The type defines two numeric values and one of the following arithmetic operators between them.

+

Adds two numbers together.

-

Subtracts the right number from the left.

 = 
   [ [ '+' | '-' ]  ]*  

 = 
   [ [ '*' | / ]  ]*  

 = 
          |
       |
      |
    |
  (  )  

 = 
  e          |
  pi         |
  infinity   |
  -infinity  |
  NaN        

The operands in the expression may be any syntax value. You can use , , , , , , or .

The two operands' types must be consistent. For lengths, you can't use 0 to mean 0px (or another length unit). Instead, you must add an explicit unit: margin-top: calc(0px + 20px); is valid, while margin-top: calc(0 + 20px); is invalid. Percentage value types are resolved based on the context. For example, margin-top: calc(50% + 20px); is valid because margin-top resolves percentages to lengths.

Including CSS variables in calc-sum expressions is also allowed. The following code calc(10px + var(--variable)), is a valid expression.

The + and - operators must be surrounded by whitespace. For instance, calc(50% -8px) will be parsed as "a percentage followed by a negative length" — which is an invalid expression — while calc(50% - 8px) is "a percentage followed by a subtraction operator and a length". Likewise, calc(8px + -50%) is treated as "a length followed by an addition operator and a negative percentage".