How does auto property work in margin: 0 auto in CSS?

HTML CSS Web Development Front End Technology

The "margin: 0 auto" property is a commonly used CSS property that allows developers to horizontally center elements within their container. The "auto" value for the margin property is what enables this centering to occur.

In this article, we will look into how the "auto" value works in the margin property, and how it can be used to achieve horizontal centering. We will also discuss some potential errors and best practices when using the "auto" value in the margin property.

Margins in CSS

Before moving on to the topic, we shall try and learn the basics that we will be needing so as to understand the question. First, we will learn what does margin mean in CSS, and then, we will move to understanding the auto property. Only after learning all that, we will be able to come up with the answer to our original question.

We know that CSS is used for the purpose of styling a web page to make it easily usable and visually pleasing, so that the overall user experience of a user is smoother and better. This styling includes many things like color, font, font size, etc. One such styling method is the use of proper spacing between elements.
Whenever we are talking about the space that is outside the defined borders of an element, we are actually talking about its margin. Margin lets us create an invisible border that segregates one element from the other. It is kind of like padding, but padding is actually the space between the child elements of an element and its surrounding elements.
CSS allows us a very high control and customizability over setting the margin of an element we can use margin normally to create a margin that is equal on all sides, but we can also define the margin on a specific side individually by specifying which margin are we actually referring to. For example,

margin : 0 // statement 1
margin top : 25px // statement 2

The margin set by statement 1 will set the margin of 0 to all sides of the element while the margin set by statement 2 will modify the top margin only and set it to 25 pixels.

We can specify a margin in different ways –

we can make use of custom lengths.
we can specify a percentage value that will change depending on the screen size of user.
we can also make the margin to be inheritable which will set the margin of the current element equal to the margin of its parent element.

But what if we do not know what value we should use while defining the margin.

The auto property

CSS provides us a property that will make it so that the browser will calculate and set the margin for that element, and that property is the auto property. This property makes it easier for a developer to use margins as we do not need to know beforehand the actual values that we will use but let the browser calculate that instead.

Let us understand, how it works first. If we specify the margin of an element to auto. It gives all the sides equal margin by calculating the width and size of the element first.

Example

Let us consider, a div of dimensions 500 pixels by 300 pixels. If we do not specify any margin to it will align itself to the top left corner of the screen. On the other hand, specifying the margin as auto will make it centered to its parent container, body tag in this case.

<!DOCTYPE html>
<html lang="en">
<head>
   <title>Margin: 0 auto example</title>
</head>
<body style="background-color: coral; margin: 0; padding: 0; font-size: larger;
font-weight: bold;">
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: aqua;">No set margin</div>
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: burlywood; margin: auto;">Margin set to 0</div>
</body>
</html>

Using margin: 0 auto

Now what will happen if we try and use margin with two values. Whenever we try to use the margin and provide it with two values, the first one is taken as the value for top and bottom margin, while the second value is used for left and right margins.

Our question was to explain, ‘how the auto property will work if we use it as the second value to margin.

The answer is, “It will vertically align the element to the center of its parent element by automatically calculating the left and right margins.”

Example

Consider the same example as above but with margin set to 0 auto.

<!DOCTYPE html>
<html lang="en">
<head>
   <title>Margin: 0 auto example</title>
</head>
<body style="background-color: coral; margin: 0; padding: 0; font-size: larger; font-weight: bold;">
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: burlywood; margin: 0 auto;">Margin set to 0 auto</div>
</body>
</html>

Conclusion

In this article, we saw what we mean by margin in CSS, what the auto property in CSS does and how does it change in behavior when we use it as the second value to margin. The answer to our original answer being, aligning the element vertically to its parent element by automatically calculating the left and right margins.