Using modulus in css calc function

Simply..."NO".

MDN

The expression can be any simple expression combining the following operators, using standard operator precedence rules:

+ Addition.

- Subtraction.

* Multiplication. At least one of the arguments must be a <number>.

/ Division. The right-hand side must be a <number>.

Unfortunately, there is no more mention of the mod operator in recent specs.

The calc() function allows mathematical expressions with addition (+), subtraction (-), multiplication (*), and division (/) to be used as component values.

You may want to resort to using javascript to achieve such behaviour.

var el = document.getElementById('element-id');
el.style.width = (100 % 5) + '%';