Metrics & KPIs

ACoS vs MER

In short: Both are efficiency ratios, but they operate at opposite ends of the funnel and even point in opposite directions on the scale. ACoS is Amazon's native vocabulary for a single campaign or product - ad spend over attributed sales, where lower is better and it needs the same click-level attribution ROAS relies on. MER ignores attribution entirely and divides total company revenue by total marketing spend across every channel, where higher is better and the number survives even when tracking inside any one platform breaks. Use ACoS to manage bids inside Amazon, and MER to sanity-check whether the whole marketing budget is pulling its weight.

By the AdFlint research team · Fact-checked against current Google and Meta platform behavior · Last reviewed July 2026

ACoS

Advertising spend divided by attributed sales revenue, expressed as a percentage; the inverse of ROAS, and the standard vocabulary on Amazon.

Lower is better here, the opposite direction from ROAS, which is where most confusion starts when teams report both. Twenty-five percent equals a four-times ROAS. It also says nothing about profitability until you set it against your break-even margin. The usual misreading is reading a falling number as growth, when it often accompanies shrinking volume instead.

Full definition

MER

Total company revenue divided by total marketing spend across all channels, a blended efficiency ratio rather than any per-campaign measurement.

It sidesteps attribution entirely by refusing to assign credit, asking only what every marketing dollar coincided with in revenue. That makes it stable when tracking degrades, but blunt: it also moves with organic demand, email, retention, and seasonality. The common misreading is diagnosing a single channel from it, which by construction it cannot tell you anything about.

Full definition

Side by side.

The differences that actually change what happens in your account.

 ACoSMER
Which direction is goodLower is better - it's spend over sales, the inverse of ROAS.Higher is better - it's revenue over spend, the same direction as ROAS.
ScopeA single campaign, ad group, or product within Amazon.The entire company across every marketing channel, paid and unpaid.
Attribution dependencyYes - it relies on Amazon's own attributed sales data for the campaign or product.None - it deliberately assigns no credit to any channel.
Native toAmazon Advertising specifically - it's the platform's standard vocabulary, distinct from ROAS-speaking platforms like Google or Meta.No single platform - it's a business-level calculation pulled from total revenue and total marketing cost.
What a falling number meansImproving efficiency on paper, but often accompanies shrinking sales volume, not growth.Marketing spend generating less total revenue relative to itself, or spend rising faster than revenue - always worth investigating.
Tells you about profitabilityNothing on its own - a low ACoS on thin-margin products can still lose money without a break-even comparison.Nothing on its own either - it moves with organic demand and retention just as much as with paid media.
Where you'd act on itInside Amazon campaign manager, adjusting bids, keywords, and placements.In an executive or finance-facing report, setting overall marketing budget.

What actually separates them.

01

ACoS and MER move in opposite directions when performance improves - ACoS goes down, MER goes up - so stating both without labeling which is which invites confusion.

02

ACoS is scoped to whatever Amazon can attribute inside its own ecosystem; MER deliberately covers revenue Amazon ads never touched, like organic search or direct traffic on your own site.

03

ACoS needs Amazon's attributed sales data to exist at all; MER needs none, which is why it survives platform-level tracking problems that would break ACoS.

04

Twenty-five percent ACoS is mathematically the same efficiency as a four-times ROAS, but the two numbers look nothing alike on a dashboard, which is a frequent source of misread reports.

05

You can compute ACoS for a single keyword or ASIN; MER only exists at the aggregate business level and cannot be broken down that way.

Which one should you use?

Use ACoS when

  • You're managing Sponsored Products, Sponsored Brands, or Sponsored Display campaigns inside Amazon Advertising and need the platform's own vocabulary.
  • You're deciding bid adjustments at the keyword or ASIN level.
  • You're comparing performance across your own Amazon campaigns and need a consistent, platform-native number.
  • You're reporting Amazon-specific results to a team that already thinks in ACoS rather than ROAS.

Use MER when

  • You want to check whether total marketing spend across Amazon, Google, Meta, email, and everything else is efficient as a whole.
  • Amazon attribution, or attribution on any single platform, feels unreliable and you want a number that doesn't depend on it.
  • You're setting an overall marketing budget for the business, not a bid inside one channel.
  • You need a metric that keeps working even when a specific platform's tracking degrades or changes.

Common questions.

How do I convert ACoS to ROAS?

Divide one by the ACoS decimal. A twenty-five percent ACoS equals a four-times ROAS, and a fifty percent ACoS equals a two-times ROAS. They're mathematically the same efficiency stated as inverse percentages.

Why did my ACoS improve but my MER stay flat?

ACoS only reflects what happened inside your Amazon campaigns, while MER reflects revenue and spend across the whole business. If Amazon got more efficient while another channel got less efficient, or organic demand softened, the two numbers can move independently.

Is a low ACoS always good?

Not by itself. A falling ACoS often comes with falling sales volume as you narrow targeting to only the cheapest, most efficient traffic - and a low ACoS on a low-margin product can still be unprofitable. Check it against your break-even ACoS, not just the direction it's moving.

Can I use MER to manage Amazon bids?

No. MER has no resolution below the whole-company level, so it can't tell you anything about a specific campaign, keyword, or ASIN. Use ACoS for bid decisions inside Amazon and reserve MER for judging overall marketing health.

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