Choose a CSS unit by deciding what a value should respond to: use px for CSS-pixel precision, rem for a consistent scale tied to the root font size, em for sizing that follows local typography, % when a property should use its defined percentage reference, and viewport units when sizing should follow the viewport. The key is that these units have different reference frames: a percentage does not always mean “of the parent,” and a CSS pixel is not necessarily one physical screen pixel.
CSS units at a glance
| Unit | What it is relative to | Good fit | Watch for |
|---|---|---|---|
px |
A CSS pixel, an absolute CSS length unit | Values intentionally specified in CSS-pixel terms, such as a fine border | Not necessarily one physical device pixel; density, zoom, and rasterization affect how it appears. MDN: CSS <length> |
em |
The element’s computed font size for most font-relative sizing; for font-size itself, the parent’s font size |
Component spacing or sizing that should scale with its typography | Nested font-size declarations can compound as local font sizes change. MDN: CSS <length> |
rem |
The root element’s font size | A consistent document-wide size scale tied to root text size | Changing the root font size changes every rem-based value. MDN: CSS <length> |
% |
The reference defined for the particular property | A value proportional to the relevant reference dimension | The reference varies by property; check that property’s definition. MDN: CSS percentage values |
vw, vh |
One percent of the viewport width or height | A value intended to respond to viewport dimensions | On mobile, browser interface changes can affect full-height layouts; default vh corresponds to the large viewport height in MDN’s account. MDN: CSS <length> |
svh, lvh, dvh and width equivalents |
One percent of the small, large, or dynamic viewport dimension | Layouts that need an explicit response to browser chrome changing size | svh can leave unused space when browser controls retract; dvh tracks the changing viewport. MDN: CSS <length> |
What is a CSS pixel?
px is an absolute CSS length unit, but “absolute” is a classification within CSS units—not a promise that one CSS pixel equals one hardware pixel. A high-density display may map a CSS pixel to multiple device pixels, and fractional CSS lengths may be rasterized to device-pixel increments. Use px when a value is deliberately specified in CSS-pixel terms, not as a guarantee of a particular physical screen dot. MDN explains the CSS-pixel mapping; the W3C Values and Units Level 4 Working Draft calls px the canonical unit when computed lengths are serialized. W3C CSS Values and Units Level 4 (Working Draft)
How do em and rem work?
em: local font-relative sizing
em follows the applicable element’s font size. There is an important distinction for font-size: an em value in a font-size declaration is calculated from the parent’s font size. For other font-relative sizing, the element’s own computed font size is the basis. That makes em useful when a component’s padding or other dimensions should grow along with its typography, but nested font-size rules can compound as each level establishes a different local size. MDN: CSS <length>
rem: root-relative sizing
rem follows the root element’s font size. It is useful when headings, spacing, or other dimensions should share a scale tied to the root rather than to each component’s local font context. If the root font size changes, rem-based values change together. MDN: CSS <length>
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What does a percentage mean in CSS?
A percentage has no single universal reference. Its basis depends on the property: one property may define a percentage using a containing dimension, while another may use a different reference. Do not assume that width: 50%, font-size: 50%, and every other percentage declaration are all “half of the parent” in the same sense. Check the relevant property definition before relying on a percentage’s basis. MDN: CSS percentage values
How do viewport units work?
Classic viewport units
1vw is one percent of the viewport width; 1vh is one percent of the viewport height. They suit values intended to change with viewport size. However, mobile browser controls can expand or retract and alter the available viewport. MDN describes the default vh unit as corresponding to the large viewport height, so a full-height mobile layout based on 100vh may not match the visible area while browser UI is present. MDN: CSS <length>
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Small, large, and dynamic variants
The sv*, lv*, and dv* families make the intended viewport reference explicit: small, large, or dynamic. For height, svh uses the small viewport, lvh the large viewport, and dvh the changing dynamic viewport; width equivalents follow the same pattern. A small viewport unit can leave unused space after browser controls retract, while a dynamic unit tracks the changing viewport. For full-screen mobile layouts, test the chosen behavior in the actual target browsers. MDN: CSS <length> W3C CSS Values and Units Level 4 (Working Draft)
How to choose the right unit
- Choose
remwhen values should follow the root text size and share a document-wide scale. - Choose
emwhen sizing should follow local typography, such as component spacing that grows with that component’s text. - Choose
%when the property’s specified percentage reference is the dependency you want; verify the reference for that property. - Choose
vw,vh, or a small/large/dynamic variant when the value should follow viewport dimensions; select the variant according to how browser interface changes should affect the layout. - Choose
pxwhen the intended value is expressed in CSS-pixel terms and should not instead track a font or viewport reference.
This is a practical decision framework inferred from the units’ definitions, not a standards-mandated prescription. The W3C specification also gives the conversion convention 1in = 96px; that describes CSS unit conversion, not a physical ruler measurement on every display. W3C CSS Values and Units Level 3
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
What is 1rem in CSS?
It is the root element’s font size. The pixel value depends on that root size.
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Does 1em always mean the parent font size?
No. For a font-size declaration, em is relative to the parent’s font size. For other font-relative sizing, it is based on the element’s own computed font size.
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Is a CSS px the same as a screen pixel?
Not necessarily. CSS pixels can map to multiple device pixels on high-density displays, and rendering can involve fractional CSS lengths.
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