---
id: css-units
title: "CSS Units"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["CSS units", "CSS length", "length values", "absolute units", "relative units"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.89
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["css", "web", "frontend", "w3schools", "units", "length", "measurement"]
raw_sources: ["https://www.w3schools.com/css/css_units.asp"]
applied_in: []
github_commit: ""
---

# [[CSS Units]]

## 🎯 한 줄 통찰 (One-line insight)
CSS units express the length of properties such as `font-size`, `width`, `margin`, `padding`, and `border`, and they split into two families: absolute units (fixed) and relative units (scale with parent, root, or viewport). [S1]

## 🧠 핵심 개념 (Core concepts)
- **Length values** — several CSS properties take "length" values, such as `font-size`, `width`, `margin`, `padding`, and `border`. [S1]
- **No whitespace rule** — there must be no space between the number and the unit (e.g., `10px`, not `10 px`); however, if the value is `0`, the unit can be omitted. [S1]
- **Two categories** — Absolute units are fixed lengths; Relative units scale based on a parent element, the root element, or the viewport. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Absolute for print, relative for screen** — absolute units suit fixed/print contexts; relative units (`em`, `rem`) scale better across screen sizes. [S1]
- **Omit unit only for zero** — `0` is the single value where the unit may be dropped. [S1]

## 📖 세부 내용 (Details)
**Introduction**
Several CSS properties take "length" values, such as `font-size`, `width`, `margin`, `padding`, and `border`. There must be no whitespace between the number and the unit; however, if the value is `0`, the unit can be omitted. [S1]

CSS has two types of length units: **Absolute** and **Relative**. [S1]

**Absolute Lengths**
Absolute length units are fixed lengths. [S1]

| Unit | Description |
| --- | --- |
| `cm` | centimeters [S1] |
| `mm` | millimeters [S1] |
| `in` | inches (1in = 96px = 2.54cm) [S1] |
| `px` | pixels (1px = 1/96th of 1in) — the most-used absolute unit for screens [S1] |
| `pt` | points (1pt = 1/72 of 1in) — a typographical unit [S1] |
| `pc` | picas (1pc = 12 pt) — a print unit [S1] |

A typical absolute-unit usage sets font sizes in pixels: [S1]
```css
h1 { font-size: 40px; }
h2 { font-size: 30px; }
p  { font-size: 17px; }
```

**Relative Lengths**
Relative length units specify a length relative to another length property, and scale better between different screen sizes. [S1]

| Unit | Description |
| --- | --- |
| `em` | Relative to the font-size of the element [S1] |
| `ex` | Relative to the x-height of the current font (height of lowercase "x") [S1] |
| `ch` | Relative to the width of the "0" (zero) character [S1] |
| `rem` | Relative to the font-size of the root element [S1] |
| `vw` | Relative to 1% of the width of the viewport [S1] |
| `vh` | Relative to 1% of the height of the viewport [S1] |
| `vmin` | Relative to 1% of the viewport's smaller dimension [S1] |
| `vmax` | Relative to 1% of the viewport's larger dimension [S1] |
| `%` | Relative to the size of the parent element [S1] |

**Tip:** The `em` and `rem` units are perfect for creating scalable and responsive websites. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's applied case is the pixel font-size example styling `h1`, `h2`, and `p`. Detailed per-family examples live on the dedicated [[CSS Absolute Units]] and [[CSS Relative Units]] pages. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Pixel (absolute) font sizing (language: CSS):
```css
h1 { font-size: 40px; }
p  { font-size: 17px; }
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **Absolute units** (`cm`, `mm`, `in`, `px`, `pt`, `pc`) — fixed lengths; pixels are the most-used for screens, the rest are mainly for print. [S1]
- **Relative units** (`em`, `ex`, `ch`, `rem`, `vw`, `vh`, `vmin`, `vmax`, `%`) — scale relative to another length, so they adapt across screen sizes; `em` and `rem` are recommended for scalable, responsive sites. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.89
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[CSS Tutorial]]
- **관련 개념:** [[CSS Absolute Units]], [[CSS Relative Units]], [[CSS Font Size]]
- **참조 맥락:** Referenced whenever specifying any length value (size, spacing, borders) in CSS.

## 📚 출처 (Sources)
- [S1] W3Schools — CSS Units — https://www.w3schools.com/css/css_units.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "CSS Units" page (Astra wiki-curation, P-Reinforce v3.1 format).