MER vs ROAS: which metric should your e-commerce store actually run on?

What is the difference between MER and ROAS?

Both metrics compare revenue to ad spend. The difference is scope and who does the counting.

ROAS, or return on ad spend, is campaign revenue divided by campaign cost. The revenue comes from the ad platform’s own attribution. Google’s help center gives the textbook example: $5 in sales for every $1 in ad spend is a 500% ROAS, or 5x.

MER, or marketing efficiency ratio, is total revenue divided by total marketing spend. The revenue comes from your store, Shopify or your payment processor. The spend includes every paid channel, plus anything else you count as marketing, like influencer fees or email software. Some people call it blended ROAS.

How do you calculate MER?

1. 1Pick a period, ideally a full month so weekday swings even out.
2. 2Pull total net revenue from your store for that period. Use the same definition every time: after discounts and refunds is best.
3. 3Add up all marketing spend for the same dates: Meta, Google, TikTok, Pinterest, Amazon ads, affiliate and influencer fees.
4. 4Divide revenue by spend. $200,000 in revenue on $50,000 of spend is a MER of 4.0.
5. 5Track it weekly as a trend line, not as a one-off number.

Why platform ROAS looks better than your bank account

Every ad platform gets credit for an order on its own terms. On Meta, the default attribution setting for website conversions is 7-day click, 1-day view, plus a 1-day engage-through window. That means a shopper who saw your ad yesterday without clicking, then bought from a Google search, can show up as a sale in both Meta and Google.

Google Ads uses data-driven attribution as the default model for most conversion actions. It splits credit across Google touchpoints only. It does not know about the Meta ad, the email or the friend who recommended you. Each platform is honest inside its own walls. Added together, they often claim more revenue than you actually made.

Meta has also kept changing the rules. In March 2026 it narrowed click-through attribution to link clicks only and moved likes, comments, shares and video views into the separate engage-through bucket. When definitions shift, your reported ROAS shifts with them, even if nothing changed in your business.

When is ROAS still the right metric?

ROAS is not useless. It is the right tool for decisions inside a single platform, where the attribution rules are the same for every campaign you compare.

• Ranking ad sets, creatives or audiences against each other in Meta.
• Setting a Target ROAS bid strategy in Google Ads, which needs conversion values and, for Search and Shopping, at least 15 conversions in the past 30 days.
• Spotting a campaign that suddenly broke, like a product feed error or a landing page that went down.
• Comparing product categories with very different margins.

The mistake is using ROAS to decide how much to spend in total, or comparing Meta ROAS directly with Google ROAS. The windows and models are different, so the numbers are not the same unit.

What is a good MER for e-commerce?

There is no universal good MER. A store with 70% gross margins can be very profitable at 2.5. A store selling electronics at 20% margins can lose money at 4.0. The right target comes from your own numbers, not an industry average.

Start with contribution margin: revenue minus cost of goods, shipping, payment fees and returns, before marketing. Then work out two targets.

Breakeven MER and breakeven ROAS

Breakeven is 1 divided by your contribution margin. At 40% contribution margin, breakeven is 1 ÷ 0.40 = 2.5. Below 2.5, every marketing dollar loses money on the first order.

Target MER

Your target has to cover fixed costs too: payroll, rent, software. If you want marketing to leave room for those costs and a profit, your target MER sits above breakeven.

The example targets assume roughly a quarter of contribution margin is left after marketing. Adjust them to your own fixed costs and profit goal.

That figure comes from mostly billion-dollar companies, so treat it as context, not a target. Growing e-commerce brands often spend a much larger share of revenue to buy new customers.

Why does MER drop when you scale ad spend?

MER includes revenue you would have earned anyway: repeat customers, organic search, word of mouth. When you double ad spend, that base revenue does not double. So MER usually falls as you scale, even when the new spend is profitable.

This is why MER alone can scare owners off good growth. The better question is marginal: if you spent $10,000 more last month, how much more revenue came in? If the extra revenue times your contribution margin is bigger than $10,000, the extra spend paid for itself.

MER can also hide problems. A big email promo or a viral post can lift total revenue and make paid ads look great when they did nothing. Watch MER next to new customer revenue, not instead of it.

How to use MER and ROAS together

1. 1Set a monthly target MER from your contribution margin and fixed costs.
2. 2Track MER weekly from store revenue and total spend. This is your north star for total budget.
3. 3Split revenue into new and returning customers. Many stores also track new-customer MER: first-order revenue divided by total spend.
4. 4Inside each platform, use ROAS only to rank campaigns, ads and audiences against each other.
5. 5When MER is above target, test more spend in the campaigns with the best relative ROAS.
6. 6When MER falls below target for two or three weeks, cut the weakest campaigns first and check if total revenue really drops.
7. 7Run a holdout or geo test a few times a year to see what ads add beyond what would happen anyway.

Most owners already look at revenue. The gap is connecting it back to spend and margin in one view. A simple weekly sheet with revenue, spend, MER, new customers and contribution margin does more than any dashboard full of platform ROAS.

Common mistakes with MER and ROAS

• Adding platform-reported revenue together and calling it total ad revenue.
• Comparing Meta ROAS with Google ROAS as if they used the same rules.
• Setting a ROAS target without knowing your margins.
• Counting gross revenue before refunds in MER, which flatters stores with high return rates.
• Reading MER daily. Shipping delays and paydays make daily numbers noisy.
• Ignoring lifetime value. A low first-order MER can be fine if customers reorder within a few months.

Frequently asked questions