Less file for setting font sizes in rem

font-sizing rems.less

//mixin declaration

.font-size(@sizeValue) {
  @remValue: @sizeValue;
  @pxValue: @sizeValue;
  font-size: ~"@{pxValue}px"; 
  font-size: ~"@{remValue}rem";
}

//media queries set

font-size: .9px;

/* Small devices (tablets, 768px and up) */
@media (min-width: @screen-sm-min) { 
	html {
		font-size: .8px;
	}
}

/* Medium devices (desktops, 992px and up) */
@media (min-width: @screen-md-min) {  
	html {
		font-size: 1px;
	}
}

/* Large devices (large desktops, 1200px and up) */
@media (min-width: @screen-lg-min) {  

}

font-sizing rems.scss

This of course assumes you are using font-size = 1px.

@mixin font-size($sizeValue) {
  font-size: ($sizeValue) + px;
  font-size: $sizeValue + rem;
}