title: SASS Calc Interpolation
body: <p>I have a SASS variable:</p>
  
  <pre><code>$default-padding: 15px
  </code></pre>
  
  <p>I want to use this to set a CSS width property using the calc function. </p>
  
  <p>I have various calculations to make based on the value of the $default-padding variable. For Example, the width would be 100% - (2 * $default-padding). As there is padding on the left and on the right.</p>
  
  <p>I am trying to set a local variable (using Bourbon's strip-unit method):</p>
  
  <pre><code>$default-padding-left-and-right: #{(strip-unit($default-padding) * 2)}px;
  </code></pre>
  
  <p>I would expect the result of this variable to be 30px in this use case, and I want to use it in the following rule:</p>
  
  <pre><code>.popup {
      width: calc(100% - #{$default-padding-left-and-right});
  }
  </code></pre>
  
  <p>This doesn't work, the resultant CSS is:</p>
  
  <pre><code>.popup {
      width: calc(100% - strip-unit(15px)px);
  }
  </code></pre>
  
  <p>Any ideas what I'm doing wrong?</p>