The Ecomm Analyst

Ask ten operators what their ROAS is and you will get ten confident answers and very little agreement about what the word means. Some are reading the number Meta reports. Some mean blended return across everything. Some mean a target they were told to hit by an agency three years ago. The confusion matters because the metric you steer by determines the decisions you make, and the two most common choices, platform ROAS and MER, pull in different directions.

Platform-reported ROAS is revenue a channel claims it generated divided by what you spent on that channel, as measured by the platform itself. Meta tells you its campaigns returned 4x. Google says 5x. The appeal is obvious. It is channel-specific, it updates in real time, and it lets you decide where to push budget. The problem is everything covered by signal loss and double counting. That 4x is part measured and part modeled, it claims sales other channels also claim, and it never hears about the refund two weeks later. Platform ROAS is useful, but it is a number with a thumb on the scale, and the thumb belongs to the company selling you ads.

MER, marketing efficiency ratio, is total revenue divided by total marketing spend across every channel. Some people call the same idea blended ROAS. Whatever you call it, the defining feature is that there is one revenue number and one spend number, so nothing can be double-counted and no platform gets to grade its own work. If you did 200,000 in revenue and spent 50,000 on marketing, your MER is 4. That number ties to reality because the revenue half comes from your store and the spend half comes from your bank statements. It does not care about attribution windows or opt-out rates because it never tried to track anybody in the first place.

So which should run the business. For the top-line question of whether you are spending profitably, MER wins and it is not close. It is the number to put on the wall, review weekly, and tie to your actual margins. The reason is simple. You can hit great platform ROAS on every channel and still lose money, because platform ROAS double-counts and MER does not. When each channel claims credit for the same brand-search buyer, the sum of your platform numbers looks healthy while your blended reality is thin. MER is the only one of the two that cannot lie to you that particular way.

There is a sharper version of MER worth knowing, sometimes called contribution MER or aMER, where you divide contribution margin rather than revenue by spend. Revenue MER tells you the marketing is producing sales. Margin-based MER tells you the sales are worth producing. For a store with healthy margins the two move together. For a store discounting heavily or carrying high product cost, revenue MER can look fine while you are buying sales that lose money after the cost of goods. If you only run one number, run the margin-aware one, because revenue you cannot keep is not a win.

None of this means platform ROAS is useless. It has a real job, it is just a smaller one than people give it. Use it for direction inside a channel, not for truth across channels. If you change a Meta campaign and its reported ROAS climbs from 3 to 5 while spend holds steady, the level is probably wrong but the direction is real, and direction is what you need to decide whether the change worked. That is the right way to use a biased instrument. You do not trust its readings, you trust its movements.

The practical setup most operators land on is a hierarchy. MER, ideally margin-aware, is the number you run the business on and the one that triggers a real conversation when it slips. Platform ROAS is a diagnostic you drop into when MER tells you something is wrong and you need to find which channel caused it. And a sanity check sits underneath both, the simple comparison between what your platforms collectively claim and what your store actually did, so you always know how inflated the channel numbers are this month.

The mistake to avoid is steering by platform ROAS because it feels more precise and more actionable. It is more precise looking. It is less true. Optimizing every channel to a great reported ROAS while your MER quietly erodes is one of the most common ways a store convinces itself it is scaling profitably right up until the cash stops adding up. The channel dashboards all looked green. The blended number, the one nobody made the hero metric, was telling a different story the whole time.

Pick MER as the metric of record. Make it margin-aware if you can. Demote platform ROAS to the diagnostic it should have always been. The business runs on the number that reconciles to your bank account, not the one an ad platform hands you with a confident green arrow next to it.