What is a tracking pixel?
A tracking pixel is a tiny image request used to record that something was viewed. How pixels work, where they are still used, and how to block them.
What is a tracking pixel?
A tracking pixel is a tiny, usually invisible image embedded in a web page or an email. When the client loads the image, the request itself tells the server that the page or message was opened, along with whatever the request reveals: an IP address, a user agent, a timestamp and any parameters in the URL.
A worked example.
What one pixel request actually transmits.
| Markup | an img tag, 1 by 1 pixel, pointing at a tracking URL with an id parameter |
|---|---|
| The image itself | A 43-byte transparent GIF, or a 1x1 transparent PNG |
| Sent automatically by the client | IP address, user agent, referrer, timestamp |
| Sent in the URL | The campaign or recipient id encoded by the sender |
The sender learns that recipient 8f21c opened the message, roughly where from and on what
No JavaScript runs, nothing is stored on the device, and the recipient sees nothing. The measurement is the request itself, which is why pixels still exist in places where scripts cannot.
How a pixel works
The mechanism is as old as the web and has not changed. The page or email contains an image tag pointing at a URL on the tracking server. Loading the image causes an HTTP request. The server logs it, then returns a transparent one-pixel image so nothing is visible.
Everything measured comes from that single request. The IP address and user agent are sent by the client automatically; anything else, such as a campaign id or a recipient id, is encoded into the URL by whoever created the pixel.
Because it needs no JavaScript, a pixel works in contexts where a script cannot run at all, which is why email marketing depends on it entirely.
Where pixels are still used
Email open tracking
Every open rate in every email tool comes from a pixel. It is the only measurement available inside a mail client, and it is also the least reliable one.
Advertising conversion pixels
Ad platforms drop a pixel on your confirmation page so a purchase can be matched back to an ad impression on their side.
Retargeting audiences
A pixel on your pages adds visitors to an advertising platform's audience, which is the cross-site behaviour that browser privacy work is aimed at.
Noscript fallbacks
Some analytics tools include a pixel inside a noscript tag so visitors with JavaScript disabled are still counted, at a much coarser level.
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Why open rates are unreliable now
Apple Mail Privacy Protection, introduced in 2021, pre-fetches every remote image in a message on Apple's servers regardless of whether the recipient opened anything. Gmail has proxied and cached images for far longer.
The consequences are direct:
- Opens are inflated, because the pixel loads whether or not anyone read the message.
- Location is wrong, because the request comes from a proxy rather than the recipient.
- Open time is meaningless, because it reflects when the proxy fetched, not when a human looked.
The practical answer is to judge campaigns on clicks and on what happened after the click, and to treat open rate as a directional trend at best.
Pixels, privacy and Mrkr
A tracking pixel stores nothing on the device, which is often used to argue it needs no consent. That argument does not hold when the pixel is part of building a cross-site profile: ePrivacy covers accessing information on the device, and data protection law covers the profiling that follows. Advertising and retargeting pixels are firmly in consent territory.
Blocking is straightforward from the visitor's side: disabling remote image loading in a mail client stops open tracking outright, and most content blockers already carry the well-known advertising pixel domains.
Mrkr does not use tracking pixels and does not build advertising audiences. Measurement is a first-party script on your own site, cookieless by default, with a first-party proxy option so the request comes from your own domain rather than a third-party host.
Questions, answered.
Related terms.
- What is cookie banner?
A cookie banner is the notice that asks a visitor's permission before storing or reading information on their device.
- What is first-party data?
First-party data is information you collect directly from your own audience on your own properties: site behaviour, purchases, support conversations, and anything customers tell you.
- What is server-side tracking?
Server-side tracking sends analytics events from your own backend rather than from the visitor's browser.
- What is cookieless tracking?
Cookieless tracking is any method of recognising visits without storing an identifier in the browser.
- What is event?
An event is any recorded action beyond loading a page: a button click, a form submitted, a video played, a subscription upgraded.
Where this shows up in Mrkr
- Measure without pixels or cookies in Mrkr
A first-party script, cookieless by default, with no advertising audiences behind it.
- Browse the full analytics glossary
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