To track return on ad spend by SKU using ecommerce analytics, assign ad spend to the specific products it promoted, calculate net revenue from orders attributed to those campaigns by SKU, then divide SKU-level net revenue by the ad spend attributed to it, using margin-adjusted revenue rather than gross revenue where possible. Blended ROAS tells you how the account performed. SKU-level ROAS tells you which products are paying for themselves and which are quietly subsidized by the ones that are.

A campaign running four products with a blended 3.2x ROAS might include one SKU at 6.1x that is cross-subsidizing two SKUs running below 1.5x. The blended number masks this entirely. The SKU-level breakdown is where every profitable scaling and cut decision actually lives.