What is MER in ecommerce?

MER, short for marketing efficiency ratio, is usually all your revenue divided by all your ad spend over the same dates. It counts each sale once, no matter which platform claims it. So it tells you whether your advertising pays for itself overall, but not which channel, campaign or ad earned the money.

The formula is the easy part. Three choices hide inside it: which revenue goes on top, which costs go underneath, and which dates both cover. Change any one of them and the same month earns a different MER.

MER exists because ad platforms grade their own homework. Each one counts the sales it touched, by its own rules, and two of them can claim the same order. Your store booked that order once. Divide the store's revenue by everything you spent, and nobody gets to count a sale twice.

What one store's data shows

One store's anonymised GA4 export, 1 January 2024 to 21 August 2026. It holds shares of revenue only: no ad spend, no order counts.

The top row is the whole case for MER, and its catch. In one store's export, Direct holds 57.7% of revenue on the Channels sheet, in every view. Direct has no budget line of its own, so a ROAS per channel has nothing to divide that revenue by.

MER divides it anyway. If an ad sent people who later came back by typing your address, MER rightly gives the ads a share of that money. If those buyers were regulars who would have returned regardless, MER gives the ads the same share. The ratio cannot tell the two apart.

The journey rows add a clock. On the Journeys sheet, 79.5% of revenue came from one-touch journeys that took 0.5 days. On the same Journeys sheet, journeys of 2 to 3 touches took 12.5 days, and 4 to 9 touches took 16.9 days. So in that store, part of any week's revenue started with touches from earlier weeks. A weekly MER sets this week's spend against sales that last week's ads began.

The last row is arithmetic, not a result. The Break-even sheet's sum works for MER too: at a 40% margin, an MER under 2.5 means the ads outspend their gross profit.

What the export cannot give you is this store's MER. It holds no ad spend at all. Shares of revenue show where sales landed, not what the advertising was worth.

Why can a healthy MER still mislead?

Because the top of the fraction moves for reasons that have nothing to do with ads. A busy season, a press mention or a wave of returning customers lifts revenue. MER rises with it, and the ads get the applause for standing nearby.

The revenue line also comes in three sizes. Shopify's gross sales is price times quantity, before anything comes off. Net sales takes off discounts and sales reversals, which cover returns, cancellations and order edits. Total sales then adds taxes, duties, shipping charges and fees back on.

For illustration, take a month with €60,000 of total sales, €50,000 of net sales and €10,000 of ad spend. On total sales the MER is 6; on net sales it is 5. Same month, same ads, two answers. Taxes were never yours to keep, so net sales makes the fairer top line.

The cost line has the same problem. Some teams count ad spend only. Others add agency fees, creative, creator payments and software. Either works, as long as you never switch between them halfway through the year.

Then the dates. Meta's help says the amount spent in Ads Manager is an estimate, because ad results can take up to 48 hours to process. Yesterday's MER is a draft.

What can MER not tell you?

It cannot tell you which channel did the work. MER is one number for the whole store, so it cannot rank Google against Meta, or one campaign against another.

It cannot tell you whether the next euro pays. MER is an average, and averages hide the edge. For illustration, say €10,000 of spend sits next to €50,000 of net sales: an MER of 5. If you double spend to €20,000 and net sales reach €70,000, MER falls to 3.5, still above a 2.5 line. In that worked example, the extra €10,000 brought in €20,000: a ratio of 2. If your margin is 40%, the extra spend lost money while the average still looked healthy.

And it cannot tell you what the ads caused. A rising MER fits better ads, and it fits more people coming back on their own. To separate the two, switch ads off for some regions or people and compare their sales with the rest. That is a holdout test, and it answers the question MER skips.

What to do this week

1. Write your MER definition in one line. Name the revenue (Shopify net sales, under Analytics > Reports > Total sales over time), the costs and the window. Pass: a colleague can rebuild last month's MER from that line alone. Fail: two people bring two MERs to the same meeting.
2. Find the line your MER has to clear. In Shopify, go to Analytics > Reports, filter the Category to Profit Margin and open Gross profit by product. Divide 1 by your margin after shipping and payment costs. Pass: you know your break-even MER. Fail: the profit rows are blank, because no cost was recorded when those products sold.
3. Check how long your buyers take. In GA4, click Advertising, then Attribution paths under Attribution, keep only purchase selected and read Days to key event in the top row. Pass: your MER window is longer than that. Fail: you read MER weekly while buyers take two weeks or more.

Check the homework. Your GA4 Attribution paths export already holds the evidence. Causality Engine reads that one file and shows what each channel caused next to what last-click gave it, in 1 to 2 minutes, for €99 once (excluding VAT), refundable within 30 days. Check the homework

Sources, 1 October 2026: Sales reports (Shopify Help Center); Profit reports (Shopify Help Center); Why amount spent is different in ad account spending limit, Meta Ads Manager and billing history (Meta Business Help Center); Get started with attribution (Google Analytics Help); Key events attribution paths report (Google Analytics Help).

Related answers

• All ROAS & margin questions
• How to calculate MER for your store, step by step
• What does MER tell a home goods store?
• Blended MER vs platform ROAS
• Incrementality testing, explained