What Are Indices? S&P 500, Dow, NASDAQ
Stock indices track the performance of groups of stocks — the S&P 500, Dow, and NASDAQ are the most followed and the foundation of index trading.
What Are Indices? S&P 500, Dow, NASDAQ
An index measures the performance of a group of stocks — the S&P 500, Dow Jones, and NASDAQ Composite are the three most followed in the world.
Every night the news reports whether "the market" went up or down. But which market? They're usually referring to one of three indices — each measuring a different slice of US equities. Understanding what each measures is foundational for any trader.
What is a stock index?
A stock index is a statistical measure of the value of a group of stocks. It tracks the aggregate performance of its constituents, giving investors a quick read on a market, sector, or theme.
Indices serve three purposes:
- Benchmark — measure portfolio or fund performance
- Investment vehicle — index funds and ETFs track them
- Economic signal — broad market health as a macro indicator
The three major US indices
S&P 500 (SPX)
- 500 largest US public companies
- Covers ~80% of available US market cap
- Market-cap weighted (larger companies move it more)
- Considered the best single proxy for "the US market"
- ETF tracker: SPY, IVV, VOO
Dow Jones Industrial Average (DJIA)
- 30 large established companies
- Price-weighted (higher-priced stocks move it more)
- Oldest US index, founded 1896
- Includes blue chips like Apple, Microsoft, Coca-Cola
- ETF tracker: DIA
NASDAQ Composite (IXIC)
- All stocks listed on NASDAQ (~3,000+ companies)
- Heavy tech and growth exposure
- More volatile than S&P or Dow
- Home to Apple, Microsoft, Amazon, Google, Meta
- ETF tracker: QQQ (NASDAQ 100, the largest non-financials)
How indices are weighted
| Method | How it works | Example |
|---|---|---|
| Market-cap weighted | Larger companies have more weight | S&P 500 |
| Price-weighted | Higher-priced stocks have more weight | Dow |
| Equal-weighted | Each stock same weight | RSP (equal-weight S&P) |
Weighting matters — the top handful of stocks can dominate an index. In the S&P 500, the top 10 stocks often make up 30%+ of the total weight.
Other important indices
- Russell 2000 — small-cap US stocks (IWM)
- Wilshire 5000 — broadest US index
- VIX — the volatility index (see our VIX guide)
- DXY — US dollar index (see our DXY guide)
Global indices
- MSCI EAFE — developed markets ex-US
- MSCI Emerging Markets — China, India, Brazil, etc.
- Nikkei 225 — Japan
- DAX — Germany
- FTSE 100 — UK
- Hang Seng — Hong Kong
How indices are calculated
An index's value reflects the combined performance of its constituents, adjusted for the weighting method:
- Market-cap weighted: sum of (shares × price) ÷ divisor
- Price-weighted: sum of prices ÷ divisor
- The divisor is adjusted for stock splits and changes to keep continuity
Why traders care about indices
- Quick market read — one number summarizes thousands of stocks
- Tradeable — ETFs let you trade the entire index in one click
- Sentiment signal — index moves reflect broad risk appetite
- Macro insight — index trends reflect economic conditions
- Benchmark — measure your own performance against "the market"
How to trade indices
- Index ETFs — SPY, QQQ, DIA (easiest)
- Index futures — ES, NQ, YM (leveraged)
- Options on ETFs or futures — flexible strategies
- CFDs — popular outside the US
Tip: ES (S&P futures) and NQ (NASDAQ futures) are among the most liquid futures in the world — used by institutions worldwide.
Common mistakes
- Treating the Dow as "the market" (it's only 30 stocks)
- Ignoring market-cap concentration (S&P top 10 dominate)
- Forgetting dividends affect total return
- Confusing price return with total return indices
- Assuming index performance = your stock's performance
Bottom line
Indices are the simplest way to understand and trade "the market." The S&P 500 is the gold standard, the Dow is the newsroom favorite, and the NASDAQ is the tech trader's benchmark. Knowing what each measures — and how to trade them — is foundational for every trader.