您可以使用 calc 函数.

.selector  {
  $width: '10px';
  width: calc(#{$width}/2);
}

更新:

基于在Sass中将字符串转换为数字的解决方案"文章.

Sassmeister 演示.

Sassmeister demo.

转号码功能
输入:'10px'，输出:10;
输入:10px，输出:10px

to-number function
input: '10px', output: 10;
input: 10px, output: 10px

@function to-number($value) {
  @if type-of($value) == 'number' {
    @return $value;
  } @else if type-of($value) != 'string' {
    @error 'Value for `to-number` should be a number or a string.';
  }

  $result: 0;
  $digits: 0;
  $minus: str-slice($value, 1, 1) == '-';
  $numbers: ('0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9);

  @for $i from if($minus, 2, 1) through str-length($value) {
    $character: str-slice($value, $i, $i);

    @if (index(map-keys($numbers), $character) or $character == '.') {
      @if $character == '.' {
        $digits: 1; 
      } @else if $digits == 0 {
        $result: $result * 10 + map-get($numbers, $character);  
      } @else {
        $digits: $digits * 10;
        $result: $result + map-get($numbers, $character) / $digits;
      }
    }
  }

  @return if($minus, -$result, $result);;
}

到单位功能
输入:'20px'，输出:1px;
输入:'35%'，输出:1%

to-unit function
input: '20px', output: 1px;
input: '35%', output: 1%

@function to-unit($value) {
  @if type-of($value) != 'string' {
    @error 'Value for `to-unit` should be a string.';
  }

  $units: ('px': 1px, 'cm': 1cm, 'mm': 1mm, '%': 1%, 'ch': 1ch, 'pc': 1pc, 'in': 1in, 'em': 1em, 'rem': 1rem, 'pt': 1pt, 'ex': 1ex, 'vw': 1vw, 'vh': 1vh, 'vmin': 1vmin, 'vmax': 1vmax);
  $parsed-unit: false;

  @each $unit in $units {
    // str-index - find substring in a string
    // 'px' in '10px' for example

    // $unit is a pair of ['px': 1px] (item in $units)
    // nth(['px': 1px], 1) returns 'px'
    // nth(['px': 1px], 2) returns 1px

    @if (str-index($value, nth($unit, 1))) {
      $parsed-unit: nth($unit, 2);
    }
  }

  @if (not $parsed-unit) {
    @error 'Invalid unit `#{$value}`.';
  }

  @return $parsed-unit;
}

函数用法.首先从字符串中获取数字.其次，从字符串中获取单位.然后将数字乘以单位:

Functions usage. At first get the number from string. Second, get the unit from string. Then multiply the number by the unit:

.selector {
  $size: '10px';

  $number: to-number($size);
  $unit: to-unit($size);
  width: ($number * $unit) / 2;
}

生成的css:

.selector {
  width: 5px;
}