How is open rate calculated?
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The formula is straightforward: unique opens divided by delivered messages, multiplied by 100. Send 1,000 emails, get 200 unique opens, that's a 20% open rate.
The "unique" part matters. If one person opens your email three times, that counts as one unique open, not three. Most ESPs report both unique opens and total opens, but open rate is almost always the unique figure.
"Delivered" means the email was accepted by the mailbox provider's servers, not that it actually reached the inbox. An email that lands in spam still counts as delivered in this calculation.
How does an "open" get registered? Your email contains a tiny invisible image (a tracking pixel) hosted on a server. When someone's email client loads that image, it sends a request to your server and you record an open. The problem: email clients don't always load images automatically, and Apple Mail Privacy Protection pre-loads these pixels in the background regardless of whether the person actually read the email. This inflates open rates on Apple Mail users significantly.
As a rough guide, open rates above 20-25% used to indicate a healthy list. Post-MPP, those benchmarks are less reliable. Click rate and click-to-open rate (CTOR) are better signals of actual engagement now.
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