The Importance of Email File Size
Bird
27 Aug 2018
1 min read
Key Takeaways
Premise: The effectiveness of an email campaign depends heavily on file size — both for deliverability and user experience. Oversized HTML or image assets can cause clipping, slow load times, and lower engagement, especially on mobile.
Goal: Help marketers balance design ambition with technical constraints by optimizing HTML, external assets, and fully loaded size.
Highlights:
HTML file size:
Keep under 100 KB to avoid Gmail clipping (ideal target ≈ 50 KB).
Oversized HTML can break layouts and affect deliverability.
Reduce size through:
Code cleanup (remove outdated markup and unused elements).
Avoiding over-nesting of tables — 5–10 tables are usually sufficient.
Moving safe inline styles to embedded CSS where possible.
Simplifying layout (single-column designs) and minifying code.
External assets (mainly images):
Recommended max = 100 KB per image; animated GIFs may reach 500 KB.
Optimize with ImageOptim, Caesium, or CDN compression.
Choose correct formats: JPG for photos, GIF for graphics, PNG for transparency.
Use fewer, lighter images — display :none still downloads assets.
Fully loaded size:
Ideal total load ≤ 1 MB for smooth performance.
On 3G networks, large emails can take 2–14 seconds to load; even longer on 2G.
Optimize for slow or limited-data users (especially in developing regions).
Testing and balance:
A/B test to find the trade-off between visual richness and performance.
Ensure the email is legible without images for faster perceived load.
Poor size optimization can amplify deliverability and engagement problems.
Outcome: Clean, compact code and optimized media lead to faster loading, higher inbox placement, and better user experience across devices.
Q&A Highlights
Why does email file size matter?
Large emails risk clipping in Gmail and cause long load times — both harm engagement and deliverability.
What’s the recommended maximum HTML size?
Keep your HTML under 100 KB (ideally < 50 KB). Gmail clips messages at 102 KB.
How can I reduce my HTML size?
Remove redundant code, minimize table nesting, consolidate CSS, and minify safely.
What’s the best way to handle image weight?
Compress files using CDNs or optimization tools, use proper formats, and limit each to ≤ 100 KB.
How can I ensure accessibility for users with slow connections?
Keep total email size < 1 MB, design for no-image fallback, and prioritize essential content loading first.
What’s the ultimate goal of email size optimization?
To balance design quality, deliverability, and speed — resulting in higher open and click-through rates.
When talking about file sizes in email, we’re really talking about 3 topics: the size of the HTML file sent, the size of external assets, and the fully loaded size. We want to keep all of these file sizes as small as possible, but what are the maximum sizes and what should we aim for?
HTML File Size
HTML file size is the size of the .html file that is actually sent. This file size does not include the size of the included images, as these would be linked from this html file and downloaded when opened.
The maximum size we’d recommend is 100KB, as anything over can lead to deliverability and display issues. For example, Gmail will clip your html at 102KB. Clipping will hide the end of your email and can break your layout. Gmail's handling of emails has other considerations too, such as image prefetching impact on email tracking.
Most emails should easily come in under 50KB. However, sometimes complex progressive enhancements or a large amount of campaign content results in a larger file size. If this happens there are a number of ways to help reduce the size of your HTML file.
Clean your code. Email is constantly changing and we see a lot of templates with outdated code in them. A quick clean up can save a few KBs.
Don’t over code. Some templates we see have extra code that simply isn’t needed. For guidance on creating efficient templates, see our guide on advanced email templates. The worst offender is adding additional table elements. Most layouts can be achieved with 5-10 tables, but we often see code with 30-40 tables. Fewer tables also has the added benefit of making your code easier to read.
Move some code from HTML to CSS. It’s not yet safe to move all your styles to CSS, but if done correctly moving some inline styles to embedded styles can help reduce code.
Simplify your design. Another option is to go back to the design stage. Single column emails use a lot less code.
Minify your code. Minifying code will remove unnecessary spaces and comments. This can have a huge effect but be careful as many ESPs enforce a maximum line length that could force a new line and break the code.
External Assets Size
Fully Loaded Email Size
This is the combined size of everything when the email is opened and everything is downloaded. This can vary depending on the environment where the email is opened.
The main issue here is the time it takes the email to load. You’ll likely be testing your emails in an office with a very fast connection but many of your recipients may be on mobile devices with slow 3G connection.
On a good connection, a total email size of 1mb would load in a fraction of a second and seem instant to the user. However, on a 3G connection it could take between 2 and 14 seconds to load the full email. And, if your target audience is likely to be on a 2G connection that download could take a couple of minutes. When emails load slowly, compelling subject lines become even more critical for engagement.
This is also a potential issue with users with restrictive data plans who have to consider every MB they download. This is particularly a consideration in developing nations.
A couple of things you can do to help:
Firstly, look at all the recommendations we mentioned regarding html and image size optimization
If you have the time it’s always good to experiment with code and test what is really needed and what you can leave out. Experiment with images and try different ways to compress them. Make sure your email looks good and makes sense without images, so your users can start looking at your email while they wait for the images.
Putting it All Together
With file sizes, as with a lot of things, it all comes down to compromise. You may do some A/B testing and find large high resolution images perform well. You may find that a lot of detailed content performs well. You may find that complex interactivity performs well. However, when you put all three into a single campaign– you may see that things start to perform poorly. Email performance issues can also stem from deliverability problems like email address typos.
