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Can you explain the difference between em, rem, px, and viewport units (vh, vw)?
Can you explain the difference between em, rem, px, and viewport units (vh, vw)?
Jun 19, 2025 am 12:51 AM
The main difference between em, rem, px, and viewport units (vh, vw) lies in their reference point: px is fixed and based on pixel values, em is relative to the font size of the element or its parent, rem is relative to the root font size, and vh/vw are based on the viewport dimensions. 1. px offers precision but lacks scalability. 2. em scales with the local font size, though it can compound in nested elements. 3. rem ensures consistent scaling across the site without compounding issues. 4. vh and vw adjust based on screen height and width, making them ideal for responsive layouts. Knowing when to use each unit helps create flexible, accessible, and well-structured designs.
Sure! The difference between em, rem, px, and viewport units (vh, vw) comes down to what they’re based on—and that affects how your layout behaves across different screens and user settings.
px – The Fixed Unit
Pixels are the most straightforward. When you set something to, say, 16px, it’s going to be 16 pixels tall (or wide), regardless of anything else.
- ? Good for precision
- ? Doesn’t scale with user settings or zoom levels in some cases
- ? Not responsive by default
This makes px a solid choice when you want things pixel-perfect, like borders or small icons. But if you're aiming for a fully scalable design, this might not be your best bet.
em – Relative to Font Size
em is relative to the font size of the element itself, or inherited from its parent if not explicitly set.
- ? So if an element has
font-size: 20px, then1em=20px - ?? Can compound if nested — for example, a child inside an element using
emwill base its size on the parent's computed font size
.parent {
font-size: 20px;
}
.child {
font-size: 1.5em; /* 1.5 * 20px = 30px */
}This can get tricky quickly if you have multiple levels of nesting, so use it carefully.
rem – Root em
Think of rem as a cleaner version of em. It’s always based on the root element’s font size, which is usually <html>.
- ? Default browser root font size is
16px - ? You can change it globally via CSS, e.g.,
html { font-size: 18px } - ? No compounding issues like
em
It’s great for consistent scaling. For example:
html {
font-size: 16px;
}
.title {
font-size: 2rem; /* Always 32px */
}This makes rem ideal for building designs that need consistent sizing while still being flexible for accessibility (like allowing users to adjust text size).
vh and vw – Viewport-Based Units
These two are all about screen size:
1vh= 1% of the viewport height1vw= 1% of the viewport width
So on a screen that’s 1000px high, 10vh equals 100px.
Useful for:
- Full-screen sections (like hero banners)
- Elements that should scale directly with the window size
Examples:
.fullscreen {
height: 100vh;
width: 100vw;
}Just keep in mind that mobile browsers sometimes handle these inconsistently due to toolbars affecting viewport height.
In short:
- Use
pxfor fixed sizes - Use
emwhen you want something relative to a local font size (but watch out for nesting) - Use
remfor global scalability and consistency - Use
vh/vwfor layouts tied to screen dimensions
And yeah, mixing them isn't bad—as long as you know what each one does, you can combine them smartly.
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