Amazon ACOS and ROAS: one number, two notations
Amazon reports in ACOS while the rest of the advertising world talks in ROAS. Both measure the same thing from opposite ends. An ACOS of 25 % is a ROAS of 4x. The calculator below turns it either way and adds the line at which a campaign starts losing money, which depends on a margin only you know. If you only want the ROAS with no Amazon attached, the ROAS calculator does that.
ACOS, ROAS and break-even
ACOS
—
Ad spend ÷ ad revenue × 100
ROAS
—
The same figure as a multiple: 100 ÷ ACOS
Contribution after ad cost
—
Revenue × margin − ad spend
The zero line sits exactly on your gross margin: at a 30 % margin, a 30 % ACOS is the point where the third card reads nothing. That is not a calculation, it follows from the definition.
What is ACOS, and which revenue belongs in it
ACOS stands for advertising cost of sales. It is the share of your ad-attributed revenue that went straight back out as ad spend, written as a percentage: spend $25 to earn $100 and your ACOS is 25%.
Both numbers come off the same row of the same campaign report. That sounds obvious until you watch someone drop total store revenue into the denominator, which is the most common way an ACOS ends up looking healthier than it is. A seller turning over $50,000 a month who spends $5,000 on ads does not have a 10% ACOS. Nowhere close. If those ads drove $12,000 of the turnover, the real figure is 42%.
The revenue Amazon sets against your spend is ad-attributed only, and only inside its own attribution window. Organic orders from the same week sit in a different column. They answer a different question.
Converting ACOS to ROAS: the table
This conversion falls straight out of the formula and needs no market data. ROAS = 100 ÷ ACOS, and the reverse holds too. The third column names the margin at which it just about holds.
| ACOS | equals ROAS | break-even at margin |
|---|---|---|
| 10% | 10.00x | 10% |
| 15% | 6.67x | 15% |
| 20% | 5.00x | 20% |
| 25% | 4.00x | 25% |
| 30% | 3.33x | 30% |
| 40% | 2.50x | 40% |
| 50% | 2.00x | 50% |
| 75% | 1.33x | 75% |
| 100% | 1.00x | 100% |
That third column is not a second calculation, it is the same one: break-even ACOS is gross margin. Which makes ACOS more convenient in daily use than ROAS, because you do not have to work the threshold out, you already know it. At a 35 % margin, a 45 % ACOS costs money. No arithmetic needed. Expressed as ROAS the same line is 2.86x, and that one you do have to compute — the break-even ROAS calculator handles it for other channels.
Why Amazon reports ACOS, and what gets lost in it
Amazon documents both metrics and states outright that they are inverses of one another: ACOS is ad spend divided by ad revenue, ROAS is ad revenue divided by ad spend. The same two numbers, one way round and then the other.
The practical difference is the reading direction. With ROAS higher is better; with ACOS lower is. That sounds trivial and yet it produces bad calls in mixed reporting, when somebody presents a lower ACOS as progress without mentioning that revenue fell alongside it. ACOS can always be lowered. Switch off the expensive campaigns.
What neither metric sees. ACOS and ROAS both refer to ad-attributed revenue only. The organic sales the same campaign helps along appear in neither number: better placement, more reviews, more visibility. Steer by ACOS alone and you are optimising a slice, not the business.
Which is why the margin field above is not decoration. Without it a 30 % ACOS says nothing at all; with it the same figure is either comfortable or a slow loss. And because Amazon's fees have already taken a visible bite before any advertising starts, the number that belongs in that field is the margin after fees, not the mark-up from your price list.
Outside Amazon the same quantity is called ROAS again, but it is measured differently. Google Ads rests on self-reported conversion values, while Meta uses an attribution window that differs from account to account.
Frequently asked questions
- How do you convert ACOS to ROAS?
- ROAS = 100 ÷ ACOS. An ACOS of 25 % is a ROAS of 4x, and an ACOS of 50 % is a ROAS of 2x. The reverse is ACOS = 100 ÷ ROAS. Both metrics use the same two figures, only in swapped order.
- What is a good ACOS on Amazon?
- Not answerable without your own margin, which is exactly why break-even ACOS is the more useful figure: it equals gross margin after cost of goods and Amazon fees. At a 30 % margin, a 30 % ACOS is the zero line. Below it you earn, above it you pay.
- Why does break-even ACOS equal gross margin exactly?
- Because at the zero line the ad cost consumes the entire contribution. Contribution is revenue × margin and ad cost is revenue × ACOS. Set them equal, the revenue cancels, and ACOS = margin is what remains. ROAS has no such property; there you have to form 1 ÷ margin.
- Does ACOS cover total revenue?
- No, only the ad-attributed part. Organic sales arising from better placement or additional reviews are not counted. To see the whole effect you put total revenue against total ad spend, which is the logic behind TACOS and behind the marketing efficiency ratio.
Background & sources
- Definition of ACOS, the formula ad spend ÷ ad revenue × 100, and the explicit statement that ROAS is its inverse: Amazon Ads – «What is advertising cost of sales (ACOS)?».
- Definition of ROAS as ad revenue ÷ ad spend and how Amazon frames the metric: Amazon Ads – «What is Return on Ad Spend?».