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Declaring font sizes in viewport units can produce really interesting results, but it does come with challenges. There are no min-font-size or max-font-size properties in CSS, so edge cases where text gets too small or too big are difficult to deal with.

This Sass mixin makes use of media queries to define a minimum and maximum size for the font, in pixels. It also supports an optional parameter to serve as a fallback to browsers that don’t support viewport units.

As an example, this is how you’d define a font as 5vw constrained between 35px and 150px (with a fallback of 50px for non-supporting browsers):

@include responsive-font(5vw, 35px, 150px, 50px);

And here’s the full mixin:

/// Viewport sized typography with minimum and maximum values
/// @author Eduardo Boucas (@eduardoboucas)
/// @param {Number}   $responsive  - Viewport-based size
/// @param {Number}   $min         - Minimum font size (px)
/// @param {Number}   $max         - Maximum font size (px)
///                                  (optional)
/// @param {Number}   $fallback    - Fallback for viewport-
///                                  based units (optional)
/// @example scss - 5vw font size (with 50px fallback), 
///                 minumum of 35px and maximum of 150px
///  @include responsive-font(5vw, 35px, 150px, 50px);
@mixin responsive-font($responsive, $min, $max: false, $fallback: false) {
  $responsive-unitless: $responsive / ($responsive - $responsive + 1);
  $dimension: if(unit($responsive) == 'vh', 'height', 'width');
  $min-breakpoint: $min / $responsive-unitless * 100;
  @media (max-#{$dimension}: #{$min-breakpoint}) {
    font-size: $min;
  @if $max {
    $max-breakpoint: $max / $responsive-unitless * 100;
    @media (min-#{$dimension}: #{$max-breakpoint}) {
      font-size: $max;
  @if $fallback {
    font-size: $fallback;
  font-size: $responsive;


See the Pen Viewport sized typography with minimum and maximum sizes by Eduardo Bouças (@eduardoboucas) on CodePen.