Substack detects opens by watching users load the images in their messages or opening a post after receiving a mobile app notification about it. Often this happens when the user clicks an email, but sometimes images can load in previews, or even be eagerly loaded by email clients for messages at the top of a user's mailbox. In addition, users can change their mailer settings to decline to load images or read text-only versions of the messages that don't load images at all.
Each email sent by Substack is marked with its intended recipient. We track opens by the intended recipient, not by the email messages themselves, so a reader who forwards their email to a friend will appear as a single user, opening a message twice.
The "open rate" percentage we show in the interface is the proportion of intended recipients who have opened the email. It is based on the number unique individuals who opened your email. This means if the same reader opens your email five times, only one of those opens counts towards the open rate.
Below the open rate we show the total opens not unique opens. This is not the number which is used to calculate your open rate. An individual opening the same email many times (or sending it to their friends) may count for multiple entries in that absolute number.