RoAS

Return on ad spend: ad-attributed sales divided by ad spend, expressed as a multiple, and the exact inverse of ACoS.

You flip the ACoS fraction. Where ACoS is spend over sales, RoAS is sales over spend, so the two carry the same information in opposite directions. Amazon and most DSP reports surface both, and one is fully determined by the other.

An ACoS of 25 percent is a RoAS of 4. Spend, and get four times that back in attributed sales.

It is not a second data point. Because RoAS is one divided by ACoS, reporting both side by side as if they are separate findings adds nothing. A brand that fixates on a big RoAS number is looking at the same efficiency an ACoS-watcher sees, just scaled the other way.

Same math, mirrored. Pick the one your team reads faster and stay consistent.