Ecommerce measurement reference
MER and CAC should be reported together
MER shows revenue against marketing spend. CAC shows spend against new customers. Write the formula next to the number because teams use two opposite MER conventions.
On this page
Direct answer
Use one ledger and name every denominator.
Many ecommerce teams use MER to mean total revenue divided by total marketing spend, where a higher number is better. Some teams use the reciprocal, marketing spend divided by revenue, where a lower percentage is better. Both can describe the same month. Calling both “MER” without the formula creates a reporting error.
Blended CAC divides the agreed acquisition cost by all new customers in the same period. Paid CAC divides paid-media cost by the new customers assigned to paid acquisition under an agreed rule. Read the metrics together with contribution margin, refund timing, repeat purchase, payback, and capacity.
Apply the definitions to ecommerce marketing, connect them to paid advertising work, and use how we work when the reporting problem needs implementation.
Formulas
Three calculations, three questions
| Metric | Formula | Question | Main limit |
|---|---|---|---|
| Revenue-to-spend MER | Net revenue ÷ total marketing spend | How many revenue dollars were recorded per marketing dollar? | It mixes new and returning revenue and does not show margin. |
| Spend-to-revenue ratio | Total marketing spend ÷ net revenue | What share of recorded revenue went to marketing? | It is the reciprocal of the first formula, not a separate result. |
| Blended CAC | Agreed acquisition cost ÷ all new customers | What did each new customer cost across the business? | The cost scope and new-customer definition must stay fixed. |
| Paid CAC | Paid-media cost ÷ paid-attributed new customers | What did a paid-attributed new customer cost? | The attribution rule changes the denominator. |
Worked example
One month in a measurement ledger
The numbers below are illustrative. They are not benchmarks.
| Ledger line | Amount | Rule |
|---|---|---|
| Net revenue | $200,000 | Orders less agreed refunds, discounts, and taxes for the month. |
| Total marketing spend | $50,000 | Paid media plus the agreed agency, creative, software, and other acquisition costs. |
| Paid-media spend | $35,000 | Media cost only. |
| All new customers | 400 | First purchase under the store’s fixed customer rule. |
| Paid-attributed new customers | 250 | New customers assigned to paid acquisition under the agreed attribution rule. |
| Results | 4.0x; 25%; $125; $140 | Revenue MER; spend ratio; blended CAC; paid CAC. |
Break-even
There is no universal good MER or CAC
Contribution margin
Revenue does not pay for marketing by itself. Product cost, fulfillment, payment fees, returns, discounts, and variable service costs change what is available.
New versus returning revenue
A store with strong repeat purchase can accept a different first-order CAC than a store with little repeat revenue. Separate the cohorts.
Payback
A profitable customer can still create a cash problem if the payback takes longer than the business can finance.
Incrementality
Platform attribution can assign credit to sales that would have happened anyway. Lift tests and controlled comparisons ask a different question from attribution reports.
Set the acceptable range from the business model. Do not borrow a target from another store without matching margin, repeat rate, cash needs, product mix, and growth plan.
Monthly control
Lock the definitions before reading the trend
- Use the same date window across finance, store, media, and customer records.
- Name gross or net revenue and state the refund, tax, discount, and shipping treatment.
- List every cost included in total marketing spend.
- Define a new customer once and keep the rule stable.
- Show the attribution rule behind paid-attributed customers.
- Separate acquisition, retention, brand, and fixed operating costs when decisions require it.
- Read the month beside contribution margin, cash, stock, and capacity.
- Record any definition change so the trend is not silently rewritten.
Common questions
Answers without a sales pitch
What is a good MER?
There is no universal answer. The acceptable number depends on which MER formula is used, contribution margin, repeat purchase, product mix, refunds, cash needs, and the growth plan.
Is MER the same as ROAS?
No. MER normally uses total business revenue and a wider marketing-cost denominator. Platform ROAS uses revenue attributed to a channel or campaign under that platform’s rules.
What is the difference between blended CAC and paid CAC?
Blended CAC uses the agreed acquisition cost across all new customers. Paid CAC uses paid-media cost and new customers assigned to paid acquisition under a stated attribution rule.
Should agency fees be included in CAC?
Include them when the question is the full cost to acquire customers. A media-only CAC can also be useful, but it must be labeled and shown beside the fuller cost.
Primary sources
What the platform documentation supports
These sources support the facts on this page. They do not disclose complete ranking systems or guarantee a recommendation, citation, click, lead, or appointment.
- Shopify: Marketing efficiency ratioA current commerce explanation of revenue-to-spend MER.
- Shopify Help: Sales reportsStore sales reporting concepts and definitions.
- Shopify Help: Customers reportsNew and returning customer reporting.
- Google Ads: Attribution reportsHow Google Ads reports paths and attribution.
- Google Ads: Attribution modelsHow the credit rule changes reported conversions.
- Google Ads: Conversion LiftGoogle’s controlled lift measurement.
- Meta: Conversion LiftMeta’s experiment approach to incremental outcomes.
- Triple Whale: Marketing efficiency ratioAn ecommerce operator’s view of MER and its reporting limits.
- Google Analytics: Advertising attributionAttribution reporting outside the ad-platform view.
- Google MeridianOpen-source marketing-mix modeling documentation.
Make the report match the business books.
Stan Consulting can connect store, media, customer, margin, and reporting definitions for an ecommerce marketing project.
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