Why shouldn't I sum ACoS or ROAS across accounts?
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Quick answer
Summing ACoS (Advertising Cost of Sales) or ROAS (Return on Ad Spend) averages from multiple Amazon Ads campaigns or accounts produces misleading results. These metrics are averages derived from core data—ad spend and ad-attributed sales—not additive numbers. Accurate aggregation involves recalculating from combined totals to represent overall performance, ensuring that reporting reflects true advertising impact.
It can be tempting to add up average ACoS or ROAS figures across campaigns to gauge your total Amazon Ads efficiency, but this shortcut leads to inaccurate conclusions. Understanding why these metrics shouldn’t be summed will help you make more reliable business decisions.
How ACoS and ROAS Are Calculated
ACoS is calculated as ad spend divided by sales, while ROAS is sales divided by ad spend. Each campaign has its own proportion of spend to revenue, making these figures campaign-specific averages. If you simply total the ACoS or ROAS percentages across different campaigns or accounts, you ignore the weight of spend, which can heavily skew the result.
The Pitfalls of Adding Averages
When you sum ACoS or ROAS numbers without considering their underlying spend and sales values, you can distort your actual advertising performance. A campaign with high spend and a low ACoS can be offset by another with low spend and high ACoS—but a direct sum doesn’t reflect this relationship and can mislead reporting. To represent your total efficiency, always calculate overall ACoS and ROAS using total spend and total sales.
How to Aggregate the Right Way
To get an accurate account-level or multi-account ACoS or ROAS, add up your total ad spend across all campaigns, and your total ad-attributed sales, then use these sums in the metric formula. Tools like the Amazon Ads Dashboard by Todoza can help automate these calculations with proper data consolidation while preserving visibility into each underlying component.
When This Matters Most
Accurate rollups are essential for brands, agencies, or teams handling multiple Amazon ad accounts or large portfolios. Misrepresenting efficiency can result in poor budget decisions, missed optimization opportunities, or incorrect reporting to stakeholders. Ensuring correct metric aggregation supports better strategy and transparency.
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Understanding the right way to aggregate ACoS and ROAS aligns with best practices for Amazon Ads account rollups. For more on proper metric handling and other key reporting tips, explore our full guide on Amazon Ads Account Rollups.