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🌐 Web Development Roadmap, Part 27

CSS Units (px, %, em, rem, vh, vw and fr)

CSS units define the size of elements, text, spacing, widths, heights, and layouts.

For example, when you write:

h1 {
font-size: 32px;
}
The px defines the size of the heading.

CSS units are divided into two main categories:

• Absolute units
• Relative units

1. Pixels (px)

Pixels are commonly used to define fixed sizes.

.box {
width: 300px;
height: 200px;
padding: 20px;
}
Here:

• Width = 300 pixels
• Height = 200 pixels
• Padding = 20 pixels

Use pixels when you need a specific size.

2. Percentage (%)

Percentage defines a size relative to the parent element's corresponding dimension.

<div class="parent">
<div class="child">Hello</div>
</div>
`
.parent {
width: 600px;
}

.child {
width: 50%;
}

The child width will be 300px because it is 50% of its parent's 600px width.

3. EM (em)

em is a relative unit.

For font size, it is calculated relative to the inherited font size of the parent.

.parent {
font-size: 20px;
}

.child {
font-size: 2em;
}

The child font size becomes 40px.

Calculation:

20px × 2 = 40px

Note: For properties such as padding and margin, em is based on the font size of the element itself.

4. REM (rem)

rem stands for Root EM.

It is calculated relative to the font size of the root HTML element.

By default, browsers generally use 16px as the root font size.

html {
font-size: 16px;
}

h1 {
font-size: 2rem;
}


16px × 2 = 32px

So the heading will have a font size of 32px.

Difference between em and rem:

• em depends on the relevant element's inherited or own font size, depending on the property.
• rem depends on the root HTML font size.

5. Viewport Height (vh)

vh represents a percentage of the viewport height.

1vh = 1% of viewport height.

.hero {
height: 100vh;
}

This makes the element as tall as the viewport.

Useful for:

• Full-screen sections
• Landing pages
• Hero sections

6. Viewport Width (vw)

vw represents a percentage of the viewport width.

1vw = 1% of viewport width.

.container {
width: 80vw;
}

This makes the container 80% of the viewport width.

• Responsive layouts
• Full-width sections
• Flexible sizing

7. FR Unit

fr stands for fractional unit.

It is mainly used with CSS Grid to divide available space into fractions.

.container {
display: grid;
grid-template-columns: 1fr 1fr 1fr;
}

This creates three equal-width columns.

Another example:

.container {
display: grid;
grid-template-columns: 2fr 1fr;
}

The first column gets twice the available space of the second column.

8. Other CSS Units

Some additional units you should know:

• cm, Centimeters
• mm, Millimeters
• in, Inches
• pt, Points
• pc, Picas
• vmin, 1% of the smaller viewport dimension
• vmax, 1% of the larger viewport dimension
• ch, Based on the width of the "0" character
• ex, Based on the x-height of the font

9. Practical Example

Let's create a responsive card using different CSS units.

HTML:

<div class="card">
<h2>Learn Web Development</h2>
<p>Start learning HTML, CSS and JavaScript.</p>
</div>

CSS:

.card {
width: 80%;
max-width: 500px;
padding: 2rem;
margin: 5vh auto;
background-color: lightblue;
border-radius: 1rem;
}

.card h2 {
font-size: 2rem;
}

.card p {
font-size: 1rem;
}

`

This example combines percentages, pixels, rem, and vh to create a flexible layout.

10. Quick Comparison

• px: Meaning Pixels · Common usage Fixed sizing
• %: Meaning Percentage · Common usage Relative sizing
• em: Meaning Relative to font size · Common usage Text and spacing

- rem: Meaning Relative to root font size · Common usage Consistent typography
- vh: Meaning Viewport height · Common usage Full-screen sections
- vw: Meaning Viewport width · Common usage Responsive widths
- fr: Meaning Fraction · Common usage CSS Grid layouts

Key Takeaways

- Use

px

for fixed dimensions.

- Use

%

for relative sizing.

- Use

em

and

rem

for scalable typography and spacing.

- Use

vh

and

vw

for viewport-based sizing.

- Use

fr

to divide available space in CSS Grid.

Understanding CSS units is essential for creating responsive websites that work across mobile phones, tablets, and desktops.

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