Email Open Rate
Email open rate is the percentage of delivered emails that were opened by recipients, traditionally measured by the loading of a tracking pixel embedded in the email.
Also known as: open rate, email opens percentage
Formula
(Emails Opened / Emails Delivered) x 100
Why It Matters
Email open rate has historically been the primary metric for evaluating subject line effectiveness and send time optimization. A high open rate suggests your subject line resonated with recipients and your sender reputation is strong enough to land in the primary inbox.
However, open rate reliability has deteriorated significantly since Apple introduced Mail Privacy Protection (MPP) in iOS 15. MPP pre-loads tracking pixels for Apple Mail users, registering an "open" regardless of whether the recipient actually read the email. This has inflated open rates by 10-30% for many senders and made the metric unreliable as a standalone indicator.
Despite these limitations, open rate remains useful as a relative comparison metric. Comparing open rates between subject lines in an A/B test still works because MPP affects both variants equally. Tracking open rate trends over time can still indicate deliverability issues. But the metric should no longer be used as a primary KPI for email program health.
How to Calculate
Open rate is calculated by dividing the number of emails opened (pixel loads) by the number of emails successfully delivered (total sent minus bounces), then multiplying by 100. If 8,500 out of 25,000 delivered emails registered an open, the open rate is 34%.
Email Open Rate Calculator
(Emails Opened / Emails Delivered) x 100
Industry Applications
An ecommerce brand sees a 15% jump in reported open rates after iOS 15 launches. Instead of celebrating, they shift focus to click rate and revenue per email as primary metrics, getting a more accurate view of their email program effectiveness.
Benchmark: Average ecommerce email open rate: 15-25% (pre-MPP); 30-45% (post-MPP, inflated)
A SaaS company uses open rate A/B tests to optimize subject lines for their product update newsletter. They test personalized vs generic subject lines and find that including the user name and a specific feature mention increases opens by 18% (relative).
Benchmark: Average SaaS email open rate: 20-30% (pre-MPP)
How to Track in KISSmetrics
Email service providers report open rates automatically. Use KISSmetrics to track what happens after the open by connecting email engagement with on-site behavior. When recipients click through to your site with UTM-tagged links, KISSmetrics tracks their full journey from email click to conversion, giving you a more reliable measure of email effectiveness than open rate alone.
Common Mistakes
- -Using open rate as a primary email KPI after Apple Mail Privacy Protection made the metric unreliable.
- -Not filtering Apple Mail users from open rate calculations to get a cleaner (though smaller sample) metric.
- -A/B testing subject lines based on open rate differences that are within the margin of error.
- -Assuming a low open rate always means poor subject lines when deliverability issues (landing in spam) may be the real cause.
Pro Tips
- +Shift your primary email KPI from open rate to click rate, which is not affected by Apple Mail Privacy Protection and better measures genuine engagement.
- +Use open rate for relative comparisons (A/B tests, trend analysis) rather than absolute targets.
- +Monitor open rate trends for deliverability signals - a sudden drop often indicates a spam filtering issue that needs immediate attention.
- +Segment open rate analysis by email client to separate the inflated Apple Mail numbers from more reliable clients.
- +Focus on optimizing preview text alongside subject lines, as many recipients decide whether to open based on both.
Related Terms
Email Click Rate
Email click rate is the percentage of delivered emails in which at least one link was clicked by the recipient, serving as a more reliable measure of email engagement than open rate since it requires deliberate user action.
Unsubscribe Rate
Unsubscribe rate is the percentage of email recipients who opt out of future emails after receiving a specific campaign, serving as a critical health metric for email list quality and content relevance.
Click-Through Rate
Click-through rate (CTR) is the percentage of people who click on a link, ad, email, or call-to-action out of the total number who viewed it, measuring how effectively a piece of content drives user action.
A/B Testing
A/B testing is a controlled experiment that compares two versions of a web page, email, ad, or feature by randomly splitting traffic between them and measuring which version performs better on a defined success metric.
Conversion Rate
Conversion rate is the percentage of users who complete a desired action out of the total number of users who had the opportunity to do so, serving as the primary measure of how effectively a page, campaign, or experience turns visitors into customers.
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