The simple reported formula
Divide reported unique openers by successfully delivered emails and multiply by 100. If 350 of 1,000 delivered messages are reported as opened, the reported rate is 35%. Use unique recipients, not total repeated opens, in the numerator. Use successful deliveries rather than all attempts in the denominator, and keep both figures from one campaign and reporting window.
Why the number is uncertain
Open tracking typically depends on loading a small remote image. Some clients block it; others may preload it. Mailchimp explains that Apple's Mail Privacy Protection can inflate open metrics and make the exact opening time unreliable for affected recipients. Bot filtering and different provider definitions can also change the count. A reported open is not the same as a confirmed human reading event.
Pair it with stronger evidence
Review clicks, replies, relevant conversions, unsubscribes, and customer questions alongside reported opens. Avoid triggering important follow-up messages solely because someone appears not to have opened an email. If you compare campaigns, document any change in tracking, audience mix, or privacy handling that could shift the reported rate without a real change in attention.
Mailchimp: open tracking · Mailchimp: Apple Mail Privacy Protection