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    1004.1.2 • Public • Published

    postcss-rem-to-pixel NPM version

    A plugin for PostCSS that generates px units from u units.


    $ npm install postcss-rem-to-pixel --save-dev


    Sometimes you need to include a third-party css file that uses u's. Great pracitice! Unless you can't afford to change your body font-size just for some vendor. This script converts every u value to a px value from the properties you choose using a default font size of 8px.


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

    // input
    h1 {
        margin: 0 0 20px;
        font-size: 2u;
        line-height: 1.2;
        letter-spacing: 0.0625u;
    // output
    h1 {
        margin: 0 0 20px;
        font-size: 16px;
        line-height: 1.2;
        letter-spacing: 0.5px;


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


    Type: Object | Null

        rootValue: 8,
        unitPrecision: 5,
        propList: ['*'],
        selectorBlackList: [],
        replace: true,
        mediaQuery: false,
        minUnitValue: 0
    • rootValue (Number) The root element font size.
    • unitPrecision (Number) The decimal precision px units are allowed to use, floored (rounding down on half).
    • propList (Array) The properties that can change from u to px.
      • 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 u.
      • 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 u's instead of adding fallbacks.
    • mediaQuery (Boolean) Allow u to be converted in media queries.
    • minUnitValue (Number) Set the minimum u value to replace.

    Use with gulp-postcss and autoprefixer

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

    A message about ignoring properties

    // `u` is converted to `px`
    .convert {
        font-size: 1u; // converted to 16px
    // `U`is ignored by `postcss-rem-to-pixel`
    .ignore {
        border-width: 2U; // ignored


    npm i @startupjs/postcss-rem-to-pixel

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