What is MER in ecommerce marketing?

MER is total revenue divided by total marketing spend over the same period. Every order counts, including orders from email, organic search, word of mouth and repeat customers. Every marketing dollar counts, across every platform.

That bluntness is the point. It uses no attribution model, no conversion window and no tracking pixel, so nothing about consent banners, browser changes or app privacy settings can move it. It is arithmetic on two numbers you already know are true: what the store took, and what you spent.

Some stores call it blended ROAS, and the calculation is the same idea. The name matters less than the discipline of using one figure that cannot be argued with.

Because it includes revenue that marketing did not cause, MER is always higher than the sum of what the platforms report, and it should be. It is not measuring campaign performance. It is measuring how much revenue the business produces per dollar of marketing, which is a different and equally useful question.

Decide the definition once and write it down. Which revenue counts, before or after refunds, including or excluding shipping charged, and what counts as marketing spend. Consistency matters more than which choice you make, because the value of MER is in the trend.

MER answers one question well: is the whole marketing operation getting more or less efficient. When it rises while revenue grows, spend is compounding. When it falls while spend grows, you are buying growth at a worse rate and need to know why.

It is also the right number for a conversation with a finance team, because it reconciles with the accounts. Platform reported returns rarely do, and the gap between the two causes more arguments than any other reporting issue in ecommerce.

What it hides is everything about cause. A store can hold a steady MER while one channel quietly stops working and another carries it. It cannot tell you where the next dollar should go, and using it for that decision leads to cutting a channel that was doing the work.

It also moves for reasons unrelated to marketing. A price rise, a change in product mix, a wholesale order, a seasonal peak, or a big month of repeat orders from customers acquired last year all move it. Repeat revenue in particular flatters current spend, so a store with a large returning base can look efficient while its acquisition is failing.

And it lags. Money spent this month may produce orders next month, especially for considered purchases, so short windows are noisy.

Three habits make MER useful rather than decorative.

Watch it on a rolling window rather than day by day. A four week rolling figure smooths the noise while still reacting fast enough to matter. Daily MER on a small store is mostly randomness.

Split new customer revenue from returning customer revenue and calculate a version on new customers only. This is the number that tells you whether acquisition is working, and it is far harder to flatter. A store with a healthy overall MER and a poor new customer MER is living off past work.

Use contribution margin rather than revenue when you can. Revenue divided by spend ignores product cost, shipping and fees, so two stores with the same MER can be in completely different health. Replacing revenue with the money actually left after variable costs turns the ratio into something you can make decisions with.

Then pair it with the platform numbers rather than choosing between them. When platform reported returns hold steady and MER falls, the platforms are claiming credit for orders that would have happened anyway. When platform numbers look weak and MER holds, tracking is undercounting. That comparison is more informative than either figure alone.

There is no correct MER, because it depends on margin, on how much of your revenue is repeat business and on what you count as marketing spend. But you can derive the one your store needs.

Start from the bottom. Take your fixed costs for the period and the gross margin percentage you earn on revenue. Work out the revenue needed to cover fixed costs and leave the profit you want. Then work out how much marketing spend that revenue can support while still leaving that profit. The ratio between the two is your minimum MER.

Having derived it, use it as a floor rather than a goal. Running well above it usually means you are underspending and leaving growth on the table, since the easiest way to raise MER is to cut marketing and live off existing demand. That looks efficient for a quarter and shrinks the business over a year.

So watch the pair together: MER and absolute contribution. Growing contribution at a slightly lower MER is usually the better outcome than a high MER on a shrinking base.

And review the floor when costs change. A supplier price rise, a shipping increase or a new platform fee all move the MER you need, and the number that was comfortable last year may be losing money this year.