# What Is a Sector? Industry Groups and Market-Cap Rankings

> Sectors group companies by what they do — Technology, Health Care, and more. Here's how sector market cap and company weight work on MarketCapLens.

Source: MarketCapLens — https://www.marketcaplens.com/learn/what-is-a-sector
Updated: 2026-06-19

A **sector** is a broad industry group — Technology, Health Care, Energy, and so on — used to
sort companies by *what they mostly do*. On MarketCapLens, each [sector page](https://www.marketcaplens.com/sectors) ranks
the companies in that group by [market capitalization](https://www.marketcaplens.com/learn/what-is-market-capitalization),
largest first.

**Key takeaways**

- Sectors group companies by business activity, not by size.
- A sector’s “market cap” is the sum of its members’ market caps.
- One huge company can dominate a sector’s total; many small ones barely move it.
- Sector rankings answer “who’s biggest *in this industry?*” — not “which sector to buy.”

## Why sectors exist

Comparing Apple to a regional bank is rarely useful. Sectors give you a fairer neighborhood:
chipmakers next to software firms, insurers next to banks. Indexes, funds, and research notes
all use some version of this map (often GICS-style labels). MarketCapLens uses the sector tags
that come with the market data so the ranking and the sector hubs stay consistent.

## Sector market cap vs. company market cap

A company’s market cap is price × shares. A **sector’s** market cap is simply those company
figures added up.

Imagine a tiny “sector” with three companies:

| Company | Market cap | Weight in sector |
| --- | --- | --- |
| Alpha | $600B | 60% |
| Beta | $300B | 30% |
| Gamma | $100B | 10% |
| **Sector total** | **$1T** | **100%** |

Alpha alone is more than half the sector. If Alpha’s shares jump 5% and the others are flat,
the *sector* total moves even though two of three companies did nothing. That’s why sector
pages show both the ranked list and each company’s **weight** — so you can see concentration,
not just a name list.

## What a sector ranking is good for

- Spotting the largest companies in an industry (for example, who’s big in
  [Technology](https://www.marketcaplens.com/sectors/technology) today).
- Seeing whether a sector is top-heavy or more evenly spread.
- Jumping from the [sectors hub](https://www.marketcaplens.com/sectors) into a company page without wading through the full
  market list.

## What it isn’t

A sector ranking is not a recommendation, and “largest sector” is not “best sector.” Size
describes how much equity value sits in that group right now. For how we build the underlying
list of companies, see [How the MarketCapLens Ranking Works](https://www.marketcaplens.com/learn/how-the-ranking-works).

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*For general education only. Nothing here is investment advice.*

## Frequently asked questions

### What is a sector?

A sector is a broad industry group that classifies public companies by what they mainly do — for example Technology, Financial Services, or Health Care.

### What is sector market cap?

Sector market cap is the sum of each constituent company's market cap. A company's weight is its market cap divided by that sector total.