ACOS to ROAS Converter

Setting it out

ROAS = 100 ÷ ACOS when ACOS is expressed as a percentage, and ACOS = 100 ÷ ROAS. The two are reciprocals and describe exactly the same relationship between spend and revenue.

A 25% ACOS is a 4.0 ROAS. A 50% ACOS is 2.0. A 100% ACOS is 1.0, spending exactly what the revenue returns.

Anything above a 100% ACOS is a ROAS below 1, which means the advertising costs more than the revenue it produces before any cost of goods.

The numbers, worked through

A campaign at a 40% ACOS: 100 ÷ 40 = 2.5 ROAS.

A campaign at a 2.5 ROAS: 100 ÷ 2.5 = 40% ACOS. The conversion is symmetric.

The useful reference points worth memorising: 20% ACOS is 5.0 ROAS, 25% is 4.0, 33% is 3.0, 50% is 2.0, 67% is 1.5, and 100% is 1.0.

Those six pairs cover nearly every conversation, and knowing them removes the need to reach for a calculator when someone quotes a figure in the other convention.

What the number leaves out

Converting between the two does not make figures from different platforms comparable, because the attribution models differ. A 3.0 ROAS reported by one platform and a 33% ACOS reported by another may describe very different underlying performance.

The conversion is arithmetic; the comparison is not, and treating a converted number as directly equivalent is a common error in multi-channel reporting.

Turning it into a decision

Standardise on one convention internally and convert at the boundary, so that internal targets, reports and conversations all use the same unit.

Then keep the break-even figure in the same unit as the target. Mixing a break-even expressed as ACOS with targets expressed as ROAS is how accounts end up managed against thresholds nobody has checked.

Why the reciprocal relationship matters more than it looks

Because they are reciprocals, equal-looking changes in one are not equal in the other. Improving ACOS from 50% to 40% moves ROAS from 2.0 to 2.5, a 25% improvement. Improving from 20% to 10% moves ROAS from 5.0 to 10.0, a 100% improvement.

So a ten-point ACOS improvement is worth far more at low ACOS than at high, while a fixed ROAS improvement is worth more at high ACOS. The two metrics make the same performance change look different in size.

That asymmetry matters when setting goals. A target of 'improve ACOS by ten points' means something quite different depending on where the account starts, which is why targets are better expressed relative to break-even than as absolute movements in either unit.

State the convention explicitly in the report rather than assuming the reader shares it. A figure of 40 means excellent performance as an ACOS and poor performance as a ROAS, and the ambiguity has caused real decisions to be made backwards.

Labelling the unit costs nothing and removes a category of error that recurs whenever Amazon and Google reporting sit on the same page.

Keeping the break-even figure written at the top of any advertising report in whichever unit the team works in prevents the same confusion recurring.