MER vs ROAS: which question does each metric answer?

Define each metric and its denominator

Both are ratios of revenue to spend. The difference is which revenue and which spend.

• ROAS (return on ad spend).

  Revenue a platform attributes to a campaign, divided by that campaign's spend. The platform decides which sales count, based on its attribution settings.

• MER (marketing efficiency ratio), as used in this guide.

  Total store revenue divided by total media spend, for the same period. Every order counts once, whatever channel it came from, and every media dollar counts, whichever platform it went to.

MER is not a standardised metric. Different teams and tools use the name for slightly different calculations, and some call the same thing blended ROAS. That is fine, as long as you write down which version you use and keep to it. The main variants, calculated on month 1 of the dataset in the next section:

Also fix the revenue basis: including or excluding GST, before or after refunds, with or without shipping charged to the customer. Changing any of those changes the ratio without changing the business.

Work through one consistent dataset

Here is one hypothetical store across two months. It advertises on Meta and Google and sends email, which has no media cost. All revenue excludes GST and is net of refunds. In month 2 the owner adds $6,000 to the Meta budget and changes nothing else.

Start with month 1. The store's MER of 4.0 is higher than either platform's ROAS, because it counts orders from email, organic search and returning customers that no ad produced. Yet the channels together claim $94,000 from a store that took $80,000, because some orders were claimed more than once.

Month 2 is where the metrics tell different stories. Meta reports $12,000 of extra revenue for $6,000 of extra spend, at a ROAS of 3.0, comfortably above the store's break-even of about 2.2 (1 ÷ 0.45). Read on its own, that says scale.

The store disagrees. Total revenue rose $8,000, not $12,000. That extra $8,000 left $3,600 of contribution and cost $6,000 of media, so contribution after media fell by $2,400. On the extra spend, the store returned about 1.33 ($8,000 ÷ $6,000), below break-even.

MER fell too, from 4.0 to 3.38, but it is still well above break-even, so MER on its own would not have flagged the problem. What flagged it was the change: extra store revenue against extra spend, then contribution. The average can stay healthy while the last dollar loses money. When to increase your ad budget builds on exactly this.

One month is not proof. Seasonality, delayed purchases and other changes can all move store revenue. Treat a result like month 2 as a reason to test more carefully, not as a verdict.

Explain attribution and overlapping channel claims

The gap between $94,000 of claims and $80,000 of revenue is not an error. It is what happens when every channel counts by its own rules.

Attribution settings vary by platform and change over time, so check the current windows in each account rather than assuming them. The broader point holds either way: each report is that platform's view of its own contribution. The reports are useful for comparing campaigns inside that platform. They are not additive.

MER avoids double counting by ignoring the question of credit. That is its strength and its limit. It includes sales no media produced, such as loyal customers reordering and people who found you by word of mouth. If those grow, MER improves even if the ads got worse. If they shrink, MER falls even when the ads improved.

• Platform ROAS

  for comparing campaigns, ad sets and creative within one platform.

• MER

  for seeing whether total media spend is paying for itself across the business.

• The change in store revenue after a deliberate spend change

  for judging the next dollar.

• New-customer revenue, where you can separate it

  for checking that acquisition is working, rather than repeat orders flattering the total.

Use each metric alongside contribution and cash

Neither ratio is profit. Both need two more numbers beside them: contribution and cash.

Contribution turns a revenue ratio into money. If 45% of revenue is left after variable costs, any MER below about 2.2 means media costs more than the orders leave behind. Your break-even MER is 1 divided by your contribution margin, the same arithmetic as break-even ROAS, applied to the whole store. The break-even ROAS calculator works it out from your figures.

Contribution after media still has to pay for everything else. In month 1 the store has $16,000 left after media. If rent, software, staff and the founder's pay come to $12,000, the month made $4,000.

Cash can tell a different story again. If the same store pays a $30,000 stock order that month for the season ahead, the bank balance falls by $26,000 in a profitable month. The product cost inside contribution covers the units sold; the stock order pays for units not sold yet. A plan that scales media without allowing for that can run out of cash while every ratio looks fine. How to set an ecommerce marketing budget covers cash timing in more detail.

This is how we report Apothecary by Justin on our results page: blended MER of 0.17× in January 2026 ($548 of revenue on $3,204 of media) and 2.61× in August 2026 ($27,469 on $10,525), against a 1.20× break-even. Store revenue over total media, with the break-even beside it, because the ratio means little without it.