Open Rate

Open rate is the share of successfully delivered emails that register as opened, expressed as a percentage.

What Open Rate Means in Marketing

Open rate has been the headline email metric for two decades. The logic was simple: if nobody opens the email, nothing else in it matters. Subject line testing, send-time optimisation and list hygiene all had open rate as their north star.

That logic is still valid. The measurement is not.

Since Apple launched Mail Privacy Protection in September 2021, email clients on iPhones, iPads and Macs began pre-loading messages on Apple’s proxy servers. That pre-load fires the tracking pixel your email platform uses to detect opens, whether or not anyone reads a word. Depending on your audience, this can inflate your reported open rate by a significant margin. Mailchimp, Klaviyo and most other platforms flagged the change and began labelling Apple-impacted opens separately, but the headline number still shows up everywhere and misleads plenty of teams.

The practical effect is that two campaigns can look identical in an open rate report while one was actually read and the other was not.

How Open Rate Works

The formula is:

Open Rate = (Unique Opens ÷ Delivered Emails) × 100

Most platforms count unique opens, so if the same person opens the same message three times on their phone, that still counts as one open. Bounced emails are excluded from the denominator.

Before Apple’s change, the pixel-fire method was reliable enough. A tiny image embedded in the email would load from your platform’s server when someone opened the message, logging the event. Privacy tools now block or simulate this load, making pixel-based tracking structurally broken for a large slice of most email lists.

Open Rate Example

Consider an e-commerce brand with 50,000 subscribers. Pre-2021, an open rate of 28% meant roughly 14,000 people had seen the subject line and chosen to read the message. Post-2021, that same 28% might represent 10,000 real readers and 4,000 Apple Mail pre-fetches. The team that optimises aggressively toward open rate is chasing an inflated number.

Why Open Rate Matters for Marketers

Open rate still tells you something. A dramatic drop in open rate, especially if it precedes a revenue drop, signals a deliverability problem or a disengaged list that no subject line will fix. Use it directionally, not as a precision instrument.

The metrics that have replaced it in serious email programmes are click-to-open rate (CTOR), which measures clicks as a share of actual opens rather than delivered emails, and revenue per email, which cuts through any measurement noise. If your emails are generating revenue, the tracking imperfections stop mattering.