Margin is a very common property that is used in CSS all the time to control the spacing and layout of elements in a web page. Even in popular CSS frameworks like Bootstrap and Tailwind CSS, we have utility classes to specify margins for all or specific sides of an element.

Though we normally use lengths in units (such as rems, pixels, ems etc.) or percentages as values for margin property. Do you know margin property also accepts a keyword called "auto" as value?

Let's take a deeper look at what auto does and how it can be used!

✨ What does auto mean?

When an auto keyword is used in margin property, the browser automatically calculates a suitable margin for an element based on the space around it. auto essentially tells the browser to let the element take up the available space.

✨ What does "taking up available space" mean?

Suppose I have a <div> tag inside a <section> tag and it looks like the image below.

We know that block-level elements primarily take up the entire width of their parent so no space is left for auto to work on. Therefore, I've given the <div> tag a fixed width of 100px to create space inside its container.

💭 What do you think will happen if I assign margin-left: auto to this orange box?
The element moves all the way to the right side, using every bit of the available space for its left margin.

auto takes up all the available space for the margin it is applied for. But there are few points that should be kept in mind when using auto

• For block-level elements, it works only with margin-left and margin-right properties.
• It shows no effect for floated and inline elements.
• It works for absolute elements only under certain conditions.
• It works for margin-top and margin-bottom properties only with flexbox or grid.

✨ Using auto with block-level elements

When using with block-level elements, auto only works on margin-left and margin-right properties given that the element has a fixed width, leaving space in its parent container. We saw earlier how margin-left : auto pushed the element to the right edge of its container.

💭 What do you think will happen if I also add margin-right: auto to that orange box?
auto will divide all the available space equally between right and left margins and hence center align the element horizontally within the parent container.

This is the most popular use case of auto. With just one line we can horizontally center images, divs and other block-level elements within their parent containers.

You can use either margin shorthand property or margin-inline to center align the element horizontally.
⚠️ Do remember that it will work only for block-level elements with fixed widths!

   element {
     /* use any one of the below properties */
      margin: 0 auto; 
       /* or */
      margin-inline: auto;
  }

✨ Using auto with absolute elements

Absolutely positioned elements are out of the normal flow of a document so it feels right that margin: auto should not work for it. But there seems to be an exception which allows us to use auto to center align an absolute element within its parent both vertically and horizontally.

📎 W3C Spec specifies ⟶

If all three of "left", "width", and "right" are auto: First set any auto values for "margin-left" and "margin-right" to 0.

If none of the three is auto: If both "margin-left" and "margin-right" are auto, solve the equation under the extra constraint that the two margins get equal values

W3C Spec specifies the same for "height", "top" and "bottom" properties. This means that we can center align an absolute element ⟶

• horizontally if its left and right properties have same values and has a fixed width
• vertically if top and bottom have same values and has a fixed height

By using these exceptions we can assign below properties to center align an absolute element within its parent.

   .parent {
      position: relative;
      height: 150px; 
    }

   .child {
      position: absolute;
      margin: auto;
      inset: 0; /* sets left, right, top & bottom to 0 */
      width: 70px;
      height: 20px;
    }

inset: 0 is shorthand for setting all four properties — left, right, top and bottom as 0.

Though this method works to center align an absolute element within a parent because of the exceptions in W3C Spec. It is best to use a combination left, top and transform translate properties to achieve this.

    .parent {
        position: relative;
     }

    /* use this method to center align an absolute element */
    .child {
        position: absolute;
        left: 50%;
        top: 50%;
        transform: translate(-50%, -50%);
     }

✨ Using auto with flexbox or grid

Flexbox and grid both allows us to use auto with margin property to center align an element both horizontally and vertically.

Let's take our previous example of a <section> tag with a <div> tag inside it and this time set display: flex property on the <section> tag.

    section {
       height: 120px; /* height is required to align vertically */
       display: flex;
     }

    div {
      width: 80px;
      margin: auto;
    }

auto works on all 4 margin properties and places the orange box in the middle of its flexbox parent.
⚠️ Do remember to add height to flexbox container if you want to use auto on margin-top and margin-bottom properties on its children, otherwise they won't work.

If you replace display: flex with display: grid for <section> tag, you will get the same result.

✨ Creating Navbars with auto and CSS flexbox

One of the most interesting use case of auto that I came across recently was making navigation bars with the help of CSS flexbox!

I have prepared a very basic navigation bar with a title, few links and a sign up and login buttons. Flexbox is used to align all these elements in a row.

💭 What if I want all the links and the 2 buttons on the right side of navbar?
One of the ways is to use justify-content: space-between on flexbox container since we have title in one tag and all the links and buttons inside a <ul> tag.

Another way is to add flex-grow: 1 on <ul> tag and use margin-left: auto on the first <li> tag that is link Products

   ul {
     flex-grow: 1
   }

   /* <li> tag with link 'Products' */
   /* nth-child(1) selects first <li> tag */
   li:nth-child(1) {
     margin-left: auto;
   }

By default, flex items take up minimum space required to fit their content. So to create space for auto to take we added flex-grow: 1 on <ul> tag so that it grows to fill the remaining space in its flexbox parent.

💭 What do you think will happen if I set margin-left: auto on sign-up button also?
All the links are now in the middle and the 2 buttons are at the end. We got another version of navbar very easily with just one line of code!

In the same way, we can make sidebars where say socials icons are at the end, title at the top and all the links in the middle. We just have to give flex container a fixed height and use margin-top: auto on the first link and the social icon element.

✨ Conclusion

Throughout this article, we looked at how auto keyword can be used with margin property. We also learnt how it can be used with flexbox to easily make beautiful versions of navigation bars.

That's all for this article. Thank you for reading!! 😊

I hope you found this article helpful! Feel free to comment down your feedbacks!! I would love to read them and improve. You can also buy me a coffee. For more content around CSS, follow me on Twitter.