Aligning two flex items: one to the top, the other centered [duplicate]

Answer 1

In order to perfectly center the #center element in the flex container, while having another element at the top, there are two ways you could do this:

1. Introduce a third ("phantom") element to create equal balance, or
2. Absolutely position the #center element.

(These same methods could be used to center one item and fix another element to the bottom.)

---

It would be easy to center the #center element if it were the only flex item in the container. Either of the following two methods would suffice:

#wrap {
    display: flex;
    justify-content: center;
    align-items: center;
}

OR

#center {
    margin: auto;
}

Except there is a second flex item in the container occupying space at the top. This means that the centering methods above – which are based on the distribution of free space in the container – won't be precise. They will center the #center element inside the remaining space, but not inside the entire space (because #top is occupying space which, as a result, is excluded from the calculation).

In the following demo, #center is perfectly centered in the gray area, but not in the entire flex container, which includes the orange area:

DEMO 1

The #center element looks centered, and that may be enough for your purposes, but it's not precisely centered. It's off vertically by 50px, which is the height of the #top element.

So another centering method must be used that takes the #top space into consideration.

---

Method #1: Introduce a third "phantom" element for equal balance

This method involves creating a third flex item with the same height as #top. We place this new item on the other side of #center to create an equal balance of space on both sides. We then hide this item with visibility: hidden.

HTML

<div id="wrap">
    <div id="top"></div>
    <div id="center"></div>
    <div id="bottom"></div><!-- invisible flex item -->
</div>

CSS (relevant section)

#top {
    flex: 0 0 50px;
    width: 100%;
    background-color: orange;
}

#center {
    margin: auto;
    width: 200px;
    height: 300px;
    background-color: aqua;
}

#bottom {
    flex: 0 1 50px;
    visibility: hidden;
}

Now #center is perfectly centered in the flex container.

DEMO 2

---

Method #2: Absolutely position the `#center` element

Flexbox allows for the absolute positioning of flex items. This means we could remove #center from the document flow and position it relative to its parent.

HTML

<div id="wrap">
    <div id="top"></div>
    <div id="center"></div>
</div>

CSS (relevant sections)

#wrap {
    position: relative;
}

#center {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

Although this method doesn't require an extra flex item (like in Method #1) – and still achieves perfect centering – by removing this item from the document flow it can overlap other elements.

DEMO 3

Answer 2

You can do this with one extra element in your HTML which will basically fill the space after the header. Then your content can be centered within that filled space.

#container {
  display: flex;
  flex-flow: flex-wrap;
  flex-direction: column;
  
  border: 1px solid black;
  height: 150px;
  justify-content: flex-start;
}

#content {
  background: yellow;
  height: 100%;
  display: flex;
  justify-content: center;
}

#centered {
  background: red;
  height: 30px;
  align-self: center;
}

#header {
  background: green;
}

<div id="container">
  <div id="header">This is footer content.
  </div>
  <div id="content">
    <div id="centered">
      This is centered content.
    </div>
  </div>
</div>