Sass cannot perform arithmetic on values that cannot be converted from one unit to the next. Sass has no way of knowing exactly how wide “100%” is in terms of pixels or any other unit. That’s something only the browser knows.

You need to use `calc()`

instead. Check browser compatibility on Can I use…

```
.foo {
height: calc(25% - 5px);
}
```

If your values are in variables, you may need to use interpolation turn them into strings (otherwise Sass just tries to perform arithmetic):

```
$a: 25%;
$b: 5px;
.foo {
width: calc(#{$a} - #{$b});
}
```