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What are CSS viewport units?

Abel Lifaefi Mbula

Overview

The viewport is the visible area of a page in the browser. CSS provides us with units that relate to its size. They are known as the viewport-percentage lengths units. They are vh, vw, vmax and vmin. These units let us size things relative to the viewport of the user.

vh & vw units

In CSS, vh stands for viewport height and vw for viewport width. As you can see, the first unit is based on the viewport height, and 1vh is equivalent to 1% of the viewport height. vw works the same, but for viewport width. So, 1vw equals 1% of the viewport width.

Example

Given a box, we set the width to 20vw and the height to 20vh. Considering the different viewport scenarios listed below, what is the final pixel (px) size for the width and height?

  • Viewport width and height are respectively 1200px and 1000px.

In this scenario, the value of 20vw will be 240pxand that of 20vh will be 200px.

  • Viewport width and height are respectively 1000px and 1200px.

Here, we can see that the device is rotated. So, 20vw is now equal to 200px and 20vh equals 240px.

  • Viewport width and height are respectively 1000px and 800px.

Notice how the initial viewport is resized. 20vw = 100px and 20vh = 160px.

Example of vh and vw units

vmin units

In CSS, vmin stands for viewport minimum. The vmin function is used to set the size of an element as a percentage of the minimum value between the viewport width or height. For example, if the viewport is 1000px wide and 800px high, if we set the width of an element to 30vmin, it will be 30% of the height. So 30% of 800px is 240px.

vmax units

In CSS, vmax stands for viewport maximum. The vmax function ranks elements as a percentage of the maximum value between the viewport width or height. For example, as shown above, an element of 30vmax would be 30% of the width, or 300px.

Example

We use these viewport units in situations where we need to set the element size relative to the viewport. For this practical exercise, let’s create a section that occupies the exact height of the viewport.

First, the markup.

<section class="cover">
  <h1>Welcome to Educative, Inc.</h1>
</section>

Next, the style.

body {
    font-family: 'Lato';
}

.cover {
    width: 100%;
    height: 100vh;
    color: #fff;
    background: linear-gradient(115deg, #4e54c8, #8f94fb);
    display: flex;
    align-items: center;
    justify-content: center;
}

Note: You’ll notice that for the width, I use 100% instead of 100vw to make the section take up the full width of the page. This is because the vw part is used outside the HTML and body elements. But if you still want to use vw, you can use overflow-x: hidden on the body element.

Let's put it all together to enjoy the output.

Hands-on exercise

Conclusion

We've looked at viewpoint units in more detail. They are vh, vw, vmin and vmax. The most commonly used of these are:

  • vh, which determines its size based on the viewpoint's height.
  • vw, whose size is based on the viewpoint’s width.

In this shot, we’ve learned that when you want an element to span the full width of the page, it’s best to use percent units instead of viewpoints.

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