How ROAS is calculated
Return on ad spend is the conversion value recorded against your ads divided by what those ads cost. Google Ads reports it as a ratio, Meta as a multiple; either way it answers one question, which is how much value came back for the money that went out.
Because it is built entirely from reported figures, it inherits their assumptions. The value side depends on what your tags send and how attribution shares credit; the cost side is platform spend only, with no fees, production or discounts in it. Change the attribution model or the conversion window and ROAS moves without anything happening in the real world.
Why ROAS matters
It is the quickest way to compare unlike things. Campaigns, products, audiences and countries all have different prices and different order sizes, and ROAS puts them on one scale, so you can see which pounds, dollars or rupees are working hardest without reading a page of columns.
It is also the target for value-based bidding. Target ROAS bidding takes the ratio you set and buys what it can while holding to it, which means the number you type is effectively an instruction about how aggressive to be. Set it high and the system becomes fussy and volume falls; set it low and it spends more freely for a thinner return.
Common mistakes with ROAS
Treating it as profit is the big one. ROAS is built on revenue, and revenue is not what you keep. A business with thin margins can be losing money at a ratio that a high-margin business would celebrate, so there is no such thing as a good ROAS in the abstract — only one that clears your own break-even, and that break-even is decided by your margin.
Chasing the highest possible ratio is the next trap. Efficiency and growth pull against each other: the tightest ROAS in an account usually belongs to brand searches and remarketing, where customers had already decided. An account tuned only for the ratio shrinks towards those and stops finding anyone new.
Then there is double counting. Every platform claims the sales it touched, so adding channel ROAS figures together always flatters. And a strong ratio on a small budget may simply mean the campaign has not been asked to do much yet.
What to do about it
Work out the point at which advertising stops paying for itself, using the margin on what you actually sell rather than the sticker price. Thin margins demand a high ratio just to stand still; comfortable ones leave room to buy growth. Set your target above break-even by whatever the business needs to keep, not by what a competitor claims.
Then read it beside the figures that cover its blind spots: marketing efficiency ratio for a whole-business view that no platform can inflate, and profit-based measures where margins vary across your range. And where repeat purchase is normal, remember the first order understates what the customer is worth.