It’s good practice to use rem for font sizes rather than px but remembering which rem value to use can be tricky.

Use a Sass function to calculate the rem value and a mixin to set the value.

@function calculateRem($size) {
	$remSize: $size / 16px;
	@return $remSize * 1rem;
}

@mixin font-size($size) {
	font-size: $size;
	font-size: calculateRem($size);
}

Simply use the mixin whenever you want to set a font size value.

p {
	@include font-size(22px);
}
/* Output */
p {
	font-size: 22px;
	font-size: 1.375rem;
}

The mixin outputs the original pixel value as a fallback for old browsers where rem is not supported, and the calculated rem value after it which takes priority in all modern browsers.