ACoS vs ROAS for Amazon Sellers: Which Metric to Use and When

Amazon sellers hear both ACoS and ROAS when talking about ad performance. They measure related things but answer different questions. Here is when to use which.

What ACoS and ROAS Actually Mean

ACoS (Advertising Cost of Sales) is ad spend divided by attributed sales — the share of sales that went to ads. ROAS (Return on Ad Spend) is the opposite: sales divided by ad spend. A 30% ACoS is the same as a 3.33 ROAS; one is a cost ratio, the other a return multiple.

When to Focus on ACoS

ACoS is ideal when you have a target margin or break-even. If your contribution margin before advertising is 40%, ACoS above 40% loses money before fixed costs. Many brands set a target ACoS and optimize campaigns toward it. Use ACoS when you think in cost per dollar of sale. Work out your real margin first with the FBA profit worksheet, then turn it into ad targets with the break-even & target ACoS calculator.

When to Focus on ROAS

ROAS is useful when comparing channels or when you care about scale. Compare channels only after checking their attribution and revenue definitions.

How to avoid metric conflicts

Teams often create confusion by assigning one KPI to media buyers and another to leadership without translation rules. Define a clear conversion between targets: if finance requires margin protection, set maximum ACoS by product line; if leadership tracks growth velocity, map that to minimum acceptable ROAS by campaign type.

Use KPI pairs and trend windows: ACoS with TACoS, ROAS with contribution margin, and CTR with conversion rate. This can help identify local optimization that harms total account health.

For historical examples with their own reporting context, see our case studies. The PPC optimization service explains our ongoing work. A free audit can help identify which metrics to review for your account.