An ecommerce brand should run on contribution margin: what is left from each sale after product cost, shipping, payment fees, returns and marketing. Marketing efficiency ratio (MER), total revenue divided by total marketing spend, is the quick check on whether marketing as a whole is paying back. Return on ad spend (ROAS), revenue divided by spend for one campaign or channel, is for steering budget inside a platform. The three answer different questions, and confusing them is how brands grow revenue while losing money. This answer sets out how to calculate each, what a good number looks like and what it takes to see all three in one place. It is from Brand Media Works, an AI consulting and marketing data engineering firm for ecommerce brands.
What is the difference between MER, ROAS and contribution margin?
| Metric | Formula | What it answers | Main weakness |
|---|---|---|---|
| ROAS | Conversion value from a campaign divided by its ad spend | Which ads and campaigns to fund | Each platform counts the sales it touched, so the totals overlap |
| MER | Total store revenue divided by total marketing spend | Is all marketing paying back? | Ignores margin, so a high MER can still lose money |
| Contribution margin | Revenue minus product cost, shipping, fees, returns and marketing | Is the business making money on its sales? | Needs cost data that most stores do not keep complete |
Google defines its ROAS target as the average conversion value you want for each dollar spent on ads: $5 in sales for every $1 of ad spend is a 500% ROAS. Shopify describes ROAS as the measure for specific ads or campaigns, and MER as the measure of whether total marketing spend is producing enough revenue across the business (read 1 November 2026).
What does it take to see all three in one place, and when is a custom build worth it?
MER needs two numbers, store revenue and total marketing spend from every channel. Contribution margin needs cost data for every order. Shopify reports profit only for products that had a cost per item recorded at the time they sold, and warns that its sales and profit reports can show different net sales as a result (read 1 November 2026). Shipping, payment fees and returns usually sit in other systems.
- A profit app. TrueProfit lists $35 a month for 300 orders up to $200 for 3,500 orders, with a per-order charge above that. BeProfit lists $49 to $249 a month, or 20% less on annual billing (read 1 November 2026).
- A warehouse. Google's BigQuery Data Transfer Service loads Google Ads data with no transfer orchestration charge, Fivetran's Free plan covers 500,000 monthly active rows, and BigQuery's first 1 TiB of queries each month is free (read 18 October 2026). One data consultancy publishes about $4,000 for custom pipelines from one or two sources with automated refresh (read 25 October 2026).
A profit app is the quicker start for a single Shopify store. A warehouse pays off when the brand also sells on Amazon, wholesale or in stores, needs its own rules for allocating costs, or wants the same order-level margin for incrementality tests, marketing mix modeling and lifetime value. Marketing data engineering describes how Brand Media Works builds that view, and automated marketing reporting covers putting it into a weekly report.
What is a good marketing efficiency ratio?
Shopify, citing a 2026 benchmark study, puts a healthy blended MER at about 3 to 5 times, adjusted for margin, growth stage, industry and acquisition costs (read 1 November 2026). The adjustment for margin is the one that matters. The break-even MER is 1 divided by the contribution margin before marketing (our arithmetic):
- A brand that keeps 45 cents of each sales dollar after product cost, shipping, fees and returns breaks even at an MER of about 2.2.
- A brand that keeps 25 cents breaks even at 4.0, so an MER of 3 loses money.
That is why two brands with the same MER can have opposite results, and why MER targets should be set from margin, not from a benchmark.
How do you calculate contribution margin percentage?
Contribution margin percentage is contribution margin divided by revenue. A worked example with round numbers (our arithmetic):
- Revenue of $100,000 in a month.
- Minus product cost of $40,000, leaving $60,000 of gross profit, a 60% gross margin on Shopify's formula of net sales minus cost, divided by net sales.
- Minus $15,000 of shipping, payment fees and returns, leaving $45,000, or 45% before marketing.
- Minus $30,000 of marketing, leaving $15,000, or 15% contribution margin after marketing. MER in this example is 3.3.
Discounts change the picture quickly. Shopify's own example shows a T-shirt with a $20 price and $10 cost at a 50% margin, which falls to 33% when it sells at 25% off (read 1 November 2026).
What is a good contribution margin?
One that is positive after marketing and high enough to cover fixed costs such as salaries, rent and software. There is no single benchmark, because it depends on the brand's fixed costs and growth plan. A brand investing in new customers may accept a low or negative margin on first orders if repeat orders recover it, which is a question for lifetime value, not MER. Incrementality testing shows how to check whether the marketing behind those orders caused them.
Frequently asked questions
Is MER the same as blended ROAS?
In practice, yes. Both divide total revenue by total marketing spend across every channel, rather than using each platform's own attributed revenue.
Why does platform ROAS look better than MER?
Each ad platform counts the sales its ads touched, so the same order can be claimed by Meta, Google and email at once. Adding up platform revenue overstates the total, while MER uses the store's actual revenue once.
Which number should the CFO see?
Contribution margin after marketing, with MER beside it. ROAS belongs in the media team's reports.