MER vs ROAS: which number should you steer on?

MER (marketing efficiency ratio) is your total store revenue divided by your total ad spend across all channels. ROAS is the revenue one ad platform or campaign credits to its own ads, divided by its spend. Use MER to set the total ad budget across all channels, and use ROAS to steer campaigns inside one platform.

The two numbers often disagree, and the gap tells you something. This guide explains how MER and ROAS differ, works through an example where the platforms claim more revenue than the store took in, shows when each number is the better guide, and turns MER into a profit target.

What is MER?

MER needs no attribution. It does not ask which ad led to which order. It only asks how much the store sold for every dollar of ad spend. Because of that, it is also called blended ROAS.

Some teams also add other marketing costs to the bottom of the fraction, such as agency fees or influencer payments. Both versions are in use. Pick one definition, write it down and keep it, so this month's MER compares with last month's.

How is MER different from ROAS?

ROAS uses the same formula on a smaller scale: the revenue credited to one channel or campaign, divided by its spend. The difference is in where the revenue comes from.

Worked example: why channel ROAS adds up to more than MER

An example store advertises on three platforms in one month. Each platform reports the revenue it credits to its own ads:

Add up the platforms and advertising seems to return 3.67. The store says 3.0 at most, and that includes $26,000 of orders from email, organic search and direct visits with no paid click. The ads cannot have caused more revenue than the store took in. So at least $20,000 of the claimed revenue is counted twice, or belongs to orders the ads did not bring in.

Three causes are common. First, a shopper who clicks a Meta ad and later a Google ad is claimed by both. Second, platforms can count view-through purchases by people who only saw an ad. Third, each platform uses its own attribution window, which can be longer than the window your store uses. None of this means a platform reports wrongly by its own rules. It means the platform numbers cannot be added up. Our guide to why Ads Manager and Shopify sales differ goes through each cause.

When is MER the better number?

Setting the total ad budget

The question "can we spend more on ads?" is about the whole store. MER answers it with the revenue the store actually took in. If you raise total spend from $30,000 to $40,000 and MER falls from 3.0 to 2.4, revenue went from $90,000 to $96,000. The extra $10,000 of spend brought in $6,000 of revenue. No platform report shows that directly.

When attribution has gaps

Declined cookie consent, ad blockers, short cookie lifetimes in Safari and purchases on another device all break the link between a click and an order. Platform ROAS and your own attribution both miss some of those orders. MER does not depend on that link, so it stays usable when tracking is incomplete.

When channels overlap

The more platforms you run, the more orders get claimed twice. MER counts each order once, however many platforms touched it.

To measure what the ads really add, some platforms offer lift tests. They compare people who could see your ads with a group that was held back. Google Ads, for example, reports incremental ROAS from such a test: only the conversion value that would not have happened without the ads, divided by the spend.

When is ROAS the better number?

MER cannot tell you which campaign to cut. For decisions inside one platform, platform ROAS is the better guide:

• Per campaign, ad set and ad. Platform ROAS is available at every level and every day. MER is one number for the whole store.
• Within one platform. All campaigns on Meta are measured with the same Meta rules. That makes them fair to compare with each other, even if the level is too high.
• For bidding. Value-based bidding, such as target ROAS in Google Ads, works on the platform's own conversion values. The platform optimizes toward its own ROAS, not toward your MER.

To compare channels with each other, neither number is ideal. MER cannot split revenue by channel, and platform ROAS lets each channel grade its own work. Use ROAS from your own tracking instead: credit each order once, with one attribution model for every channel. The attribution help page explains the models.

How to use MER and ROAS together

1. Set a MER floor from your margin

   Break-even MER is 1 divided by your gross margin rate. Below it, advertising as a whole loses money.

2. Set the total budget against MER

   Raise total spend while MER stays above your target. Stop when the extra spend no longer brings in enough extra revenue.

3. Split the budget with your own attribution

   Compare channels on orders your store tracked, with one model for all, so each order counts once.

4. Optimize inside each platform on platform ROAS

   Move budget between campaigns, ad sets and ads with the numbers the platform reports.

5. Check MER every week

   If platform ROAS rises but MER does not move, the platforms are most likely claiming orders the store would have had anyway.

That last check matters most. The chart shows what it looks like when it goes wrong:

Profit versions: break-even MER and POAS

MER, like ROAS, counts revenue. To know whether it is high enough, connect it to your margin. Gross profit is revenue minus product cost, shipping you pay, payment fees and handling. The gross margin rate is gross profit divided by revenue.

For the example store: with a 45% gross margin rate, $90,000 of revenue leaves $40,500 of gross profit. After $30,000 of ad spend, $10,500 is left. Blended POAS is 3.0 × 0.45 = 1.35. To keep 10% of revenue after ads, the target MER is 1 ÷ (0.45 − 0.10) = 2.86. The store is above it.

This is the same math as break-even ROAS, applied to the whole store. The break-even ROAS guide covers which costs to include and why each product line has its own break-even. For profit per campaign instead of per store, use POAS.

A stricter variant leaves returning customers out: revenue from first orders divided by total ad spend, often called new-customer ROAS. Returning customers often buy without an ad, so this number shows what the ads add in new business.

MER and ROAS in ROASaurus

On the profit dashboard, the MER of this post is the Blended ROAS tile: revenue ÷ ad spend, with Break-even ROAS under it. The Daily summary email shows the same number as MER. Two other places show marketing spend as a share of revenue instead. This table shows every version for the example store:

Read the percentage versions as the share of revenue that went to marketing, so lower is better. The share under Contribution margin also counts the marketing costs you enter as custom costs, such as an agency fee. Contribution margin itself is gross profit minus ad spend and those marketing costs, as explained in contribution margin in ecommerce.

The other tiles: POAS is gross profit ÷ ad spend. Ad ROAS is the revenue of orders whose last click was a paid ad, divided by ad spend, with new-customer ROAS (ncROAS) under it. Per channel and campaign, the Marketing page shows revenue, ROAS and POAS from the ad clicks your store recorded, credited under any of the twelve attribution models, from last click to data-driven Markov and Shapley and Total Impact, which also uses the answer to the post-purchase survey.

Tax also changes the numbers. By default, the Revenue tile on the profit dashboard includes the taxes you collect and deducts them from net profit. Blended ROAS and the email's MER leave those taxes out, as in this post, so they show the same number. Only when you set Taxes collected to In revenue and kept in net profit under Profit › Cost settings do the taxes count in MER.

MER is only right when all spend is in it. Spend from the ad accounts you connect (Meta, TikTok, Google Ads and more) syncs automatically. Add spend from other channels under Profit › Manual ad spend, or MER and blended ROAS will look higher than they are.

Sources

1. Shopify Help Center: Measuring marketing performance
2. Google Ads Help: About attribution models
3. Google Ads Help: About Target ROAS bidding
4. Google Ads Help: Understand your Conversion Lift based on users measurement data

In ROASaurus

• Profit analytics in ROASaurus
• Attribution in ROASaurus
• ROAS and break-even ROAS calculator

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