CSS's calc() function supports addition, subtraction, multiplication and division operations to make layout values more flexible. ① It simplifies responsive layouts, such as width: calc(100% - 200px) that can dynamically calculate width; ② Automatically process mixed units such as px, %, em, rem, vw, vh, but pay attention to the spaces around the operator; ③ Suitable for a variety of attributes, such as top, left positioning, grid/elastic box spacing, transparency or transform values; ④ Compatible with modern browsers, suitable for creating intelligent adaptive layouts.
The calc()
function in CSS is a handy tool that lets you perform math operations right inside your styles. It supports addition, subscription, multiplication, and division, which makes it super useful when you need flexible layout values — like mixing percentages with fixed pixel sizes.

Why use calc()
?
A common reason to use calc()
is to make responsive layouts easier. For example, you might want a box to take up 100% of the container width but subtract a fixed sidebar. Instead of hardcoding everything or using JavaScript, you can just write:
-
width: calc(100% - 200px);
This tells the browser to calculate the width dynamically based on the current container size, minus 200 pixels for the sidebar. This kind of flexibility is where calc()
really shines.

How does calc()
work with different units?
One of the big advantages of calc()
is that it handles mixed units automatically. You can combine px
, %
, em
, rem
, and even viewport units like vw
or vh
.
For example:

-
margin-left: calc(50% - 10rem);
-
font-size: calc(1rem 2vw);
These are especially helpful for fluid typography or centering elements based on dynamic measurements. Just remember: the spacing around operators matters. You must include spaces around
, -
, and don't need them for *
or /
.
So this works:
? calc(100% - 20px)
But this doesn't:
? calc(100%-20px)
Where else can you use calc()
?
You'll see calc()
used in all sorts of properties — not just widths and heights. Some other places where it's helpful:
Top / left positioning
top: calc(50% - 25px);
to vertically center an element that's 50px tall.Grid and flexbox gaps
If you're building a grid and want spacing that adjusts based on screen size,calc()
can help figure out item widths.Opacity or transform values
Though less common, you can also usecalc()
for numeric values in transforms or opacity settings if they depend on other values.
It's supported in all modern browsers, so you don't have to worry about compatibility much these days.
That's basically how calc()
works. It's not complicated, but it's powerful when you need to mix values or create smarter, more adaptive layouts.
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