CSS - Length Value (Size)

1 - Introduction

Lengths refer to horizontal or vertical measurements.

3 - Format

The format of a length value is a number (with or without a decimal point) immediately followed by a unit identifier

number unit


In sizing attribute such as width and height, there is a default unit and this is the pixel. This is also the default Svg unit

3.1 - Negative

Some properties allow negative length values, but this may complicate the formatting model and there may be implementation-specific limits. If a negative length value cannot be supported, it should be converted to the nearest value that can be supported.

If a negative length value is set on a property that does not allow negative length values, the declaration is ignored.

4 - Example

4.1 - Default Units on CSS and attribute

This example demonstrates that:

  • the unit is mandatory in CSS (If the length does not have any unit, the property is discarded and takes the default values)
  • the unit default to pixel on attribute

Example code:

.box {
  • the html with :
    • a div where the css rule will apply
    • an img with width and height attribute without a length of 50 without unit
<div class="box">
   <img src="/_media/web/rss_icon.jpg" height="50" width="50" />


let box = document.querySelector('.box');
let styles = window.getComputedStyle(box);
let width = styles.getPropertyValue("width");
let height = styles.getPropertyValue("height");
console.log("The computed css width is "+width);
console.log("The computed css height is "+height);
let img = document.querySelector('img');
let imgStyles = window.getComputedStyle(img);
let widthImg = imgStyles.getPropertyValue("width");
let heightImg = imgStyles.getPropertyValue("height");
console.log("The computed img width is "+widthImg);
console.log("The computed img height is "+heightImg);
  • Result: we can see that:
    • the css width and height are not 10 but the default values: ie
      • the full available width
      • a zero height.
    • the img sizing has taken the default pixel unit

5 - Documentation / Reference

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