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A CSS post-processor that converts px to rem.

postcss-pxtorem NPM version

A plugin for PostCSS that generates rem units from pixel units.


Pixels are the easiest unit to use (opinion). The only issue with them is that they don't let browsers change the default font size of 16. This script converts every px value to a rem from the properties you choose to allow the browser to set the font size.


With the default settings, only font related properties are targeted.

// input
h1 {
    margin: 0 0 20px;
    font-size: 32px;
    line-height: 1.2;
    letter-spacing: 1px;
// output
h1 {
    margin: 0 0 20px;
    font-size: 2rem;
    line-height: 1.2;
    letter-spacing: 0.0625rem;


var fs = require('fs');
var postcss = require('postcss');
var pxtorem = require('postcss-pxtorem');
var css = fs.readFileSync('main.css', 'utf8');
var options = {
    replace: false
var processedCss = postcss(pxtorem(options)).process(css).css;
fs.writeFile('main-rem.css', processedCss, function (err) {
  if (err) {
    throw err;
  console.log('Rem file written.');


Type: Object | Null

    rootValue: 16,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0
  • rootValue (Number) The root element font size.
  • unitPrecision (Number) The decimal numbers to allow the REM units to grow to.
  • propList (Array) The properties that can change from px to rem.
    • Values need to be exact matches.
    • Use wildcard * to enable all properties. Example: ['*']
    • Use * at the start or end of a word. (['*position*'] will match background-position-y)
    • Use ! to not match a property. Example: ['*', '!letter-spacing']
    • Combine the "not" prefix with the other prefixes. Example: ['*', '!font*']
  • selectorBlackList (Array) The selectors to ignore and leave as px.
    • If value is string, it checks to see if selector contains the string.
      • ['body'] will match .body-class
    • If value is regexp, it checks to see if the selector matches the regexp.
      • [/^body$/] will match body but not .body
  • replace (Boolean) replaces rules containing rems instead of adding fallbacks.
  • mediaQuery (Boolean) Allow px to be converted in media queries.
  • minPixelValue (Number) Set the minimum pixel value to replace.

Use with gulp-postcss and autoprefixer

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var pxtorem = require('postcss-pxtorem');
gulp.task('css', function () {
    var processors = [
            browsers: 'last 1 version'
            replace: false
    return gulp.src(['build/css/**/*.css'])

A message about ignoring properties

Currently, the easiest way to have a single property ignored is to use a capital in the pixel unit declaration.

// `px` is converted to `rem`
.convert {
    font-size: 16px; // converted to 1rem
// `Px` or `PX` is ignored by `postcss-pxtorem` but still accepted by browsers
.ignore {
    border: 1Px solid; // ignored
    border-width: 2PX; // ignored