If you're running paid media across more than one channel and using each channel's reported ROAS to make budget decisions, you're probably working with a distorted picture.

Not because you're doing it wrong. Because the numbers the platforms give you are built to make the platforms look good.

Channel ROAS tells you what each platform claims it's responsible for. MER tells you what your marketing spend is actually producing. For most ecommerce brands managing real budgets across Meta, Google, and other channels, the gap between those two numbers is where a lot of money quietly gets misallocated.

What is MER?

MER stands for Marketing Efficiency Ratio. The formula is very simple:

MER = Total Revenue / Total Ad Spend

You take all the revenue your store generated in a given period and divide it by everything you spent on paid media across all channels combined. No attribution model. No platform-reported conversions. Just what came in versus what you spent.

If you made $200,000 in total revenue in a month and spent $40,000 across Meta, Google, and TikTok, your MER is 5.0.

The reason it's useful is precisely because it's simple. It doesn't ask which channel gets credit for a sale. It doesn't rely on pixels, click windows, or view-through attribution. It just looks at the relationship between total marketing spend and total business output. That's it.

Why channel ROAS misleads you

Every ad platform measures its own performance using its own rules. Meta counts a conversion if someone saw your ad in the last 7 days and then purchased, even if they clicked a Google Shopping ad to complete the transaction. Google counts the same sale. So does every other platform that ran an ad anywhere near that purchase window.

The result is that if you add up the attributed revenue from each platform's dashboard, the total is almost always higher than your actual revenue. Sometimes significantly higher. Each platform is taking credit for the same customers.

This creates a big problem when you try to make budget decisions based on channel ROAS. You might look at Meta at 3.8 and Google at 6.2 and conclude Google is more efficient, so you shift budget there. But a lot of Google's reported conversions are customers who were first reached by Meta and would have found you anyway through branded search. Pull back on Meta and Google's reported ROAS might drop too, because the top-of-funnel demand feeding it just got weaker.

So you just shifted spend from a weak channel to a strong channel and... made things worse? How???

Channel ROAS measures what platforms say they're doing. MER measures what's actually happening to your business.

How MER and channel ROAS work together

MER isn't a replacement for channel-level data. It's the lens you use to interpret it honestly.

At Saeba Digital, we track both. Channel ROAS and in-platform metrics tell us what's happening inside each campaign: which creatives are resonating, which audiences are efficient, where we should be testing. That granular data is still essential for day-to-day decisions.

But when a client asks whether their overall paid media investment is working, or whether they should increase total spend next month, we look at MER first. If MER is healthy and moving in the right direction, the machine is working. If MER is declining while individual channel ROAS figures look strong, something is wrong. Usually it's over-attribution, channel cannibalisation, or a product margin issue being masked by numbers that look good on the surface.

The question MER answers is simple: for every dollar we put into paid media, how many dollars of revenue come out? That's the number that means something to a founder or a CFO.

How to calculate your MER

You need two numbers from the same time period:

1. Total revenue: your store's actual revenue for the period, from Shopify or your ecommerce platform, not from any ad platform dashboard
2. Total ad spend: every dollar spent across every paid channel, combined

Divide revenue by spend. That's your MER.

A few things worth keeping in mind:

Use the same time period for both. Revenue and spend should be pulled for the exact same date range. Ad spend and the revenue it generates don't always land on the same day, so look at MER over 30-day windows at minimum rather than day by day.

Use store revenue, not platform-reported revenue. The whole point of MER is to get off platform attribution. Pull your revenue number from Shopify, your accounting system, or wherever your actual sales are recorded.

Include all paid channels. Meta, Google, TikTok, Pinterest, CTV — everything you're paying for. Leave one out and the ratio stops being useful.

What is a good MER for ecommerce?

Honestly, there's no single benchmark that means much because MER is a function of your margins, your average order value, your product category, and your business model. A brand with 70% gross margins can sustain a lower MER than a brand running at 30% and still be profitable.

The more useful question is: what MER do you actually need to be profitable, given your contribution margins?

What we do with clients is build a MER target from their actual margin structure rather than a random industry number. Once you know what MER the business needs to hit its profitability targets, you have something real to optimise toward.

When channel ROAS still matters

MER tells you whether the overall system is working. Channel ROAS, CPMs, CTRs, and cost per purchase still tell you what's happening inside the system.

If your MER drops, you use channel-level data to diagnose why. Is one channel underperforming? Is creative fatigue driving up CPMs? Is a specific audience segment becoming less efficient? MER surfaces the problem. Channel data helps you find it.

Think of MER as the check engine light and channel ROAS as the diagnostic tool you run once the light comes on.

How to start tracking MER

If you're not tracking it already, a basic version takes about ten minutes to set up in a spreadsheet.

Pull your total store revenue and total ad spend for each month, going back as far as you have clean data. Calculate MER for each month. Plot it. You now have a trend line showing whether your paid media is becoming more or less efficient as you scale.

From there, the next step is working out a MER target based on your margin structure, so you're optimising toward a number tied to profitability rather than just efficiency in the abstract. That's the layer we build into the growth modelling we do for every client at Saeba Digital, connecting the ad account to the actual economics of the business.

The bottom line

Channel ROAS is a useful signal. It's not a reliable measure of whether your paid media is working at a business level.

MER is simpler, harder to mess with, and directly connected to the question that actually matters: is the money going into paid advertising making the business more profitable?

If you're managing real spend across multiple channels and you're not tracking MER alongside your channel metrics, you're missing the clearest read you have on what's actually going on.

At Saeba Digital we build MER tracking into every paid media retainer as part of our growth modelling layer. If you want to understand what MER target makes sense for your margins and how to use it to make better budget decisions, book a discovery call.