Sass Variable in CSS calc() function

I'm trying to use the calc() function in a Sass stylesheet, but I'm having some issues. Here's my code:

$body_padding: 50px

body
    padding-top: $body_padding
    height: calc(100% - $body_padding)

If I use the literal 50px instead of my body_padding variable, I get exactly what I want. However, when I switch to the variable, this is the output:

body {
    padding-top: 50px;
    height: calc(100% - $body_padding);
}

How can I get Sass to recognize that it needs to replace the variable within the calc function?

Interpolate:

body
    height: calc(100% - #{$body_padding})

For this case, border-box would also suffice:

body
    box-sizing: border-box
    height: 100%
    padding-top: $body_padding

To use $variables inside your calc() of the height property:

HTML:

<div></div>

SCSS:

$a: 4em;

div {
  height: calc(#{$a} + 7px);
  background: #e53b2c;
}