Compile LESS using Grunt

The topic describes how to install, configure, and use Grunt JavaScript task runner to compile .less files in Magento 2.


Adding themes to Grunt configuration

To compile .less files, add your theme to module.exports in the Grunt configuration, either in the default dev/tools/grunt/configs/themes.js or in the custom configuration file. For example:

  1. Install [node.js] to any location on your machine.

  2. Install the Grunt CLI globally:
     npm install -g grunt-cli
  3. Rename the following files in your Magento root directory:
    • package.json.sample to package.json
    • Gruntfile.js.sample to Gruntfile.js
  4. Install (or refresh) the node.js project dependency, including Grunt, for your Magento instance. To do this, run the following commands in a command prompt:
     cd <your_Magento_instance_directory>
     npm install
     npm update
  5. Add your theme to Grunt configuration. To do this, in the dev/tools/grunt/configs/themes.js file, add your theme to module.exports like following:
     module.exports = {
         <theme>: {
             area: 'frontend',
             name: '<Vendor>/<theme>',
             locale: '<language>',
             files: [
                 '<path_to_file1>', //path to root source file
             dsl: 'less'

    Where the following notation is used:

    • <theme>: your theme code, conventionally should correspond to the theme directory name.
    • <language>: specified in the code_subtag format, for example en_US. Only one locale can be specified here. To debug the theme with another locale, create one more theme declaration, having specified another value for language
    • <path_to_file>: path to the root source file, relative to the app/design/frontend/<Vendor>/<theme>/web directory. You need to specify all root source files of the theme. If your theme [inherits] from a certain theme, and does not contain its own root source files, specify the root source files of the parent theme.
  6. (Optional) If you want to use Grunt for “watching” changes automatically, without reloading pages in a browser each time, install the [LiveReload extension] in your browser.

Grunt commands

The following table describes the grunt commands you can use to perform different customization tasks. Run all commands from your Magento installation directory.

Grunt task Action
grunt clean Removes the theme related static files in the pub/static and var directories.
grunt exec Republishes symlinks to the source files to the pub/static/frontend/ directory.
grunt less Compiles .css files using the symlinks published in the pub/static/frontend/ directory.
grunt watch Tracks the changes in the source files, recompiles .css files, and reloads the page in the browser.

Use cases of tracking changes using Grunt

The following shows which Grunt tasks to use for debugging:

  • After you switch the compilation mode from client-side to server-side, run the exec command.
  • After you customize the content of any .less file, except the root source files, run the less task and reload the page.
  • After you [customize the root source files or move the files included to the root files], run the exec command and reload the page.

If you have LiveReload installed, run the grunt watch command, and the flow is even simpler:

  • After you customize the content of any .less file, changes are applied and the page reloads automatically. No additional changes
  • After you [customize the root source files or move the files included to the root files], run the clean and exec commands, which reloads the page in the browser.

CSS source maps

When using Grunt for styles preprocessing, you can enable the CSS source maps generation in your browser. It will make the theme styles debugging easier.

For each theme, Magento compiles all theme .less files into two CSS files: styles-m.css and styles-l.css. So when you debug a theme, your browser only sees styles-m.css and it might be difficult to define which .css or .less file requires corrections. For example:

node declaration autocomplete

CSS source maps solve this issue. They help to find the .less file, where the style is specified. For example:

node declaration autocomplete

CSS source maps are generated automatically when you compile CSS for your theme using the grunt less: <theme> command. To use them, you need to enable the display of source maps in your browser. For example, in Chrome, you would open the Developer Tools, go to the Settings panel, select Preferences, then check the Enable CSS source maps checkbox.