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Guide to Email File Size and Load Times


When creating emails, we focus on building the best email for our audience. That often manifests itself in content curation before designing and creating the email. While we may agonize over subject linesCTAs (call-to-action), and segmentation or personalization, there are some things we often forget: Email file size and email load times. And while they may have similarities, they are different issues which need to be taken into consideration.

While not the sexiest of email topics, it’s critical to building the right subscriber experience. Besides the fact that Gmail clips messages over a certain size, your email file size can also determine the bandwidth needed to download and see your email. If your subscriber can’t see your email, or it takes forever to load on mobile, it’s unlikely they’re going to engage or convert.

Read on to find out how to overcome your email file size and load times obstacles.

What is an email file size?

There are two ways to measure email file size:

  • The weight of the code
  • Loaded email weight

The weight of the code

We’re talking about bytes in the email HTML where each character is between 1 to 2 bytes. This includes HTML tables, text, links, and so on.

Loaded email weight

This includes everything needed to fully render your email. This includes the HTML of your email—as mentioned above—plus all of your assets, such as images, fonts, and videos.

What are the factors that impact email file size?

The first and most obvious factor is the length of the email. Sending an email with dozens of product images and descriptions will likely result in a heavy email. The same is true if you don’t compress your images. Including just one heavy image as your entire email can result in a very heavy email.

Another popular culprit of heavy emails is animated GIFs. Every frame in an animated GIF is essentially its own image. The more frames your GIF has, the heavier it will be. Other factors that impact email file size include coding style and custom fonts. A plain email with long paragraphs is rarely a problem. That being said, we don’t recommend sending a whole novel inside an email.

Why does email file size matter?

Both types of email file size covered earlier matter.

First, the weight of the HTML is important because Gmail clips messages that weigh over 102kb.

Second, the loaded weight of your email determines how much bandwidth is needed to download—and see—your email. Heavy emails take longer to fully render, which can be a problem, especially when it comes to email engagement.

Then there’s the user experience. Subscribers won’t see the full narrative you’re trying to get across when heavy images are still loading. Things get even trickier on mobile where loading images might make it look as though you sent an empty email. The same applies to custom fonts; while fonts are loading, the text in your email won’t render.

Does email file size affect deliverability?

Sending an email with an HTML weight of under 100KB has no impact on deliverability. In some cases, sending an email over 100KB may impact deliverability, but it may not be due to the size alone. Rather, it could be the result of the poor customer experience and engagement issues. Generally, keeping emails under 80KB where possible will help decrease some of these issues.

What’s the ideal email file size? Has the rise of mobile affected the ideal file size?

The ideal file size will vary. If you’re targeting subscribers in a country with slow internet, for example, your ideal file size will be lower than someone focused on U.S. subscribers.

We analyzed the average email file size and here’s what we found:

Non-mobile optimized emails:

  • Average HTML weight: 45KB
  • Average image weight: 2.24MB

Mobile optimized emails:

  • Average HTML weight: 53KB
  • Average image weight: 2.7MB

It’s interesting to note that mobile-optimized emails tend to be heavier than non-mobile optimized email. On the HTML front, this makes sense since it requires more code to build an email that is mobile optimized (think media queries, for example).

How can I determine my email file size?

The fastest and easiest way to see your email file size is…to use Litmus! You can see image size and load times, which can help you determine your overall file size.

This will give you the weight of the HTML. You can then measure the size of all the images in your email. Add everything up, and voila!

What techniques can I use to decrease my file size? Are there any drawbacks to these techniques?

The best way to decrease your email file size is to compress your images. You can also consider using fewer images. When using animated GIFs, remove unnecessary frames. This will do a lot to reduce the GIF size.

Some great tools Litmus recommends for this are:

If you can get your team on board, use web safe fonts. Not having to download a custom font is preferable.

On the coding side, don’t create emails that show or hide large blocks of content based on media queries. This will add unnecessary weight to your emails. Instead, focus on emails that fluidly adapt to both desktop and mobile clients.

What is an email load time?

Email load time determines how long it takes for your email to fully load the HTML, fonts, and images. Essentially, it’s the time it takes to load your email.

What are the factors that impact email load time?

The loaded email weight has a big impact on email load time. This, combined with your Internet connection speed, determines the overall email load time.

What effect do images have on load time?

Images included in an email need to load from somewhere. These images are hosted on a server and, unfortunately, not all servers are created equal.

The longer your server takes to serve an image, the longer your email takes to load. This can be a problem when emailing a large list where thousands (or millions!) of subscribers are opening your email at the same time.

The best way to ensure email images load quickly is to use a Content Delivery Network (CDN). This helps reduce latency and overall load times. The faster images load, the faster the email renders.

How can I determine my email load time?

The simplest way to determine this—by far—is to test your email through Litmus. You will be able see the total image size and the load time.

What techniques can I use to decrease my email load time? Are there any drawbacks to these techniques?

The easiest way to decrease load time is to reduce the loaded email weight, as we mentioned above. This includes using compressed images or fewer images and removing frames from animated GIFs.

Another option is to improve your infrastructure, such as using a CDN with a lower latency. The challenge is that, as marketers, it’s often impossible to get buy-in for things like these.

Sending emails that are not too long, with compressed images, will do the trick 99% of the time.

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Martyn Lee

Martyn Lee

Martyn Lee is a Design Associate at Litmus