CSS Viewport Units: vh, vw, vmin, and vmax

In this article, we’ll look in detail at four handy length units that are relative to the browser viewport: vh, vw, vmin and vmax. These are truly “responsive length units” in the sense that their value changes every time the browser resizes, and they introduce incredibly useful options for sizing elements in CSS.

The browser’s viewport is that area of the browser in which website content is displayed. The ability to measure that area is very useful, as it makes once difficult tasks easy — such as setting an element’s height to that of the browser window.

Key Takeaways

The Units and Their Meaning

Let’s first look at what these units mean.

• vh stands for viewport height. This unit is based on the height of the viewport. A value of 1vh is equal to 1% of the viewport height. A value of 100vh is equal to 100% of the viewport height.
• 1vw stands for viewport width. This unit is based on the width of the viewport. A value of 1vw is equal to 1% of the viewport width.
• vmin stands for viewport minimum. This unit is based on the smaller dimension of the viewport. If the viewport height is smaller than the width, the value of 1vmin will be equal to 1% of the viewport height. Similarly, if the viewport width is smaller than the height, the value of 1vmin will be equal to 1% of the viewport width.
• vmax stands for viewport maximum. This unit is based on the larger dimension of the viewport. If the viewport height is larger than the width, the value of 1vmax will be equal to 1% of viewport height. Similarly, if the viewport width is larger than the height, the value of 1vmax will be equal to 1% of the viewport width.

Some example values

Let’s see what the value of these units will be in different situations:

• If the viewport is 1200px wide and 1000px high, the value of 10vw will be 120px and the value of 10vh will be 100px. Since the width of the viewport is greater than its height, the value of 10vmax will be 120px and the value of 10vmin will be 100px.
• If the device is now rotated so that the viewport becomes 1000px wide and 1200px high, the value of 10vh will be 120px and the value of 10vw will be 100px. Interestingly, the value of 10vmax will still be 120px, because it will now be determined based on the height of the viewport. Similarly, the value of 10vmin will still be 100px.
• If you resize the browser window so that the viewport becomes 1000px wide and 800px high, the value of 10vh will become 80px and the value of 10vw will become 100px. Similarly, the value of 10vmax will become 100px and the value of 10vmin will become 80px.

At this point, viewport units may sound similar to percentages. However, they’re very different. In the case of percentages, the width or height of the child element is determined with respect to its parent. Here’s an example:

As you can see, the width of the first child element is set to be equal to 80% of its parent’s width. However, the second child element has a width of 80vw, which makes it wider than its parent.

Applications of vh, vw, vmin, and vmax

Since these units are based on viewport dimensions, it’s very convenient to use them in situations where the width, height or size of elements needs to be set relative to the viewport.

Fullscreen background images or sections with viewport units

It’s very common to set background images on elements that fully cover the screen. Similarly, you may want to design a website where each individual section about a product or service has to cover the entire screen. In such cases, you can set the width of the respective elements to be equal to 100% and set their height equal to 100vh.

As an example, take the following HTML:

<div class="fullscreen a">
  <p>a<p>
</div>

You can achieve a fullwidth background image section using the CSS below:

.fullscreen {
  width: 100%;
  height: 100vh;
  padding: 40vh;
}

.a {
  background: url('path/to/image.jpg') center/cover;
}

Both the first and second image are taken from Pixabay.

Creating perfectly fitting headlines with viewport units

The FitText jQuery plugin can be used to scale headlines in such a way that they take up all the width of the parent element. As we mentioned earlier, the value of viewport units changes directly based on the size of the viewport. This means that, if you use viewport units to set the font-size for your headings, they’ll fit perfectly on the screen. Whenever the viewport width changes, the browser will also automatically scale the headline text appropriately. The only thing that you need to do is figure out the right initial value for the font-size in terms of viewport units.

One major problem with setting font-size this way is that the size of the text will vary greatly depending on the viewport. For example, a font-size of 8vw will compute to about 96px for a viewport width of 1200px, 33px for a viewport width of 400px and 154px for a viewport width of 1920px. This can make the font either too large or too small for it to be properly readable. You can read more about properly sizing the text using a combination of units along with the the calc() function in this excellent article about viewport unit-based typography, and you can read about using the clamp() function to achieve a similar result.

Centering elements with viewport units

Viewport units can be very helpful when you want to put an element exactly at the center of your user’s screen. If you know the element’s height, you just have to set the top and bottom value of the margin property to be equal to [(100 - height)/2]vh:

.centered {
  width: 60vw;
  height: 70vh;
  margin: 15vh auto;
}

However, nowadays we can use Flexbox or CSS Grid to center elements, both vertically and horizontally.

Things to Keep in Mind with Viewport Units

If you decide to use viewport units in your projects, there are a few things you should keep in mind.

Be careful when setting the width of an element using viewport units. This is because, when the overflow property on the root element is set to auto, browsers will assume that the scrollbars don’t exist. This will make the elements slightly wider than you expect them to be. Consider markup with four div elements styled as follows:

div {
  height: 50vh;
  width: 50vw;
  float: left;
}

Normally, you’d expect each <div> to occupy a quarter of the available screen. However, the width of each div is computed with the assumption that there is no scrollbar. This makes the div elements slightly wider than the required width for them to appear side by side.

Changing the width of the divs from 50vw to 50% will solve this problem. The conclusion is that you should use percentages when setting width for block elements so that the scrollbars don’t interfere with the computation of their width.

A similar issue can also occur on mobile devices because of the address bar, which may appear or disappear depending on whether the user is scrolling or not. This will change the viewport height and the user will notice sudden jumps when viewing the content.

To help with this situation, some new viewport units have been added to CSS, such as svw, svh, lvw, lvh, dvw, and dvh. You can read about them in our comprehensive article on CSS sizing units.

Conclusion

In this article, we’ve briefly covered the meaning and applications of the vh, vw, vmin and vmax viewport units. If you’d like to take your knowledge of viewport units and other CSS length units ti the next level, check out An Overview of CSS Sizing Units.

You can also take all of your CSS skills to the next level with our book CSS Master, 3rd Edition by Tiffany B. Brown – covering CSS animations, transitions, transformations and much more.