Calculating width from percent to pixel then minus by pixel in LESS CSS

You can escape the calc arguments in order to prevent them from being evaluated on compilation.

Using your example, you would simply surround the arguments, like this:

calc(~'100% - 10px')

Demo : http://jsfiddle.net/c5aq20b6/

---

I find that I use this in one of the following three ways:

Basic Escaping

Everything inside the calc arguments is defined as a string, and is totally static until it's evaluated by the client:

LESS Input

div {
    > span {
        width: calc(~'100% - 10px');
    }
}

CSS Output

div > span {
  width: calc(100% - 10px);
}

Interpolation of Variables

You can insert a LESS variable into the string:

LESS Input

div {
    > span {
        @pad: 10px;
        width: calc(~'100% - @{pad}');
    }
}

CSS Output

div > span {
  width: calc(100% - 10px);
}

Mixing Escaped and Compiled Values

You may want to escape a percentage value, but go ahead and evaluate something on compilation:

LESS Input

@btnWidth: 40px;
div {
    > span {
        @pad: 10px;
        width: calc(~'(100% - @{pad})' - (@btnWidth * 2));
    }
}

CSS Output

div > span {
  width: calc((100% - 10px) - 80px);
}

Source: http://lesscss.org/functions/#string-functions-escape.