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Incrementality

Incrementality is the part of sales that would not have happened without the advertising, as opposed to the sales the ads were merely given credit for.

How it works

Google Ads records a conversion when a purchase follows an ad interaction. That shows what the ads touched, not what they caused: a shopper who searched for the shop by name and clicked a brand ad might have ordered anyway. Incrementality is the difference between sales with the ads and sales of a comparable group without them. Standard conversion reports don’t show it: it takes an experiment with a control group that sees no ads.

Formula

Sales with ads − sales of a comparable group without ads

Ways to calculate

  • Holdout by users: Conversion Lift in Google Ads splits the audience into people who see the ads and people who don’t. It isn’t available to every account, and access goes through a Google representative.
  • Geo test: ads stop in some regions and keep running in comparable ones, through Conversion Lift based on geography or Google’s open-source Meridian GeoX. This is a geo lift test. Our guide explains which test a small store can run.
  • Model: marketing mix modeling estimates it from aggregated history and can be calibrated by such experiments.

Example

Example store, not client data.

The tableware shop pauses its ads in half of its regions and runs them in a matched half. In four weeks, the regions with ads bring 160 orders worth 96,000 for 20,000 of spend. Google Ads credits 150 of them, worth 90,000: a ROAS of 4.5. The regions without ads bring 100 orders worth 60,000.

Incremental value = 96,000 − 60,000 = 36,000. Incremental ROAS = 36,000 ÷ 20,000 = 1.8.

Not to be confused with

  • Attribution model — the rule that splits credit for an order among ad clicks. It can’t say whether the order would have happened anyway.
  • Marginal ROAS — the return on the next unit of spend. It compares two levels of spend; incrementality compares ads with no ads.

Right and wrong readings

  • Wrong: “ROAS is 4.5, so every unit of spend added 4.5 of sales.” Right: in the example, only 36,000 of the 90,000 credited was extra.
  • Wrong: “Sales fell 20% in the month the ads were paused, so the ads made 20% of sales.” Right: demand and season change too; without a control group, the drop mixes the ads’ effect with everything else.

Sources

  • About conversion measurement — Google Ads Help: conversions are actions taken after interacting with an ad. Checked 2 October 2026 (archived copy of 30 September 2026).
  • About Conversion Lift — Google Ads Help: test and control groups, user and geo studies, incremental metrics. Checked 2 October 2026 (archived copy of 21 September 2026).
  • Meridian GeoX — Google for Developers: open-source geo experiments, MMM calibration. Checked 2 October 2026.