Calc is a CSS3 property that allows you to calculate the size of an element based on several factors, such as its parent element's size, its child elements' sizes, and the presence or absence of constraints (such as fixed elements or percentage width values).

Calc allows you to calculate the size of an element using a simple formula. For example, if you want to calculate the size of an element based on its parent element's width and height, you can use the following code:

height: calc(100% - 40px);

This will calculate the height of the element as 100% of its parent element's width減去40px.

Calc is a powerful tool that allows you to easily calculate the size and style of elements in CSS. It is commonly used in cases where you need to calculate the size of elements based on multiple factors, or where you need to provide a custom calculation based on user input or other data.

In addition to its use in CSS, Calc is also available in JavaScript and can be used to calculate the size and style of elements in a more complex application.