Click-through rate is the share of messages whose links were clicked. It is the metric that took over from open rate for decisions that matter, because the instrument behind it did not break in the same way. It has its own limits, and one of them is the definition itself.
Which denominator?
Whichever one you say, and you have to say.
Two figures both go by this name:
| Rate | Denominator | What it answers |
|---|---|---|
| Click-through rate | Messages delivered | How much of the audience acted |
| Click-to-open rate | Messages that registered an open | How compelling the content was to those who saw it |
The second is always the larger number, often several times over. That makes it the one quoted in a deck and the one to distrust in a comparison, since it inherits every distortion in the open figure it divides by. If your opens are inflated by privacy proxies, your click-to-open rate is deflated by exactly the same amount.
A benchmark taken from one and applied to the other is meaningless. When someone quotes a click rate, ask which denominator before you compare anything to it.
How is a click measured?
Through a redirect, which is the reason the metric held up.
Bird rewrites the links in your HTML to point at a tracking hostname. The recipient's browser requests the rewritten URL, an email.clicked event is recorded with the original destination, and the browser continues to that destination. The open and click tracking guide has the shape.
That URL is served from a branded hostname on your own sending domain rather than a shared one, which matters for two separate reasons: a recipient hovering a link sees your domain, and the link reputation accrues to you rather than to a pool. Nothing is rewritten until that record verifies, which is covered in tracking domain.
The instrument is firmer than the pixel because it needs no image and no HTML rendering. A person who reads in plain text, blocks remote images, or uses a text-only client still produces a click when they act.
Why do I have more clicks than opens?
Because the two instruments have different failure modes, and this one is documented rather than surprising.
Bird's own guide names both causes:
The pixel goes unrecorded whenever it never loads. The recipient's client blocks remote images, or they read the plain-text part of the message, which carries no pixel at all. A rewritten link still works in both cases, so the click lands and the open never does.
and
Forwarding produces the same shape from a different cause. A forwarded copy carries the same rewritten links, so a click can arrive from someone who was never on your recipient list, and no open of theirs was ever yours to measure.
So a report showing clicks above opens is not a defect. It is one population reading with images off and another population being forwarded your mail. Both are good news that the open figure cannot represent.
Is a click proof someone acted?
No, and the reason is not the one people expect.
Security tooling follows links. Filters, gateways and anti-abuse systems fetch URLs to see where they lead, and each fetch looks like a click. Spamhaus states its own practice, and notes it is not alone:
Please note – Spamhaus reserves the right to engage with the spam we receive and even to follow non-subscription links. There are many anti-spam appliances that now do this also. The fact that an email is opened or a link is clicked does not mean consent has been given.
The last sentence is worth carrying beyond metrics. A click is not permission. Treating one as an opt-in, or as evidence that a recipient wanted your mail, is a consent error rather than a measurement error.
Practically, the machine clicks tend to arrive in a distinctive shape: immediately after delivery, on every link in the message, from an address unrelated to the recipient. A rate that jumps on one campaign and touches every link is worth inspecting before it is celebrated.
Does any of this apply to SMS?
The instrument does, and the surrounding assumptions do not.
An SMS click is measured the same way, through a rewritten link. What differs is everything around it. There is no open equivalent at all, so click rate is the only engagement signal the channel produces. The message is short enough that a link is usually the entire call to action, so the rate is not diluted by other content. And recipients see the link before tapping, which makes a branded, recognisable hostname more consequential than in email rather than less.
The comparison people want to draw between the two channels is the one to resist. An SMS click rate and an email click-through rate are produced by different message lengths, different consent regimes and different expectations, so a ratio between them describes the channels' formats more than it describes your programme.
What should I do with the number?
Use it for decisions, and pair it with the ones that cost you something.
Clicks are the better basis for a per-recipient decision than opens, because the signal is firmer. They are still a positive-only measure: a rising click rate tells you nothing about how many people are quietly resenting the frequency. Read it beside unsubscribe rate and complaint rate, which are the two that go the other way.