Amazon ACoS or Advertising Cost of Scale is a metric that Amazon sellers can use as a guide when evaluating the success of their PPC ads. It calculates the ratio between the expenditure for an ad and the corresponding sales. The corresponding formula is:
ACoS = ad spend / PPC sales * 100
Of course, you can also have your ACoS calculated directly in an Amazon tool like Sellerboard. By the way, there is no specific value that can be called the optimal ACoS. However, you do have the option of determining the profit margin using the break-even ACoS, for example.