What are optimal file sizes in email-for images and overall email weight-and what size targets ensure a good user experience? Here are a few tips gleaned from our experts to make sure you're doing it right.

Weight of Email Code Files
The size of an actual HTML file for email should be below 102 KB to prevent clipping in Gmail-and this weight does not include the size of images used in the email. Messages can also be truncated in the mobile inbox, usually triggered when larger than 15 KB, or if the reader is using a slower cellular data network, prompting them to download the entire message.

The total size of the HTML file plus images, sometimes referred to as total weight or loaded weight, will be dependent on the sender and the size/amount of images used. The loaded weight divided by connection speed determines load speed, so a smaller loaded weight = faster load = more eyes before distraction/apathy.

We loosely recommend keeping loaded weight below 800 KB, and ideally below 500 KB. An average image-heavy retail email may be about 600-900 KB in total weight. To see the effect file size can have on performance, see how Kitbag and Lyris optimized their emails to trim their file size and cater to their mobile subscribers successfully.

To find the loaded weight of an email, click the "view as a web page" link, select "save as" in the menu, and choose "Web page, complete", or similar. Then, go to the saved location you chose, select both the HTML file and the folder of images, and right-click to select "Properties" or "Info", depending on your operating system. The resulting number is the total file size of the email.

Image Size
Within the ExactTarget Marketing Cloud, you can host images at or less than 200 KB in size in Portfolio. Depending on the type of imagery in your campaign, you may include a variety of file types including JPGs, GIFs, and PNGs-or even animated GIFs.

When saving images for web, you'll want to ensure you know where your audience is reading your emails. You'll need to consider retina screens on newer computers, tablets, and mobile phones. Generally speaking, use about 70% JPEG compression for standard definition images to ensure good quality but small file size.

Load Time
Image size and email weight affect your load time, and it can be risky to play with the amount of time people are willing to wait to see your messages. Remember that web speeds will also affect this, so load time will vary if your audience is using a computer or mobile device over WIFI or a cellular data network. We know that subscribers only spend a few seconds reading email if they choose to open it, so if they have to wait long to even see the email, you'll lose their attention.

To simulate your load time on a computer, use a website speed test-place the "View with images" or "View as a web page" link from a test version of your email here to try it out.

Quick Reference Guide

  • Email weight: ideally 60 KB, but aim for <100 KB
  • Loaded weight: <800 KB
  • Load time: 1 second or less
  • Image size: <200 KB

Quality Assurance
Email clients will evolve and change the rules, so these are just guidelines to review for a healthy email checkup. Keep testing thoroughly, observe what others are doing, and use email analytics to know your specific audience.

Look for parts 2 and 3 to come next, covering retina displays, mobile viewing, and more specifics on reducing file size. To keep tabs on the latest industry trends and get design inspiration, be sure to bookmark our ever-evolving Email Swipe File on Pinterest. For emails that look good, work well, and get results, get tips and advice from our Email Design Toolkit.