PMax Ate Your Search Budget. Here's the Spreadsheet That Proves It.

Performance Max looks brilliant in isolation. High conversion rates. Strong ROAS. Growing volume. The dashboard tells a story of a campaign that's working.

But dashboards don't show cannibalisation. And PMax is one of the most effective cannibalisation machines Google has ever built.

The question isn't whether PMax is converting. It's whether those conversions would have happened anyway - through your brand campaigns, your organic listings, or your existing Shopping ads.

How PMax cannibalises

PMax has priority over Standard Shopping campaigns and most Search campaigns by default. When a shopper searches for your brand name, PMax can serve an ad before your dedicated brand campaign does. When someone searches for a product you already rank organically for, PMax bids into that space.

This means PMax claims credit for conversions that your other channels were already capturing. The conversion count in PMax goes up. The conversion count in Search and Shopping goes down. Net new conversions? Often zero.

But nobody builds the spreadsheet that proves it. Until now.

The cannibalisation forensics spreadsheet

Here's how to build it. It takes an hour. It will change how you think about PMax.

1. Pull total conversions by campaign type, monthly, for the last 12 months. Search, Shopping, PMax, Display, Video. Every campaign type in a single view
2. Identify the month PMax launched or scaled. Mark it on the timeline. This is your dividing line
3. Compare total account conversions before and after. Not PMax conversions - total account conversions. If PMax grew by 500 conversions but total account conversions only grew by 100, PMax cannibalised 400 conversions from other campaign types
4. Look at brand search volume independently. Use Google Search Console or a tool like SEMrush. If brand search volume stayed flat or grew, but brand campaign conversions dropped, PMax is intercepting your own demand
5. Calculate the cost per incremental conversion. Take the PMax spend and divide it by the net new conversions (total account growth, not PMax-attributed growth). This is your true cost per acquisition from PMax

When brands do this exercise, the true CPA from PMax is typically 3-5x higher than the dashboard suggests.

What the spreadsheet usually reveals

• PMax shows 800 conversions. But total account conversions only increased by 150 vs. the period before PMax launched
• Brand campaign conversions dropped 40% when PMax scaled, even though brand search volume was stable or growing
• Standard Shopping conversions collapsed - not because Shopping stopped working, but because PMax took priority
• The "incremental" cost per conversion from PMax is £45, not the £12 the dashboard shows

This doesn't mean PMax is useless. It means PMax without proper controls is an expensive way to buy your own branded demand.

What to do with this information

• Add brand exclusions to PMax. This is now available and it's the single most impactful control you can implement. Stop PMax from bidding on your own brand terms
• Run a holdout test. Pause PMax for 2-4 weeks on a subset of products and measure whether total conversions actually drop. If they don't, PMax was cannibalising, not creating
• Restructure around incrementality. Use PMax for genuine prospecting - new customer acquisition on non-branded terms - and protect your existing demand channels
• Report on incremental conversions, not attributed conversions. The dashboard number is vanity. The incremental number is the one that affects your P&L

We've written extensively about managing Performance Max with commercial rigour. The spreadsheet is where it starts. Build it, read it, and make decisions based on what it says - not what the dashboard says.

PMax is a powerful tool. But it's also a powerful cannibal. The spreadsheet tells you which one it is for your brand.