ACOS and ROAS describe the same advertising relationship from opposite directions. Sellers often prefer one or the other, but the business question is the same: how much sales revenue did the advertising generate relative to spend?
ACOS
ACOS = ad spend ÷ attributed ad sales × 100.
If £20 of advertising produces £100 of attributed sales, ACOS is 20%.
ROAS
ROAS = attributed ad sales ÷ ad spend.
The same £20 spend and £100 sales produces a ROAS of 5.0.
How they relate
ACOS of 20% corresponds to ROAS of 5. ACOS of 25% corresponds to ROAS of 4. ACOS of 50% corresponds to ROAS of 2.
Why a low ACOS is not automatically good
A very low ACOS may be profitable, but it can also mean a seller is bidding too conservatively and leaving profitable volume available. Whether more spend is sensible depends on margin, conversion, cash flow, organic impact and campaign purpose.
Why a high ACOS is not automatically bad
Launch campaigns, defensive campaigns and ranking-focused activity can sometimes tolerate different economics. The important point is that the choice should be deliberate and linked to the product's break-even point.
Margin comes first
Two products with the same 25% ACOS can have completely different outcomes. One may have enough contribution margin to remain profitable; another may lose money on every advertised sale.
Use enough data
ACOS and ROAS can swing sharply when a term has only a handful of clicks or one order. Combine the ratio with spend, clicks, conversion and order count before making permanent changes.
Use the CalcCommerce PPC Search-Term Optimiser to calculate ACOS, ROAS, CTR, CPC and conversion rate in one workflow.
Check the economics first: the free Break-Even ACOS Calculator converts your own product costs into an ACOS/ROAS limit. For search-term analysis, use the PPC Search-Term Optimiser.
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