Why it happens
Different blocking rates
Different blocking rates
Ad blockers and consent banners block the two scripts differently. A PostHog proxy on your own domain usually closes most of the gap.
Different session and user rules
GA4 sessions time out after 30 minutes and restart on a new campaign. PostHog counts sessions and persons its own way and merges persons on identify.
gclid versus UTMs
GA4 gets Google Ads campaign data from gclid. PostHog reads UTMs, so untagged Google clicks look different in each.
Different conversion definitions
A GA4 key event and a PostHog signup event can fire on different pages or steps, so they never count exactly the same thing.
How to line them up, step by step
Compare one campaign, one week
In GA4 open Reports, then Acquisition, then Traffic acquisition, filtered to one campaign. In PostHog break down $pageview by utm_campaign for the same dates.
Align the conversion event
Make the GA4 key event and the PostHog event fire on the same action. Check GA4 key events under Admin, then Data display, then Events.
Tag every ad with UTMs
Add UTMs on top of auto-tagging, so PostHog sees Google, Meta and LinkedIn clicks the same way GA4 does.
Proxy PostHog
Serve PostHog from your own domain to cut the share of visits blocked before the first pageview.
Steps checked against PostHog Docs: UTM segmentation; PostHog Docs: Web analytics; Google Analytics Help: Traffic acquisition report.
Sprites diagnoses this
Sprites diagnoses it
Sprites reads GA4 and PostHog side by side and shows where the two split for the same campaign.
What Sprites does
- Breaks GA4 sessions and conversions down by source, medium and linked Google Ads campaign.
- Breaks PostHog visits, signups and activations down by UTM source and campaign.
- Puts both next to the ad platform numbers for the same dates.
What it doesn't do
- It does not change either tool’s tracking or settings.
- It does not merge the two into one dataset.
Questions
Is PostHog a replacement for Google Analytics?
For product analytics, often yes. For paid search reporting, GA4 still has the native Google Ads link and imports conversions into Google Ads, so many teams run both.
Why does PostHog show fewer visitors than GA4?
Usually blocking. PostHog loaded from its own domain gets blocked more often than GA4. A reverse proxy on your domain closes most of the gap.
Can PostHog attribute signups to Google Ads?
Yes, through UTMs and the gclid it captures. It does not pull Google Ads cost or campaign data on its own.
Which one should I use for ad budget decisions?
Use PostHog to judge which sources bring users who activate and pay, and GA4 or the ad platform for in-flight bidding on the conversions they can see.
See where it breaks
Connect your accounts. Sprites reads them side by side and shows you exactly where the numbers split.