ACoS is advertising cost of sale: your ad spend divided by the revenue those ads produced. There is no universally good ACoS. What matters is whether yours sits below your break-even ACoS, which is your profit margin before advertising.
The two numbers you need
ACoS tells you what advertising costs as a share of ad revenue. Spend $250 to generate $1,000 in sales and your ACoS is 25 percent.
Break-even ACoS is the point where ad spend consumes exactly all your profit. It equals your profit margin before advertising. If you sell at $29.99 and everything else costs $18, your margin is about 40 percent, so your break-even ACoS is 40 percent.
Below that you are making money on ad sales. Above it, every additional sale costs you. The ACoS Calculator works out both figures and shows the headroom between them.
Why chasing a low ACoS can be wrong
A seller with a 45 percent margin running at 20 percent ACoS is leaving growth on the table. They could spend considerably more per sale and still profit. Meanwhile a seller with a 22 percent margin running at 30 percent ACoS is losing money on every ad-driven order while congratulating themselves on the traffic.
The same ACoS figure is healthy for one seller and fatal for another. That is why the number in isolation means nothing.
When running above break-even is a deliberate choice
- Product launches, where early sales velocity and reviews shape long-term ranking.
- Defending a keyword a competitor is bidding on.
- Clearing stock before long-term storage fees bite.
All defensible. What is not defensible is doing it by accident because nobody worked out the break-even point.
ACoS and ROAS are the same fact
ROAS is return on ad spend, revenue divided by spend. It is the inverse of ACoS. A 25 percent ACoS is a ROAS of 4. Use whichever your team finds more intuitive, but do not let two teams use different ones and talk past each other.
Raising your break-even ACoS
The lever is margin, not the ad account. Reduce unit cost, negotiate inbound shipping, move to a lower fee tier, or raise price. Every point of margin you add raises the ACoS you can afford, which lets you outbid competitors who have not done the work. Start with the FBA Calculator to find where the margin is going.