A stock index is a combined measure that shows how a group of stocks is moving, not a single stock and not an exchange. For the US market, the three indices that show up most often in the news are the S&P 500, Nasdaq, and Dow Jones. They can move in opposite directions on the same day because their makeup and calculation methods are different.

You have probably read headlines like "Nasdaq plunges, Dow rises instead". If you don't know what each index measures, news like that feels confusing. This article explains what each index contains, how it is calculated, how to read its numbers, and how to use it as a portfolio benchmark.

Key Points

• An index is not an exchange. The S&P 500 and Dow Jones are lists of stocks whose movements are calculated together. The stocks in them can be listed on the NYSE or Nasdaq.
• The four US indices quoted most often: the S&P 500 (500 companies), Nasdaq Composite (almost every Nasdaq stock), Nasdaq-100 (the 100 largest non-financial companies on Nasdaq), and the Dow Jones Industrial Average (30 stocks).
• They are calculated differently. The S&P 500 and Nasdaq use market capitalization weighting, while the Dow uses weighting by price per share.
• Read percentages, not points. A 100-point gain means something very different on an index at 7,000 than on one at 52,000.
• Pick a fair comparison. A tech stock portfolio makes more sense compared with the Nasdaq-100 than with the Dow.
• An index is not a buy or sell signal. It describes what has already happened, not where prices will go next.

What Is a Stock Index, and How Is It Different from an Exchange?

A stock index is a single number that sums up the movement of a group of stocks. Think of it like a class average. The average shows how the class is doing overall, but it doesn't tell you each student's score.

An exchange is where stocks are bought and sold, such as the NYSE and Nasdaq. An index is a measuring tool. You can't buy the "S&P 500" directly on an exchange the way you buy a single stock. What you buy are the stocks inside it, or a product that tracks the index, such as an ETF. The difference between the US exchanges and the IDX is covered in IDX, NYSE, and Nasdaq: stock exchange differences for Indonesian investors.

Two things are often mixed up with this topic:

• IHSG and IDX indices. In Indonesia, the term "stock index" most often refers to the IHSG (Jakarta Composite Index) or an index like the IDX30. This article focuses on US indices. For Indonesian exchange indices, read The IDX30 Index: how to read its movement and composition.
• Price indices. The consumer price index and similar measures track inflation, not stocks. They are explained in Price Index: definition, formula, and how to calculate it.

Four US Indices Quoted Most Often

S&P 500: a broad snapshot of the US stock market

The S&P 500 contains 500 US companies. The number is fixed, and the list is chosen by an index committee at S&P Dow Jones Indices, so it is not simply the 500 largest companies. The requirements include being a US company listed on an eligible US exchange, having enough size and liquidity, and being profitable: the sum of earnings over the last four quarters and the most recent quarter's earnings must both be positive.

Because it uses market capitalization weighting, the companies with the highest value have the biggest influence. According to Wikipedia, the index covers about 83% of the market capitalization of US public companies as of the end of 2025. The number of tickers on its list is sometimes more than 500 because one company can have more than one share class.

Nasdaq Composite and Nasdaq-100: two indices, one exchange

News stories often call both of them simply "Nasdaq", even though their contents are different.

Nasdaq Composite covers almost every stock listed on the Nasdaq exchange, including ADRs (shares of foreign companies traded in the US). Products such as ETFs, preferred shares, and warrants are not included. It is weighted by market capitalization.

Nasdaq-100 contains only the 100 largest non-financial companies on Nasdaq. Its weighting is modified with certain limits so that a few giants don't dominate the total. Its composition is reviewed every December and rebalanced every quarter. According to Wikipedia, the Nasdaq-100 makes up about 80% of the Nasdaq Composite's weight. This is the index that ETFs like QQQ follow, which is covered in QQQ ETF: performance analysis and outlook.

So when you see a headline saying "Nasdaq fell 2%", first check whether it means the Composite or the Nasdaq-100.

Dow Jones Industrial Average: 30 stocks calculated by price

The Dow contains 30 large-company stocks chosen by a committee. Its stocks are listed on the NYSE and Nasdaq. It is weighted by price: the stock with the highest price per share has the biggest weight, regardless of the company's size.

The Dow's number is calculated by dividing the sum of the 30 stock prices by a divisor. According to Wikipedia, this divisor is about 0.168 as of June 2026, so a US$1 change in one stock moves the index by about 5.94 points. A common criticism is that high-priced stocks have an influence out of proportion to the company's size. The name "Industrial" also no longer reflects what is inside, because the industrial sector is no longer the largest in this index.

How Indices Are Calculated: Market Cap Weighting vs Price Weighting

This is the part that best explains why indices can move in different directions. The example below is hypothetical and simplified, only meant to show the principle.

Imagine an index with two stocks:

• Stock A: price US$400, 1 billion shares, market capitalization US$400 billion.
• Stock B: price US$50, 8 billion shares, market capitalization US$400 billion.

Both companies are the same size, but their price per share is very different. Then A's price rises US$10 (up 2.5%), and B's price rises US$5 (up 10%).

With price weighting, the high-priced stock A dominates, so B's 10% rise is barely felt. With market cap weighting, both companies have equal influence because they are the same size. These different numbers come from the same data. Real indices use a divisor and extra adjustment rules, but the principle is the same.

Why Three Indices Can Move Differently in the Same Week

A real example comes from the week of 12 to 16 October 2026. According to Mobee's Weekly Market Research Report, the Nasdaq Composite fell 4.48%, the S&P 500 fell 1.95%, and the Dow Jones actually rose 0.67%. That week the pressure came mainly from tech stocks, including concerns about financing AI infrastructure.

This pattern makes sense once you know what is inside each index. Nasdaq is heavy on tech, so it was hit hardest. The S&P 500 is more diverse, so its drop was smaller. The Dow has only 30 stocks and a different sector mix, so it can move the other way. No index is "wrong". Each one measures a different group.

How to Read Index Ups and Downs

1. Look at percentages, not points. Suppose the S&P 500 is around 7,600 and the Dow is around 52,000, the levels in October 2026. A 100-point gain means about +1.32% on the S&P 500, but only about +0.19% on the Dow. Points can't be compared across indices.
2. Make sure which index is meant. "Nasdaq" can mean the Composite or the Nasdaq-100. Check the full name before drawing conclusions.
3. Check the period. Rising 1% in a day means something different from rising 1% since the start of the year. Compare two indices over the same period.
4. Remember the currency. US indices are calculated in US dollars. If you calculate your results in rupiah, exchange rate moves affect them too.
5. Tell price apart from total return. The figure usually quoted in the news is the price index, which doesn't include dividends. There is a total return version that does, so make sure you compare the same type.
6. See who is driving the move. In market cap weighted indices, a few giant stocks can push the index up or down even when most stocks move the other way.

How to Use an Index as a Portfolio Benchmark

A benchmark is a point of comparison for judging whether your portfolio is doing better or worse than similar peers. The key is that the comparison must fit what is in your portfolio.

A few simple rules when comparing:

• Use exactly the same period, including the start and end dates.
• Match the currency. Don't compare results in rupiah with an index in dollars.
• Remember that an index doesn't account for transaction costs and taxes, while your portfolio bears them.

Hypothetical example. Your US stock portfolio rises 6% in a year, while the S&P 500 rises 8% over the same period. That means you trailed by 2 percentage points. This isn't automatically a sign you picked wrong. Your portfolio might be more concentrated in certain sectors, or costs and exchange rates might have played a part. Use the gap as material for evaluation, not as a verdict.

Common Mistakes When Reading Indices

• Thinking an index is an exchange. The S&P 500 and Dow Jones are not places where trading happens.
• Comparing points across indices. What is comparable is the percentage change.
• Treating the Nasdaq Composite and Nasdaq-100 as the same. Their contents differ, so their movements can differ too.
• Assuming a rising index means all stocks rose. In market cap weighted indices, a handful of big stocks can lift the index.
• Assuming the Dow represents the whole market. The Dow is only 30 price-weighted stocks.
• Using an index as a buy or sell signal. An index describes what has already happened and doesn't guarantee what comes next.

How to Follow US Indices from Indonesia

You can't buy an index directly. The usual way is through products that track the index, such as ETFs or index mutual funds. The concept of ETFs is covered in ETF Investing in Indonesia: opportunities and risks.

On Mobee, there are ETF tokens designed to track the price of their reference ETF and that can be bought with rupiah, for example SPYX, which tracks the S&P 500 ETF, and QQQX, which tracks the Nasdaq-100 ETF. These tokens are not direct ownership of the shares or ETF and don't give shareholder rights. There are also risks involving the issuer, the custodian, and price gaps against the original price, especially when US markets are closed. Product availability may change, so check directly in the app. Read the difference between US stock tokens and regular stocks and how ETF tokenization works first.

As for timing, US markets are open from 09:30 to 16:00 New York time. That is about 21:30 to 04:00 WIB during standard time and 20:30 to 03:00 WIB during daylight saving time. The details are in US market opening hours in WIB.

Start with Mobee

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Conclusion

A stock index is a measuring tool, not an exchange and not a prediction. The S&P 500, Nasdaq, and Dow can move in different directions because their makeup and calculation methods differ: the S&P 500 and Nasdaq are weighted by market capitalization, while the Dow is weighted by price. When reading the news, look at percentages, make sure which index is meant, and match the period and currency. When using one as a benchmark, pick a comparison that fits what is in your portfolio.

FAQ

What is a stock index?

A stock index is a number that sums up the movement of a group of stocks. Indices are used to see the overall condition of the market and as a portfolio benchmark. An index is not an exchange and not a particular single stock.

What is the difference between the S&P 500, Nasdaq, and Dow Jones?

The S&P 500 contains 500 large US companies weighted by market capitalization. Nasdaq Composite covers almost every Nasdaq stock, while the Nasdaq-100 covers only the 100 largest non-financial companies there. The Dow Jones contains 30 stocks weighted by price per share.

Is the S&P 500 a stock exchange?

No. The S&P 500 is an index managed by S&P Dow Jones Indices. The stocks in it are traded on exchanges such as the NYSE and Nasdaq.

What is the difference between Nasdaq Composite and Nasdaq-100?

Nasdaq Composite covers almost every stock listed on Nasdaq. The Nasdaq-100 holds only the 100 largest non-financial companies, with modified weighting. According to Wikipedia, the Nasdaq-100 makes up about 80% of the Composite's weight.

Does the Dow Jones represent the entire US stock market?

Not fully. The Dow holds only 30 stocks and uses price weighting, so high-priced stocks have a big influence. For a broader snapshot of the market, the S&P 500 is usually used more often.

Which index is right for benchmarking a portfolio?

It depends on what is in your portfolio. Large US stocks across sectors are usually compared with the S&P 500, and tech stocks with the Nasdaq-100. Match the period and currency so the comparison is fair.

Can I buy a stock index directly?

No. An index is just a number. You can buy the stocks inside it or a product that tracks the index, such as an ETF, an index mutual fund, or a token designed to track an ETF's price.

What time do US stock indices move in WIB?

US markets are open from 09:30 to 16:00 New York time, about 21:30 to 04:00 WIB during standard time and 20:30 to 03:00 WIB during daylight saving time. US market holidays are not the same as Indonesian holidays.

Disclaimer

This article is for information and education only, not investment advice, a recommendation to buy or sell any asset, or financial advice. The calculation examples are hypothetical, and market figures may change. Investing in stocks, ETFs, and digital assets involves risk, including losing part or all of your funds. Do your own research and consider your risk tolerance before making a decision. Mobee is operated by PT CTXG Indonesia Berkarya, which is licensed and supervised by the Financial Services Authority (OJK).