Email Open Rate
Quick definition
Email open rate is the percentage of delivered emails that were recorded as opened. Opens are usually detected by an image loading when the email is displayed.
Formula
Email Open Rate = Unique Opens ÷ Emails Delivered × 100
Formula variables
- Unique Opens
- The number of recipients with at least one recorded open.
- Emails Delivered
- Emails accepted by recipients' mail servers, which is sent minus bounced.
Worked example
- Emails delivered
- 10,000
- Unique opens
- 2,500
2,500 ÷ 10,000 × 100
Result: 25%
A quarter of delivered emails were recorded as opened. The figures are illustrative, not a benchmark.
How definitions differ
- Unique or total opens
- Total opens count repeat opens by the same person and can exceed unique opens.
- Delivered or sent
- Unique Opens ÷ Emails Sent × 100
- Dividing by emails sent instead of delivered gives a lower rate, because bounced emails are included in the base.
- Automatic image loading
- Some email apps and privacy features load images automatically, which can record an open even though the person did not read the email. Open rates may therefore overstate real reading, and they are less reliable for comparison over time.
How to interpret it
Open rate is best treated as a rough directional signal, because opens are inferred rather than directly observed. It is most useful for comparing similar sends measured the same way, such as subject-line tests.
Clicks and conversions are more reliable evidence of engagement.
When it is useful
- Comparing subject lines in an A/B test.
- Monitoring changes in a list's responsiveness over time.
- Spotting deliverability problems when opens drop sharply.
Limitations
- Opens are inferred and can be inflated by automatic image loading.
- Recipients who block images may be missed.
- Definitions differ between email platforms.
Common mistakes
- Comparing open rates from different platforms.
- Treating open rate as proof of reading.
- Optimising subject lines for opens without checking clicks.