Cookieless inflation
When you run GA4 and a cookieless tool (Plausible, Fathom, Umami) in parallel for two weeks, the cookieless tool reports 5 % to 12 % more unique visitors and pageviews on the same site, same period. The gap is real, the gap is predictable, and the gap is not a bug. It is two effects stacked on top of each other.
Effect 1: consent-banner decline rate
GA4 needs a consent banner in the EU. The banner asks the user to accept cookies. Some say no. The percentage who say no is your decline rate — typically 8–15 % in Western Europe, lower in North America (3–7 %), higher in Germany and Netherlands (15–25 %).
When a user declines the banner, GA4 either drops the event entirely or runs in degraded "consent mode" with modeled estimates. Cookieless tools do not need a banner — Article 6(1)(f) legitimate interest covers the lawful basis — so they capture every visit.
Result: if your site has a 10 % decline rate, the cookieless tool will report roughly +10 % more pageviews than GA4, all else equal.
Effect 2: daily-salt rotation inflates uniques
This one is smaller and only affects the unique-visitor metric, not pageviews. Cookieless tools (Plausible, Fathom) build the visitor identifier by hashing IP + User-Agent + daily salt + domain. The salt rotates at midnight UTC. The same person visiting Monday and Friday is two distinct hashes — counted as two visitors.
GA4's _ga cookie persists for two years. The same person visiting Monday and Friday is one client_id. One visitor.
On a typical content site with 30 % returning visitors over a 7-day window, the daily-salt method inflates monthly-unique counts by 1.5–3 % above the cookied count. On a heavily-returning site (newsletter audience, daily readers), it can reach 5 %.
Stacked totals from two-week parallel runs
| Site type | Decline rate | Salt-rotation effect | Total cookieless inflation |
|---|---|---|---|
| US e-commerce | 4 % | +1.2 % | +5.2 % |
| EU SaaS marketing | 11 % | +1.8 % | +12.8 % |
| German publisher | 18 % | +2.5 % | +20.5 % (outlier) |
| Mixed-geo content site | 8 % | +1.5 % | +9.5 % |
The German publisher row is the one to flag in a stakeholder meeting. Decline rates above 15 % produce a gap large enough to look like a real traffic increase, and someone will eventually misread it.
How to communicate the gap
Three lines in the migration writeup, before the dashboard goes to the team:
- "GA4 dropped consented sessions only. Cookieless captures all sessions (Article 6(1)(f) legitimate interest)."
- "Expected gap on this site, based on banner decline rate: +X %."
- "Compare YoY against the new baseline starting cutover day, not against pre-cutover GA4."
If you skip these three lines, the first month-over-month review will produce a "we grew 10 %" claim that is methodology, not growth.