Email Open Rate
The percentage of email recipients who open a given email. It is a key indicator of subject line effectiveness and sender reputation, though tracking accuracy has declined significantly since Apple Mail Privacy Protection (iOS 15) began pre-loading tracking pixels in 2021. The formula is unique opens divided by delivered emails (excluding bounces) multiplied by 100. Industry average open rates range from 15-25% depending on sector, with nonprofit and government emails averaging highest at 28% and retail averaging around 18%.
Why It Matters
Open rate is the first gatekeeper in email marketing performance. If recipients do not open your emails, no other metric matters. Tracking open rates by segment helps identify which audiences engage most and which subject line strategies work best. Because Apple MPP inflates open rates by 10-30%, savvy marketers now pair open rate with click-based metrics for a more accurate engagement picture. Consistently low open rates below 15% also damage sender reputation, causing ISPs to route future emails to spam folders and creating a downward spiral of declining deliverability.
Example
A retailer sends a promotional email to 50,000 subscribers. The campaign records 12,500 opens for a 25% open rate. The subject line 'Your favorites are back in stock' outperforms the A/B variant 'New arrivals this week' by 8 percentage points. After segmenting out Apple Mail users who show inflated 85% open rates, the true open rate on Gmail and Outlook recipients is 19.5%. The team uses this adjusted metric to optimize send times, discovering that Tuesday at 10 AM drives 22% higher opens than their previous Friday afternoon sends.
Related Terms
Click-Through Rate (Email)
The percentage of email recipients who click on one or more links within an email. It measures how effectively email content drives engagement and moves recipients toward a desired action. CTR is calculated by dividing unique clicks by total delivered emails and multiplying by 100. A related metric, click-to-open rate (CTOR), divides clicks by opens to isolate content effectiveness from subject line performance. Industry average email CTR is 2-3%, though triggered and transactional emails often achieve 5-8%.
Unsubscribe Rate
The percentage of email recipients who opt out of future emails after receiving a campaign. A rising unsubscribe rate signals issues with content relevance, frequency, or audience targeting. The formula is unsubscribes divided by delivered emails multiplied by 100. Industry benchmarks consider anything below 0.5% per campaign healthy, while rates above 1% indicate a serious problem. Under CAN-SPAM and GDPR regulations, every marketing email must include a visible unsubscribe mechanism that is honored within 10 business days.