MER formula: marketing efficiency ratio explained

MER (marketing efficiency ratio) is total revenue divided by total marketing spend over the same period. It's a whole-business number, so it includes sales from every channel, and it doesn't depend on any ad platform's attribution.

The MER formula

MER is commonly defined as:

MER = total revenue ÷ total marketing spend

There's no official standard for it, so two choices are yours to make and write down:

1. What counts as marketing spend. Some teams use ad spend only. Others add agency fees, tools and content. Pick one and keep it the same every month. Our MER calculator uses ad spend, so if your own MER also counts fees, tools or content, expect the calculator's number to come out higher than yours.

2. Which way round. Some write MER the other way, as spend ÷ revenue, where lower is better. Our MER calculator always uses payment revenue ÷ ad spend, so higher is better.

Illustrative example

Round numbers, not a customer. In one month a brand spends €20,000 on ads and collects €80,000 in payments.

MER = €80,000 ÷ €20,000 = 4.0

Now split the €80,000. Say €50,000 came from returning customers and €30,000 from new ones. New-customer revenue ÷ spend = €30,000 ÷ €20,000 = 1.5. Same month, same spend, a very different picture of how well the ads are finding new buyers.

MER vs ROAS

ROAS is usually measured per campaign or per platform, using the revenue that platform credits to its own ads. MER uses all revenue and all spend, so a sale claimed by two platforms is still only counted once.

That's also MER's limit. It tells you whether the whole engine is working. It can't tell you which campaigns are carrying it.

Where MER falls short

It mixes new and returning customers. One r/FacebookAds reply: "Split MER by new customer vs. returning." (r/FacebookAds, March 2026). The example above shows why.

It's slow. The same thread: "I only treat MER as real on full-cycle windows (at least monthly) because daily MER swings with lag and returns."

It can hide weak acquisition. From an r/PPC thread on switching to MER, a reply warned it's "a lagging, aggregate number, so pair it with cost per NEW customer (nCAC)" (r/PPC, August 2026). Our blended CAC page covers that partner metric.

It has no per-campaign view. For that, each payment needs to be matched to the click that led to it. That's what payment-matched tools do. Thexly matches each payment to the visit and credits one channel, first or last, chosen per website (docs).

Using the MER calculator

Put your payment revenue and ad spend for the same dates into the MER calculator. For the longer argument on why MER and blended ROAS beat adding up platform numbers, read blended ROAS and MER vs platform ROAS.

Running MER for client accounts? See Thexly for agencies. There's a 14-day trial, no card needed.