Advertising cost of sales, or ACOS, is a metric that represents ad spend as a percentage of sales for Amazon Ads campaigns, such as Sponsored Products or Sponsored Brands. You calculate ACOS by dividing your ad spend by your ad-attributed revenue, and then multiplying by 100 to get a percentage. For example, if you spent $100 on an ad campaign that generated $500 of ad-attributed sales, your ACOS would be 20% (divide $100 by $500 to get 0.2, and then multiply by 100 to get 20%).
A lower ACOS indicates a better performance when you are measuring success based on ad-attributed sales. However, just like ROAS, ACOS is not the only measure of success and may not even be one of the primary metrics you use for your ad campaign, depending on your goals.