mc_cid

mc_cid is a URL parameter that Mailchimp adds to every tracked link in a campaign email to identify which campaign the click came from. The value is a short alphanumeric string unique to that send, such as mc_cid=8f2b3c9a1d. Mailchimp uses it for its own click reporting, not for Google Analytics.

mc_cid vs mc_eid

mc_cid identifies the campaign. mc_eid identifies the recipient.

Mailchimp adds both parameters together, so a tagged link usually looks like this:

https://example.com/pricing?mc_cid=8f2b3c9a1d&mc_eid=6ac41ef25b

Parameter	Full name	Identifies	Changes when
mc_cid	Mailchimp campaign ID	The email campaign	You send a new campaign
mc_eid	Mailchimp email ID	The individual subscriber	Every campaign

The mc_cid value corresponds to the campaign’s unique ID inside Mailchimp, the same identifier the Mailchimp API returns as the campaign id. Every recipient of a given send gets the same mc_cid.

mc_eid once stayed stable for a subscriber across sends, which let sites recognize the same person from one campaign to the next. Mailchimp changed that. The value is now unique to each campaign, so the same subscriber receives a different mc_eid in every email. Treat it as a per-send identifier, not a customer ID.

Where mc_cid Comes From

Mailchimp appends mc_cid automatically when click tracking is enabled on the campaign. There is no separate switch for the parameter itself.

Mailchimp rewrites the links at send time, which has three consequences:

• The parameters appear on every tracked link, including footer, button, and image links.
• They are appended to whatever query string the link already carries.
• Turning off “Track clicks” in the campaign’s tracking settings removes them, along with Mailchimp’s click reports.

If the campaign also has Mailchimp’s Google Analytics link tracking switched on, Mailchimp adds UTM parameters separately. The two systems sit on the same URL and do different jobs.

Why mc_cid Causes Analytics and SEO Problems

GA4 ignores mc_cid and mc_eid for attribution. They are not UTM parameters for email, so a click carrying only mc_cid arrives with no source or medium set. GA4 files it under Direct or a referral, and the email campaign vanishes from channel reports.

The parameters still reach your page reports. GA4 records the full page location, so /pricing?mc_cid=8f2b3c9a1d and /pricing?mc_cid=4d7e1b0c22 count as two different pages. A year of newsletters can split one landing page across dozens of rows.

Search engines face the same duplication. Each mc_cid variant is a crawlable URL serving identical content, which wastes crawl budget and splits ranking signals whenever subscribers paste those links publicly. Google does not penalize the duplication, but it will index the variants if nothing tells it not to.

mc_eid adds a privacy dimension. It points to one subscriber in your audience. When that person forwards the link or shares it in a group chat, everyone who clicks arrives carrying that subscriber’s identifier.

How to Handle mc_cid

Four fixes, in order of impact:

1. Add UTM parameters. Enable Mailchimp’s Google Analytics tracking, or tag links yourself with utm_source, utm_medium=email, and utm_campaign. This is the step that restores email in GA4 reports. Generating them with linkutm’s UTM builder keeps campaign names consistent from send to send.
2. Set a self-referencing canonical tag so search engines collapse the parameter variants into one page.

<link rel="canonical" href="https://example.com/pricing" />

1. Strip the parameters before GA4 stores the page. Rewrite page_location in Google Tag Manager, or use a Modify event rule on the GA4 data stream, so page reports show clean paths.
2. Do not store mc_eid. Keep it out of form fields, CRM records, and server logs. It identifies a person, changes every send, and carries almost no analytical value.

Frequently Asked Questions

What does mc_cid mean?

mc_cid stands for Mailchimp campaign ID. Mailchimp appends it to links in an email so its click reports know which campaign produced the click. The value is unique to a single campaign and identical for every recipient of that send.

What is the difference between mc_cid and mc_eid?

mc_cid identifies the campaign, mc_eid identifies the individual recipient. Mailchimp’s click tracking adds both together. mc_cid is the same for everyone who receives a given email, while mc_eid differs for each subscriber and now changes with every campaign.

Does Google Analytics track mc_cid?

No. GA4 does not read mc_cid or mc_eid as source or medium. Traffic carrying only those parameters is normally classified as direct. Use utm_source, utm_medium, and utm_campaign to attribute Mailchimp traffic correctly.

Can I remove mc_cid from Mailchimp links?

Only by turning off click tracking in the campaign’s tracking settings, which also removes Mailchimp’s click reporting. Most teams keep tracking on and handle the parameters on their own site instead, with a canonical tag and a GA4 rule that strips query parameters.

Is mc_eid personal data?

Treat it as personal data. mc_eid maps to a single subscriber in your Mailchimp audience, so it can identify an individual when combined with your list. Avoid storing it in analytics tools, CRM fields, or logs without a specific reason.

To tag Mailchimp sends so GA4 reports them as email instead of direct, build the links with the free UTM builder at linkutm.