Definition
The open-to-click ratio (OTC) is the number of unique clicks divided by the number of unique opens. It measures how effectively an email's content and call to action converted the recipients who chose to open it, holding the open decision constant.
OTC is a useful diagnostic because it focuses purely on the email body. If open rate is healthy but OTC is low, the problem is in the content, not the subject line. It is essentially the same underlying ratio as click-to-open.
OTC vs Click Rate
| Measure | Denominator | What It Shows |
|---|---|---|
| Click rate | Total delivered | Overall click performance |
| Open-to-click | Opens | Content performance among openers |
| Click-to-open | Opens | Same relationship, alternative naming |
Both OTC and CTO highlight body quality rather than inbox or subject performance.
How to Read OTC
- High opens + low OTC: The subject worked but the body failed to convert.
- Low opens + high OTC: Fewer people opened, but those who did were convinced.
- Consistent OTC: Body content reliably converts a steady share of openers.
How to Use OTC
- Test body changes: Since OTC removes the open decision, it isolates content impact.
- Diagnose weak bodies: A falling OTC flags a copy or CTA problem.
- A/B test content: Use OTC to judge which body performed better.
- Benchmark post-open health: Track it alongside open rate for the full picture.
Related Glossary Terms
A/B Testing
A/B testing in email marketing is the practice of sending two variations of an email to a small sample of your list to determine which version performs better before sending the winner to the remaining subscribers.
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
Abuse Complaint
An abuse complaint is a report from a recipient who marks an email as spam, which negatively affects sender reputation and deliverability.
AI Email Summary
An AI email summary is a short, machine-generated overview of an email's key points, shown by Gmail, Outlook and Apple Mail before a recipient opens the message. It is reshaping how email marketers think about subject lines, preview text and open rates.
ARPU (Average Revenue Per User)
ARPU (Average Revenue Per User) is a metric that measures the average revenue generated per email subscriber over a specific period, used to evaluate list value and campaign effectiveness.
Attention Rate
Attention rate is the percentage of email opens that last longer than 5 seconds, distinguishing genuine reads from passive opens, preview-pane views, or Apple MPP auto-loads.
Frequently Asked Questions
The open-to-click ratio is clicks divided by opens, measuring how effectively an email's content converted the recipients who opened it. It isolates body quality from subject line performance.
Click rate divides clicks by the total delivered audience, including people who never opened. Open-to-click focuses only on those who opened, showing how well the content converted that engaged group.
There is no universal figure; it varies by industry and audience. The most useful approach is to track OTC against your own campaigns and industry benchmarks, watching for changes that point to content quality.
High opens with low OTC suggest the subject line succeeded but the body failed to move recipients to click — for example, weak copy, an unclear CTA, or content that did not match the subject. Use OTC to diagnose and fix the body.