Note: the font-size: 62.5% thing on the html element is for switching to base 10 for the ease of calculations (default 16px * 62.5 / 100 = 10). From there, it’s easy to say 1.3rem + 13px.

root em with pixel fallback with scss

.font-size($pxValue){
    @remValue: (@pxValue / 10);
    font-size: ~"@{pxValue}px";
    font-size: ~"@{remValue}rem";
}
 
html { font-size: 62.5%; }
 
.my-element {
    .font-size(13px);
}
 
/* Output CSS */
 
.my-element {
    font-size: 13px;   /* Internet Explorer 6/7/8 + Opera Mini */
    font-size: 1.3rem; /* Other browsers */
}