Describe different CSS units and when to use them
Jul 27, 2025 am 04:24 AMIn web development, the choice of CSS units depends on design requirements and responsive performance. 1. Pixels (px) are used to fix sizes such as borders and icons, but are not conducive to responsive design; 2. Percentage (%) is adjusted according to the parent container, suitable for streaming layout but attention to context dependence; 3. em is based on the current font size, rem is based on the root element font, suitable for elastic fonts and unified theme control; 4. Viewport units (vw/vh/vmin/vmax) are adjusted according to the screen size, suitable for full-screen elements and dynamic UI; 5. Auto, inherit, initial and other values are used to automatically calculate, inherit or reset styles, which helps to flexibly layout and style management. The rational use of these units can improve page flexibility and responsiveness.
In web development, CSS units determine the performance of elements' size, spacing and layout. Different units are suitable for different scenarios. Choosing the right units can make the page more flexible and responsive.

1. Pixel (px): the first choice for fixed size
px
is the most basic and common CSS unit, representing a dot on the screen. It is suitable for scenes that require precise control, such as border width, icon size, etc.
- When certain parts of the page design need to be fixed, using
px
is more reliable. - But overuse of pixels can lead to poor responsive designs because they don't automatically adjust with the screen or font size.
For example:

border: 1px solid #000;
If you want the button to always be 100 pixels wide, regardless of how the user device changes, use px
.
2. Percentage (%): Set size relative to parent container
%
is a relative unit and is usually used for width, height, inner margins, and outer margins. Its value is based on the corresponding size of the parent element.

- Suitable for building streaming layouts, allowing elements to scale with the parent.
- For example, set a box to
50%
width, it will occupy half the width of its parent container.
Note: The calculation of percentages depends on the context and can sometimes lead to unexpected results, such as padding when using percentages, relative to width.
3. em and rem: elastic units related to font size
-
em
relative to the font size of the current element. If the current font is16px
, then1em = 16px
. -
rem
is more stable relative to the font size of the root element (usually<html>
).
Recommended usage:
- Use
em
when the internal proportions of components are required - Use
rem
when building a unified accessibility design. For example, the font size of the entire page can be enlarged and reduced by modifying the root font.
For example:
html { font-size: 16px; } .title { font-size: 1.5rem; /* 24px */ }
This way, changing a place can affect the font size of the entire site, which is very suitable for theme switching or user-defined font size.
4. Viewport unit vw/vh/vmin/vmax: proportional unit based on viewport size
-
vw
represents 1% of viewport width,vh
is 1% of height -
vmin
andvmax
respectively take 1% of the minimum/maximum value of viewport width and height, respectively
These units are especially suitable for responsive titles, full-screen backgrounds, or UI elements that are dynamically adjusted to the screen.
for example:
.fullscreen-section { height: 100vh; }
This setting will make the height of the element just equal to the height of the browser window, and is often used for the first screen display.
5. Special values such as auto, inherit, initial: auxiliary control layout
Although not a numerical unit, it is very practical in actual layout:
-
auto
: automatically calculated by the browser, often used in margin or width -
inherit
: inherits the style value of the parent element -
initial
: Restore to the default initial value
For example, if you want a div to center, you can write:
margin: 0 auto;
Or want the font size of a child element to be the same as that of a parent:
font-size: inherit;
Basically that's it. Each unit has applicable occasions, and the key is to understand the differences and application scenarios between them. Only by using it reasonably can you create a beautiful and flexible web layout.
The above is the detailed content of Describe different CSS units and when to use them. For more information, please follow other related articles on the PHP Chinese website!

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