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.
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.
Live demo
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Open the live demoWhy 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.
FAQ
Is a tracking pixel the same as a cookie?
No. A cookie stores a value in the browser. A pixel stores nothing: the measurement is the image request itself, and everything it learns comes from the request headers and the URL. They are often used together, with the pixel request setting or reading a cookie.
Can you tell if an email has a tracking pixel?
Usually. Turn off automatic image loading in your mail client and inspect the message source for a 1x1 image pointing at a tracking domain. If images are blocked, the pixel never loads and the open is never recorded.
Do tracking pixels require consent?
For advertising and retargeting pixels in the EU and UK, yes: they access device information and feed profiling. A first-party pixel used only for aggregate counting is a weaker case, but the safe position is that anything feeding an ad platform needs prior consent.
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