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});
}