How do you use a CSS calc function in an HTML element?

[SubhanUllah]

The calc() CSS function lets you perform calculations when specifying CSS property values. It can be used anywhere a <length>, <frequency>, <angle>, <time>, <percentage>, <number>, or <integer> is allowed.

Example:

width: calc(100% - 80px); 
width: calc(10px + 100px); 
width: calc(100% - 30px); 
width: calc(2em * 5); 
width: calc(var(--variable-width) + 20px); 
Usage with integers:

.modal { 
  z-index: calc(3 / 2); 
}