Open rates – what about accuracy?

Open rates are a basic measure of email marketing success. However, there are (at least) two problems with them: accuracy and standardization.

As you might know: Email opens measure how many times an emailing got opened (total opens), or how many recipients opened it (unique opens). Open rates put opens in relation to the number of potential readers. They account for the fact that opens increase with an increasing number of recipients and thus make different campaigns over time comparable.

Problem: Lack of standards

First problem: different email software could report a different open rate for the same success. Some use the number of emails sent in the denominator, others use the number of accepted emails (= sent emails minus bounces). Some use rendered webbugs in the numerator, and others add clicks of subscribers who didn’t fire a webbug. Depending on how you calculate open rates, they get bigger or smaller:

Conventions suggest naming only the one in the middle open rate: opens / accepted emails. The one to the left is called confirmed open rate. However, those standards are not particularly prevalent across the globe.

Problem: Open rate accuracy

Second problem: Opens are inaccurate measures. Opening an email in terms of email marketing does not necessarily mean opening the envelope and “viewing an email”. It means “having loaded an email with images on”. If images are blocked on the subscriber side, no open can be tracked. That is because opening tracking is – in most cases – based upon an invisible image that has been included into the email. No image rendering, no tracking.

As many email clients like Gmail, Outlook or block images by default, people open emails untracked. And they might also trigger opens without actually having viewed the email. Think of quickly skimming through the preview pane.

The understatement in numbers

After all, marketers have good reasons to believe that undertracking outweighs overtracking. I was curious to what extend tracked opens understate real opens. For fun, I looked at how many recipients clicked without triggering an email open per campaign:


The dots show the open rates as reported by my email service provider. The red bars mark the unique open rates based on my own webbug. And the cyan bars on top are users who clicked a link, although no email open had been recorded. Both bars together form the confirmed open rates.

Dividing cyan by red leads to an average of 27 % untracked opens. So, to better approximate my real open rates, I would multiply the tracked ones by 1.27. This finding is consistent with other studies, like here or here [both PDFs in German, see also p486 in my book].


Measured open rates can greatly understate real opens. Don’t underestimate your email marketing success. One should also be aware of how open rates are calculated – espcially when comparing them between email service providers or list owners.

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