Marketing efficiency ratio (MER) vs ROAS: the formula, and when to use each.
MER is total revenue divided by total marketing spend. ROAS is the revenue one ad platform credits to its own ads, divided by what you spent there. They answer different questions, and the gap between them tells you how much your platforms are double counting. Here is the formula for each, how to set an MER target from your margin, and a worked example.
01
What MER is, and the formula
What happens
Marketing efficiency ratio, or MER, is total revenue divided by total marketing spend over the same period. Revenue comes from your store or order system, not from any ad platform. Spend is every channel added together. An MER of 4 means four dollars of revenue for every dollar spent on marketing. Some teams call the same number blended ROAS.
What to do about it
Compute it from two numbers you already own: revenue from your order data and spend from your invoices or ad accounts. Pick one period, a week or a month, and use the same period for both. Because no platform touches either number, MER cannot be inflated by attribution.
02
How MER differs from ROAS
What happens
ROAS, return on ad spend, is the revenue an ad platform attributes to its own ads divided by what you spent on that platform. It is a channel number, and it is the platform's own view of its results. MER is a business number: every dollar in, every dollar out, with no view on which channel caused which sale.
What to do about it
Use each for the decision it fits. ROAS is useful inside a platform for comparing campaigns, ads and audiences day to day. MER is the check on whether the total marketing budget is paying for itself. When the two disagree, MER is the one tied to money you actually booked.
03
Why platform ROAS adds up to more than MER
What happens
Each platform counts the sales it touched under its own rules, so one customer who saw a social ad and then clicked a search ad can be claimed by both. View-through credit and different attribution windows add more. Add up the revenue every platform claims and it often comes to more than the revenue you actually made.
What to do about it
Put the platforms' claimed revenue next to your real revenue every month. The gap is the size of the double counting. A rising MER with flat platform ROAS is real growth; rising platform ROAS with a flat MER usually means more overlap, not more sales.
04
Decide what counts as marketing spend
What happens
Published definitions disagree. Some put only paid media in the denominator. Others include agency fees, creative production, software and the marketing team's salaries. The same business can show two very different MERs depending on which version it uses, and month-on-month comparisons break if the definition drifts.
What to do about it
Choose one definition, write it down, and say which one sits under every MER you report. Media-only MER suits weekly budget pacing. Fully loaded MER, with fees, production and tools included, is the right one for deciding whether marketing as a whole is profitable.
05
MER moves for reasons that are not marketing
What happens
Because MER uses all revenue, anything that changes revenue changes it: a price rise, a seasonal peak, a stock-out, a big wholesale order, or loyal customers who would have bought anyway. A strong MER can hide paid channels that are only recapturing existing customers.
What to do about it
Track a second version alongside it: revenue from new customers divided by the same spend. That shows whether marketing is bringing in new buyers or mostly taking credit for repeat orders. Note big non-marketing events, such as a price change or a sale, next to the month they happened.
A worked example, with illustrative numbers
Say a store books $500,000 of revenue in a month and spends $100,000 on ads: $50,000 on Meta, $40,000 on Google and $10,000 on TikTok. MER = $500,000 / $100,000 = 5.0.
Now the platforms. Meta claims $280,000 of sales, a ROAS of 5.6. Google claims $260,000, a ROAS of 6.5. TikTok claims $60,000, a ROAS of 6.0. Every channel beats the MER, and together they claim $600,000, which is $100,000 more than the store actually made. That $100,000 is sales counted by more than one platform.
The decision MER supports: the budget as a whole returns five dollars per dollar. The decision it cannot support is which channel to grow. For that you need the overlap removed, or a test that switches a channel off in some regions and measures what the store loses. Numbers changed for illustration; the arithmetic is the point.
What is a good MER? Work it out from your margin
There is no single good MER, because it depends on what each dollar of revenue is worth to you. The useful number is your break-even MER: 1 divided by your gross margin. At a 40% gross margin, break-even MER is 1 / 0.40 = 2.5. Below 2.5, marketing costs more than the gross profit it brings in. At a 60% margin, break-even is about 1.7.
Set your target above break-even by enough to cover the other costs of running the business and the profit you want. Then judge new-customer MER separately, because a first order often earns back its cost only over later repeat orders.
Connect your ad platforms and your order data, and Decifer measures efficiency on revenue you actually booked, not on what each platform claims for itself.