size

An index quote is in points, and one point is worth whatever the contract says it is worth. On a common US 500 CFD, one lot is 1 USD per point. With the index at 5,500 that is 5,500 USD of notional exposure per lot. If the daily average true range is 60 points, a two-ATR stop is 120 points, or 120 USD per lot.

On a 5,000 USD account risking 1%, the budget is 50 USD, which allows 0.41 lots. Most platforms will let you open ten times that without complaint. At roughly 275 USD of margin per lot at 1:20, four lots ties up 1,100 USD of a 5,000 USD account, so the screen presents it as comfortably affordable while the stop behind it risks close to 500 USD, or 10% of the balance. Margin is not a sizing decision the platform can make for you. Work it out first with the position size calculator and the margin calculator.

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The criteria that decide who is in the basket

Every index is defined by a rule book: a minimum market capitalisation, a listing venue, a free-float requirement, sometimes a profitability test, sometimes a sector exclusion. A committee or an algorithm applies those rules at scheduled reviews, and companies that no longer qualify are replaced by ones that do.

That mechanism has two effects worth understanding. Additions and deletions create forced flow, because every fund tracking the index has to buy the new name and sell the old one on the same day, which is why an announced inclusion often moves a share price well before the change takes effect. And the rule book quietly shapes what the index measures. An index that requires a profit history excludes the loss-making growth companies that another index welcomes, so the two can tell opposite stories about the same economy in the same quarter.

What an index does not protect you from

It is not diversification. A basket removes single-company risk, which is real, but it leaves you fully exposed to the market factor. In a broad selloff every constituent falls together, and that is exactly when the basket was supposed to help.
Sector concentration hides inside the label. Two national indices can sit in the same region and behave differently because one is 40% technology and the other is 40% banks and miners.
Composition changes. Constituents are reviewed periodically, and the index you backtested five years of data on is not made of the same companies today.
Overnight gaps are normal. Earnings and policy decisions land outside cash hours, and a stop resting inside the gap fills at the first available price, not at your level.

Three checks before your first index trade

First, look up the top ten constituents and their combined weight. If it is above 30%, you are trading a concentrated position wearing a broad name, and you should know which earnings dates matter. Second, confirm whether your broker prices the cash index or the future, and find the daily maintenance break and the roll date. Third, write down the point value, the average daily range in points, and what a two-ATR stop costs per lot.

Those three answers take about fifteen minutes to find and they change how every subsequent decision is made. An index is a clean instrument to trade, with tight spreads in cash hours and a deep, well-understood market. It is just not the neutral average of an economy that its name implies.

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Written and reviewed by the MarketPro research desk

MarketPro is a trading-signal and trading-education app operated by Harajuku Holdings LTD (Cyprus). Our signal desk publishes and tracks every trade idea in-app, and every page here is checked against the app's live behaviour before publishing. Last reviewed 15 September 2026. How we produce signals and content · About MarketPro

FAQ

What Are Stock Indices? An Overview: questions

Do you own shares when you trade a stock index?

No. An index is a calculated number rather than a security, so there is nothing to own directly. Retail exposure comes through derivatives such as CFDs, spread bets or index futures, which reference the calculation. You have no voting rights and receive no dividends, although brokers apply a dividend adjustment to reflect the mechanical drop in the index when constituents go ex-dividend.

Is trading an index safer than trading a single stock?

It removes one risk and leaves another. A basket of hundreds of companies means no single earnings miss or fraud can wipe out the position, which genuinely lowers idiosyncratic risk. But the market factor remains in full, and leverage on an index CFD can easily make the position more dangerous than an unleveraged share holding. The instrument is diversified, the trade is not automatically safer.

What is a point worth on an index CFD?

It depends on the contract specification, not on the index level. A common US 500 CFD is quoted at one unit of currency per point per lot, so with the index at 5,500 one lot carries 5,500 units of notional exposure. Check the point value in your broker's contract specification before sizing, because the same index can be offered in several different contract sizes.

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