The CSS z-index property
How to work with the `z-index` property in CSS
In the positioning post I mentioned that you can use the z-index
property to control the Z axis positioning of elements.
It’s very useful when you have multiple elements that overlap each other, and you need to decide which one is visible, as nearer to the user, and which one(s) should be hidden behind it.
This property takes a number (without decimals) and uses that number to calculate which elements appear nearer to the user, in the Z axis.
The higher the z-index value, the more an element is positioned nearer to the user.
When deciding which element should be visible and which one should be positioned behind it, the browser does a calculation on the z-index value.
The default value is auto
, a special keyword. Using auto
, the Z axis order is determined by the position of the HTML element in the page - the last sibling appears first, as it’s defined last.
By default elements have the static
value for the position
property. In this case, the z-index
property does not make any difference - it must be set to absolute
, relative
or fixed
to work.
Example:
.my-first-div {
position: absolute;
top: 0;
left: 0;
width: 600px;
height: 600px;
z-index: 10;
}
.my-second-div {
position: absolute;
top: 0;
left: 0;
width: 500px;
height: 500px;
z-index: 20;
}
The element with class .my-second-div
will be displayed, and behind it .my-first-div
.
Here we used 10 and 20, but you can use any number. Negative numbers too. It’s common to pick non-consecutive numbers, so you can position elements in the middle. If you use consecutive numbers instead, you would need to re-calculate the z-index of each element involved in the positioning.
→ I wrote 17 books to help you become a better developer, download them all at $0 cost by joining my newsletter
→ JOIN MY CODING BOOTCAMP, an amazing cohort course that will be a huge step up in your coding career - covering React, Next.js - next edition February 2025