Definition
Click-to-open (CTO) and open-to-click (OTC) are related ratios that measure click performance relative to the number of opens rather than the total audience. They isolate how effectively the email's content and call to action converted the people who chose to open it.
- Click-to-open (CTO): clicks divided by opens, showing how well content converted openers.
- Click-to-open ratio (also CTO): sometimes used to mean the same thing — clicks as a percentage of opens.
Because these ratios hold the number of opens constant, they strip out the effect of the subject line and focus on the email body's performance.
How the Ratios Are Calculated
CTO = Clicks ÷ Opens
OTC = Clicks ÷ Opens (same ratio, alternative naming)
Both describe how effectively openers were converted to clickers.
Click Rate vs Click-to-Open
| Metric | Denominator | What It Isolates |
|---|---|---|
| Click rate | Total delivered | Full journey including open decisions |
| Click-to-open | Opens only | Content quality after the open |
| Open-to-click | Opens only | Persuasiveness of the body |
A strong CTO indicates the content convinced those who opened.
Why These Ratios Matter
- Focuses on content: Excludes subject line and inbox factors.
- Reveals weak bodies: A good open rate with poor CTO shows a weak email body.
- Guides testing: Helps separate subject line and content improvements.
- Complements open rate: Gives a fuller view of email performance.
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 click-to-open ratio is clicks divided by opens. It measures how well an email's content converted the recipients who opened it, isolating body performance from subject line performance.
Click rate divides clicks by the total delivered audience, including everyone who saw the email in the inbox. Click-to-open divides clicks by opens only, focusing on how the email's content performed among those who chose to open it.
Typical values vary by industry and audience. A helpful way to read it is to compare the CTO across your own campaigns and against your industry benchmarks, watching for drops that indicate content is weaker than your subject lines.
Because it removes the open decision, CTO isolates whether a change to the email body — copy, layout or CTA — improved conversion among openers. That makes it a cleaner measure of content changes than overall click rate.