One of the greatest powers a flex container has is the ability to control the alignment of its child containers. You can specify the horizontal and the vertical alignments with two different properties that are explored below.


When flex children have different height you can specify the vertical alignment with the align-items property:

  • flex-start aligns to top.
  • center centers vertically.
  • flex-end aligns to bottom.
  • stretch makes the height of the child containers equal to the highest of the them. Child containers with the specified height property won’t stretch. It’s the default value.


The justify-content property allows you to align child containers horizontally:

  • flex-start aligns to the left. It’s the default value.
  • center centers horizontally.
  • flex-end aligns to the right.

There are three more values that distribute the horizontal space evenly:

  • space-between distributes the space evenly between the containers.
  • space-around distributes the space evenly around the containers.
  • space-evenly distributes the space evenly between the containers and the edges of the parent container.

Turning tables

The specs above are true for flex parent container with the default direction, which is row. When you change the direction to column, all the properties of the flex container rotate 90 degrees clockwise:

  • align-items switches to vertical alignment. For example, align-items: flex-start aligns children to the left, and align-items: flex-end aligns them to the right.
  • justify-content switches to horizontal alignment. For example, justify-content: flex-start aligns children to the top, and justify-content: flex-end aligns them to the bottom.

This can be confusing at first, and in reality it’s easy to mix up the alignments even after years of practice. The good news is, since there are only two alignment properties, if one of them did not work properly, you just need to switch to another one 🤷‍♂️.



A centered container that doesn’t expand beyond a fixed width is a very popular layout in responsive design. Open this prototype and try resizing the window. You should notice that on larger screens the white container has the fixed width, and on smaller screens it adjusts to the width of the viewport. Let’s create this layout step-by-step:

  1. Create a new pen. In CSS, apply margin: 0 to the body tag.
  2. In HTML, create a parent flex container with light-gray background.
  3. Make it full-screen using height: 100vh.
  4. Center the content horizontally using justify-content: center.
  5. Create a child container with white background, and subtle borders on the sides.
  6. As described in the max-width section of the Box model article, you can prevent the container from expanding beyond a particular size by applying width: 100% and max-width: 560px. Do it and try resizing the Preview now: you should see a centered container that takes the full screen width on smaller screens, and has a fixed width on larger screens.

There’s a number of ways to center containers horizontally, but using a flex parent with the justify-content: center property is particularly useful for prototyping. If you decide that you want a sidebar next to the main area, you will only need to add another child container and they will be both centered:


Design a tab bar:

  1. Create a new pen. In CSS, apply margin: 0 and a light-gray background to the body tag.
  2. In HTML, create a flex parent container with a white background and a gray bottom border.
  3. Create three child containers. Apply a text color and a colored bottom border to one of them to indicate the selected state.
  4. Add the justify-content property to the parent container and try different options of space distribution to find the one that works best for you.