Short answer: In the ROAS vs MER vs CAC debate, no single metric wins. Use ROAS to optimise inside each ad platform, MER to judge whether total marketing spend is working for the business, and CAC (with LTV) to decide how much you can afford to pay for a new customer. Scale when MER holds above your break-even level and new-customer CAC stays under your limit, not just because one platform reports a high ROAS.
Below are the formulas, one worked example you can copy, the ways each metric misleads, and a simple framework for deciding when to scale.
ROAS vs MER vs CAC: the three formulas
| Metric | Formula | Answers the question |
|---|---|---|
| ROAS (return on ad spend) | Revenue attributed to ads divided by ad spend | Which campaign, ad or channel returns the most per unit spent? |
| MER (marketing efficiency ratio) | Total revenue divided by total marketing spend | Is our marketing, as a whole, paying off? |
| CAC (customer acquisition cost) | Total marketing spend divided by new customers acquired | How much does one new customer cost us? |
Google describes target ROAS as the average conversion value you want for each unit of ad spend; its own example is $5 in sales for $1 spent, a 500% target ROAS. MER uses the same maths but zooms out: Northbeam defines it as total revenue divided by total marketing or ad spend, a top-down view that sidesteps attribution gaps.
A worked example (hypothetical numbers)
Example only: the figures below are made up to show the maths. They are not from a real client or a benchmark.
An online store has one month that looks like this:
- Total store revenue: $100,000 from 1,250 orders (average order value $80)
- Ad spend: $15,000 on Meta and $10,000 on Google, $25,000 in total
- Meta reports $60,000 in purchases; Google reports $50,000
- New customers this month: 500
- Gross margin after product cost, shipping and payment fees: 40%
Now the maths:
- Platform ROAS: Meta 60,000 / 15,000 = 4.0. Google 50,000 / 10,000 = 5.0.
- Spot the double count: the platforms claim $110,000 between them, but the store only made $100,000. Both are taking credit for some of the same orders.
- MER: 100,000 / 25,000 = 4.0.
- Break-even ROAS or MER: 1 / 0.40 = 2.5. Above 2.5, ad spend is covered by gross profit.
- Profit after ads: $100,000 x 40% = $40,000 gross profit, minus $25,000 ad spend = $15,000 left for overheads and profit.
- CAC: 25,000 / 500 = $50 per new customer.
- First-order profit: $80 x 40% = $32. The store loses $18 on each new customer’s first order and needs repeat purchases to profit.
Look at how different the story is depending on which number you read. ROAS says “great”. MER says “healthy”. CAC says “we lose money on the first order”. All three are true, and together they tell you what to do: protect repeat purchases and keep an eye on how much you pay for new customers.
Break-even ROAS: work it out from your margin
Your break-even point is the minimum return where ads pay for themselves. Shopify gives the formula as break-even ROAS = 1 / gross profit margin, where gross profit margin = (revenue minus variable costs) / revenue.
| Gross margin | Break-even ROAS |
|---|---|
| 25% | 4.0 |
| 40% | 2.5 |
| 50% | 2.0 |
| 70% | 1.43 |
Include every cost that grows with each order: product, packaging, shipping, payment fees, returns and discounts. Leaving them out makes break-even look lower than it is. Also remember that break-even only covers ad spend. Your target should sit above it so you can pay rent, salaries and still profit.
When each metric misleads
ROAS misleads when…
- Platforms double count the same sale, as in the example above. Each platform uses its own attribution rules.
- It is driven by brand search or retargeting, which reach people who were likely to buy anyway.
- Margins differ by product. A 4x ROAS on a low-margin product can earn less than 3x on a high-margin one.
- Tracking is broken or values are wrong. See Google Ads conversion tracking with GA4 and GTM.
MER misleads when…
- Revenue from email, organic search or loyal repeat buyers props it up while new-customer acquisition is getting worse.
- Seasonality, a big sale or PR coverage moves revenue for reasons unrelated to ads.
- You need to know which campaign to cut. MER cannot tell you; it only shows the total.
CAC misleads when…
- The definition changes. Blended CAC (all spend divided by all new customers) and paid CAC (ad spend divided by customers from ads) give different numbers. Pick one and keep it.
- It ignores order value. A $50 CAC is cheap for a $300 order and expensive for a $40 one.
- Your “new customer” count is inaccurate, for example when guest checkouts create duplicate customers.
LTV:CAC: how much can you pay for a customer?
Lifetime value (LTV) is the gross profit a customer brings over time. Compare it to CAC to see whether acquisition is worth it. Continuing the hypothetical example: if an average customer places three $80 orders in their first year, their gross-profit LTV is 3 x $32 = $96. Against a $50 CAC, that is an LTV:CAC of about 1.9.
The often-quoted 3:1 target comes from SaaS investor David Skok. In a later piece on For Entrepreneurs, he stresses that LTV:CAC is only meaningful once a business has a repeatable way to acquire customers, and that the number of months it takes to recover CAC can be an even more useful early metric. For eCommerce, I recommend focusing on payback: how many months until a new customer’s gross profit covers what you paid to acquire them. Cash-tight brands need a short payback; well-funded brands with strong repeat purchase can wait longer.
Be careful with LTV. Use real cohort data (customers acquired in a given month and what they actually spent since), use gross profit rather than revenue, and do not assume next year’s customers will behave like your most loyal ones.
A simple decision framework for scaling
- Set guardrails from margin. Calculate break-even ROAS and a target MER above it. Calculate the maximum CAC you can accept from first-order profit plus expected repeat profit within your payback window.
- Make MER and new-customer CAC your business-level scaling signals. Review them weekly from your store data, not the ad platforms.
- Use platform ROAS and CPA for decisions inside each platform. Which campaign, ad set or creative gets more budget.
- Scale in steps. Raise total spend, then watch MER and CAC over the next one to two weeks. If both hold within your guardrails, keep going. If MER drops toward break-even or CAC passes your limit, you have found the current ceiling.
- Check incrementality. Every few months, run a lift test or a holdout to confirm the channels you rely on add real sales.
What I recommend: pick one source of truth for revenue and new customers (usually your store or backend), agree on the definitions with whoever manages your ads, and keep a simple weekly sheet with spend, revenue, MER, new customers and CAC. That one sheet will prevent most bad scaling decisions. If you need help setting up reliable data, see my GA4 and Google Tag Manager service.
Frequently asked questions
ROAS vs MER vs CAC: which one should I scale on?
What is a good MER?
How do I calculate break-even ROAS?
Is a 3:1 LTV:CAC ratio right for eCommerce?
Who can help me set up ROAS, MER and CAC reporting?
Need help choosing the right metrics?
I’m Saifur Saif. I run performance marketing for eCommerce brands and build reporting around profit, not platform vanity numbers. If you are unsure whether your ads are really paying off, get a free ads audit. New to the topic? Start with what is performance marketing.