---
id: css-relative-units
title: "CSS Relative Units"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["CSS relative units", "em rem", "viewport units", "vw vh", "scalable units"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.88
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["css", "web", "frontend", "w3schools", "units", "relative", "responsive"]
raw_sources: ["https://www.w3schools.com/css/css_units_relative.asp"]
applied_in: []
github_commit: ""
---

# [[CSS Relative Units]]

## 🎯 한 줄 통찰 (One-line insight)
Relative units specify a length relative to another length property — a parent element, the root element, or the viewport — so they scale better across different screen sizes, making `em` and `rem` ideal for scalable, responsive websites. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Relative to another length** — relative units specify a length relative to another length property (parent element, root element, or viewport). [S1]
- **Better scaling** — relative length units scale better between different screen sizes. [S1]
- **`em`** — relative to the font-size of the parent element. [S1]
- **`rem`** — relative to the font-size of the root HTML element. [S1]
- **Viewport units** — `vw`/`vh` are relative to 1% of viewport width/height (100vw = full width, 100vh = full height); `vmin`/`vmax` to 1% of the viewport's smaller/larger dimension. [S1]
- **`%`, `fr`, `ch`** — `%` is relative to the parent's size; `fr` is a fractional unit (1fr = 1 part of available space); `ch` is relative to the width of the "0" (zero) character. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **`em` chains to the parent** — child sizes multiply against the parent's font-size, so nested `em` values compound. [S1]
- **`rem` anchors to the root** — `rem` always references the root element, giving consistent sizing independent of nesting. [S1]

## 📖 세부 내용 (Details)
**Relative Units**
Relative units specify a length relative to another length property (like parent element, root element, or viewport). Relative length units scale better between different screen sizes. [S1]

| Unit | Description |
| --- | --- |
| `em` | Relative to the font-size of the parent element [S1] |
| `rem` | Relative to the font-size of the root HTML element [S1] |
| `vw` | Relative to 1% of the width of the viewport; 100vw = full width [S1] |
| `vh` | Relative to 1% of the height of the viewport; 100vh = full height [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] |
| `fr` | A fractional unit; 1fr equals 1 part of the available space [S1] |
| `ch` | Relative to the width of the "0" (zero) character [S1] |

**Em Font Sizing**
Set font sizes with `em` (relative to the parent's font-size; with a 16px base, 2.5em = 40px): [S1]
```css
body {
  font-size: 16px; /* Base font size */
}

h1 {
  font-size: 2.5em; /* 2.5 * 16 = 40px */
}

h2 {
  font-size: 1.875em; /* 1.875 * 16 = 30px */
}

p {
  font-size: 1em; /* 1 * 16 = 16px */
}
```

**Rem Font Sizing**
Set font sizes with `rem` (relative to the root element rather than the parent, giving more consistent results): [S1]
```css
html {
  font-size: 16px; /* Set the root font size */
}

h1 {
  font-size: 2.5rem; /* 2.5 * 16 = 40px */
}

h2 {
  font-size: 1.875rem; /* 1.875 * 16 = 30px */
}

p {
  font-size: 1rem; /* 1 * 16 = 16px */
}
```

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

## 🛠️ 적용 사례 (Applied in summary)
The page's applied cases are the `em` and `rem` font-sizing examples, each computing `h1`/`h2`/`p` sizes from a 16px base. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
em sizing relative to parent (language: CSS):
```css
body { font-size: 16px; }
h1   { font-size: 2.5em; }   /* 40px */
```
rem sizing relative to root (language: CSS):
```css
html { font-size: 16px; }
h1   { font-size: 2.5rem; }  /* 40px */
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **`em`** — relative to the parent element's font-size, so nested values compound; use when sizing should respond to the local context. [S1]
- **`rem`** — relative to the root element's font-size, giving consistent results regardless of nesting; use for predictable global scaling. [S1]
- **Viewport units (`vw`/`vh`/`vmin`/`vmax`)** — size relative to the viewport for full-screen-aware layouts. [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:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[CSS Tutorial]]
- **관련 개념:** [[CSS Units]], [[CSS Absolute Units]], [[CSS Font Size]]
- **참조 맥락:** Referenced when building scalable, responsive sizing that adapts to parent, root, or viewport.

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

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