A stock market index is one number that tracks the combined price of a basket of stocks. The three US indexes traders watch are the S&P 500 (500 large companies, weighted by size), the Nasdaq 100 (100 large non-financial Nasdaq stocks, tech-heavy) and the Dow (30 blue chips, weighted by share price). You trade an index through ETFs, futures, options or CFDs.
When a news anchor says "the market fell 1.2% today," they almost never mean every stock fell. They mean an index fell. Knowing which index, and how it's built, tells you a lot about what actually happened, and why your own stock or trade may have done something completely different.
What an index actually measures
An index is a rulebook plus a calculation. The rulebook decides which stocks are in the basket (size, liquidity, sector, country) and how much each one counts. The calculation turns all of those prices into one number that is published continuously while the market is open.
The weighting method matters more than most beginners realise, because it decides which companies move the number:
| Weighting method | How it works | Example | What it means for you |
|---|---|---|---|
| Market-cap weighted | Bigger companies (price × shares outstanding) count more | S&P 500, Nasdaq 100 | A handful of mega caps can drag the whole index |
| Price weighted | Higher share price counts more, regardless of company size | Dow Jones | A $500 stock moves the Dow more than a $50 stock |
| Equal weighted | Every stock counts the same | S&P 500 Equal Weight | Better picture of the "average" stock |
The S&P 500
The S&P 500 tracks about 500 large US companies chosen by a committee at S&P Dow Jones Indices. It is market-cap weighted and float-adjusted, so a company's weight depends on the value of shares that actually trade.
Because it is cap-weighted, the S&P 500 is heavily influenced by its largest members. When the biggest technology names move together, they can account for a large share of the index's daily change. That's why you'll see days where the S&P 500 is up while most individual stocks in it are down. Market breadth (how many stocks are rising vs falling) is the check on that.
For traders, the S&P 500 is the benchmark for US equities. It's what most "is the market risk-on today?" questions are really about.
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The Nasdaq 100 holds the 100 largest non-financial companies listed on the Nasdaq exchange. It is also cap-weighted, but its sector mix is very different: heavily technology, communication services and consumer names, with no banks.
Two things follow from that. It usually moves more than the S&P 500 in both directions, and it is more sensitive to interest rates, because growth companies' valuations lean on earnings far in the future. On a day when bond yields jump, the Nasdaq 100 often falls harder than the Dow.
Don't confuse it with the Nasdaq Composite, which includes more than 3,000 Nasdaq-listed stocks. Headlines often say "the Nasdaq" and mean the Composite. Traders usually mean the Nasdaq 100, which is what NQ futures and the QQQ ETF track (and what many platforms label NAS100).
The Dow Jones Industrial Average
The Dow is the oldest of the three and the strangest by modern standards: 30 large US companies, weighted by share price rather than company size. A $10 move in a high-priced Dow stock moves the index far more than a $10 move in a low-priced one, whatever the size of the two companies.
That quirk is why the Dow can diverge from the S&P 500 on a given day. It also explains why a stock split in a Dow component reduces its influence on the index. The Dow still matters because it's the number the general public hears, and because Dow futures (YM) are actively traded. Most professionals treat the S&P 500 as the better measure of the broad market.
Why the three indexes move differently
On most days all three move in the same direction. The differences show up in size and on days with a clear theme:
- Rate or yield shock: Nasdaq 100 usually moves most, because of its long-duration growth stocks.
- Mega-cap earnings: one or two giant companies can swing the S&P 500 and Nasdaq 100, while the Dow barely notices.
- Rotation into value: banks, industrials and energy rally while tech lags, so the Dow can outperform.
- Broad sell-off: everything falls together, and correlations rise toward 1.
When you read "why is the market down today," check which index the headline uses and look at breadth before drawing conclusions. We walk through that process in Why Is the Stock Market Down Today?.
Other indexes worth knowing
- Russell 2000: US small caps. Often a read on domestic growth and on how easy credit is.
- VIX: not a stock index at all, but a measure of expected S&P 500 volatility over the next 30 days, derived from options prices. Traders call it the fear gauge. See VIX Futures Explained.
- FTSE 100 (UK), DAX 40 (Germany), Euro Stoxx 50 (eurozone blue chips), Nikkei 225 (Japan), ASX 200 (Australia): the local benchmarks if you trade outside US hours.
How traders actually trade an index
You can't buy "the S&P 500" itself. You trade something that tracks it:
| Instrument | Example | Good for | Watch out for |
|---|---|---|---|
| ETF | SPY, VOO, QQQ, DIA | Simple, trades like a stock, works in cash accounts | Only during stock market hours (plus extended sessions at some brokers) |
| Futures | ES/MES, NQ/MNQ, YM/MYM | Nearly 24-hour trading, capital-efficient, 60/40 tax treatment in the US | Leverage: one ES point is $50 per contract |
| Options | SPX, SPY, QQQ options | Defined-risk strategies, hedging | Time decay, implied volatility |
| CFDs / spread bets | "US500", "NAS100" | UK/EU retail access, small sizes | Not available to US residents; overnight financing |
If you're starting out, an index ETF in a cash account is the lowest-complexity way to trade the index. Futures are where many active index traders end up, because they trade nearly around the clock and micro contracts (MES, MNQ, MYM) keep position sizes small. What Is Futures Trading? covers how those work.
Using index context in your own trades
Even if you only trade individual stocks, the index is part of your setup:
- Trade with the tide. Long setups in a stock work more often when the S&P 500 or Nasdaq 100 is trending up on your timeframe. Fighting a falling index is a lower-probability trade.
- Measure relative strength. A stock that holds up while its index falls is showing strength. That's often the stock that leads when the index turns.
- Size down on high-VIX days. Bigger index swings mean bigger swings in almost everything. Wider stops need smaller positions. Our position sizing guide shows the math.
- Know the calendar. CPI, Fed decisions and mega-cap earnings move the whole index, and your stock with it.
TRADZY's Void Engine scores higher-timeframe alignment as one of its variables, so "is this setup fighting the index?" gets checked before you enter, not after.
FAQ
Is the S&P 500 the same as the stock market?
No. It's 500 large US companies, which make up most of the US market's total value but not all of it. It's the most common stand-in for "the market" because it's broad and weighted by company size.
Why did the Dow go up while the Nasdaq went down?
They hold different stocks and use different weighting. The Dow is price-weighted and leans toward industrials, financials and healthcare, while the Nasdaq 100 is tech-heavy. On days when money rotates out of growth stocks, the two can move in opposite directions.
What is NAS100?
NAS100 is the label many brokers and CFD platforms use for the Nasdaq 100 index. Futures traders use NQ (full size) or MNQ (micro) for the same index.
Can I invest in an index directly?
Not directly. You buy a product that tracks it, such as an index ETF or mutual fund, or trade futures, options or CFDs based on it.
Which index is best to day trade?
Most active index day traders use S&P 500 or Nasdaq 100 futures (or their micro versions) or the SPY and QQQ ETFs, because they're extremely liquid with tight spreads. The Nasdaq 100 moves more, which suits some traders and punishes others.
Put This Into Practice
- Score your next setup 0–100 with the Void Engine before entering
- Log the trade and tag the setup in the TradLog
- Review weekly: keep what pays, cut what doesn't
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Educational content, not financial advice. Trading involves substantial risk of loss.