Use em or rem font-size in Bootstrap 3

bootstrap-em.less

/**
 * Use em or rem font-size in Bootstrap 3
 */

@font-size-root: 14px;
@font-unit: 0rem; // Pick em or rem here


// Convert all variables to em

@font-size-base:  1 + @font-unit;
@font-size-large: 1.25 + @font-unit;
@font-size-small: 0.85 + @font-unit;

@font-size-h1:            2.6 + @font-unit;
@font-size-h2:            2.15 + @font-unit;
@font-size-h3:            1.7 + @font-unit;
@font-size-h4:            1.25 + @font-unit;
@font-size-h5:            @font-size-base;
@font-size-h6:            0.85 + @font-unit;

@line-height-computed:    (@font-size-base * @line-height-base);

@padding-base-vertical:     ((6px / @font-size-root) + @font-unit);
@padding-base-horizontal:   ((12px / @font-size-root) + @font-unit);

@padding-large-vertical:    ((10px / @font-size-root) + @font-unit);
@padding-large-horizontal:  ((16px / @font-size-root) + @font-unit);

@padding-small-vertical:    ((5px / @font-size-root) + @font-unit);
@padding-small-horizontal:  ((10px / @font-size-root) + @font-unit);

@padding-xs-vertical:       ((1px / @font-size-root) + @font-unit);
@padding-xs-horizontal:     ((5px / @font-size-root) + @font-unit);

@border-radius-base:        ((4px / @font-size-root) + @font-unit);
@border-radius-large:       ((6px / @font-size-root) + @font-unit);
@border-radius-small:       ((3px / @font-size-root) + @font-unit);

@caret-width-base:          ((4px / @font-size-root) + @font-unit);
@caret-width-large:         ((5px / @font-size-root) + @font-unit);

//** Default `.form-control` height
@input-height-base:         (@line-height-computed + (@padding-base-vertical * 2));
//** Large `.form-control` height
@input-height-large:        ((@font-size-large * @line-height-large) + (@padding-large-vertical * 2));
//** Small `.form-control` height
@input-height-small:        ((@font-size-small * @line-height-small) + (@padding-small-vertical * 2));


// Set root font size

html {
    font-size: @font-size-root;
}


// Change styling for elements that mix pixels and ems

pre {
  font-size: ( @font-size-base - (1 / @font-size-root) );
}

.form-control-feedback {
  width: calc(@input-height-base ~" + 2px");
  height: calc(@input-height-base ~" + 2px");
}
.input-lg + .form-control-feedback {
  width: calc(@input-height-large ~" + 2px");
  height: calc(@input-height-large ~" + 2px");
}
.input-sm + .form-control-feedback {
  width: calc(@input-height-small ~" + 2px");
  height: calc(@input-height-small ~" + 2px");
}

.form-control {
    height: calc(@input-height-base ~" + 2px");
}

.form-horizontal {
    .radio, .checkbox, .radio-inline, .checkbox-inline {
        padding: @padding-base-vertical;
        border: 1px solid transparent;
    }
    .radio, .checkbox {
        min-height: calc((@line-height-computed + @padding-base-vertical) ~"+ 1px");
    }
    .form-control-static {
        padding-top: @padding-base-vertical;
        border-top: 1px solid transparent;
        padding-bottom: @padding-base-vertical;
        border-bottom: 1px solid transparent;
    }

    @media (min-width: @screen-sm-min) {
        .control-label {
            padding-top: @padding-base-vertical;
            border-top: 1px solid transparent;
        }
    }
}

Sounds interesting but can you please comment on how one is supposed to use this with Bootstrap (i.e. order, LESS vs CSS etc.).

@DavidCle You need to work with less and should load this after bootstrap.

@import 'bootstrap/less/bootstrap.less';
@import 'bootstrap-em.less';

jasny have you got your gist bootstrap-em.less working with bootstrap-sass in a rails environment?

@adamjgomez Nop. I'm a PHP developer and use Less myself. Though you should be able to do something similar with Sass. Please fork and modify this gist and leave a link to it here in the comments.

@adamjgomez if you still need Bootstrap's Sass version rewritten with em and rem units you can check this - https://github.com/ivayloc/twbs-rem-em

I've got this working in sass here: https://gist.github.com/neil-h/631c5e848ba1f2ae92d1
I had to alter _nav-vertical-align.scss as I was getting an error about incompatible units.

it works for someone? I don't know if this is the right way.

That's very cool, but complicated---it feels like we're fighting the framework.

I just needed a simple adjustable-width headline, and used font-size:6vw; based on the vw (viewport width/100) unit. Apparently vw is well supported these days.

@ivanthisone yes, viewport width and height are supported and preferred over percentages in certain circumstances.

Unfortunately that gist breaks all rules where @line-height-computed is used as a value for margins and paddings as those properties take the element's own font-size as the base size.

Such rules occur 31 times in Bootstrap 3.3.2 and that makes this gist useless unless all those places are patched.

And even after overriding those rules such patching need to be maintained for every new release of Bootstrap.

Hopefully BS4 will have ems/rems as base units

Are there any plans to use rems in BS4 ? We have a working rems version that we use in our company - we could contribute it to BS4. Do you know who maintains the backlog or planning of BS4 ?

More info available in a stackoverflow topic and a Github Issue

For those following along, we'll be able to change from pixels to REMs in v4 when we drop IE8 support

@pribilinskiy, could you please elaborate on why @line-height-computed here breaks those margin and padding rules? I'm not against patching my copy of bootstrap. How to make it work with this gist?

@koddo, because old @line-height-computed was already applied to the rules that use it, you need to extract all of them and add it to the gist.

Where are variables @line-height-large and @line-height-small ?

does this convert all px in bootstrap into ems or just some controls?