Growth Decisions with CAC, ROAS and MER

A performance marketing review starts with three definitions. ROAS is attributed advertising revenue divided by advertising spend. CAC is defined acquisition cost divided by new customers. Here, MER is defined total revenue divided by total marketing expenditure for the same period. Each ratio answers a different question and none alone establishes profit.

Shopify’s MER explanation recommends consistent revenue and expense definitions. Here marketing includes media, creative, agency, tools and allocated marketing-team costs. CAC includes only the acquisition allocation. Label media-only versions separately. Define the time zone, currency, refunds and valid first purchase; orders and new customers are different denominators.

Report a zero-denominator ratio as undefined. Long sales cycles can make same-month acquisition ratios misleading. Document allocation before comparing cohorts, and mark definition changes so they cannot masquerade as improvement.

For ecommerce managers, consider an original illustrative month, with no client outcome or quoted price implied. Product revenue is TRY140,000, discounts TRY10,000 and refunds TRY10,000. Defined net product revenue is TRY120,000, excluding tax and customer shipping charges. Total marketing costs are TRY30,000, including TRY20,000 media spend.

The ad report credits TRY80,000 under its own value definition and attribution window. ROAS is 80,000 / 20,000 = 4; business MER is 120,000 / 30,000 = 4. Equal ratios do not establish equal revenue scope. Allocate TRY24,000 to acquisition and TRY6,000 to retention. With 100 distinct valid first-purchase customers, scoped CAC is TRY240.

Suppose non-marketing variable costs are TRY70,000, including product, fulfilment, payment and the defined returns costs. Net revenue already removes refunds, so avoid subtracting them twice. Contribution before marketing is TRY50,000; after all marketing it is TRY20,000. This amount precedes fixed expenses, financing and tax. The arithmetic demonstrates scope, not a forecast or profit guarantee.

A measurement review reconciles orders, values and currency before budget decisions. Two platforms can credit the same sale under different windows. Adding their attributed revenue can therefore exceed unique store revenue. Keep business totals and channel views at their respective levels rather than force them into an additive sales ledger.

Google Ads defines ROAS using conversion value and advertising cost. What you supply as value matters. Check duplicate events, tax treatment, currencies and refund handling. Better capture can increase the reported value without creating additional orders. Date the extraction and identify definition changes before interpreting a trend.

MER reduces reliance on individual channel attribution but does not isolate incrementality. Established customers, organic demand, stock and season affect total revenue. Neither a high ratio nor its historical average guarantees the return on the next budget increase. Use a bounded test, with clear assumptions, to examine that proposed change.

In ecommerce growth planning, an average order value decision should include product mix and contribution. Discounts or shipping subsidies can enlarge a basket while reducing what remains. Compare new and repeat customers separately, then examine variable costs and cash timing. A revenue ratio cannot establish whether the operating plan can fund the next order.

In a separate simplified example, assume a 30% contribution rate before advertising on the same net-revenue basis. The threshold to cover advertising alone is 1 / 0.30 ≈ 3.33. It applies only when attributed value represents that same revenue basis. It does not cover fixed costs, other marketing expenses, financing or tax, and is not a universal target.

Agree targets with finance using actual fulfilment, product and return costs. Repeat-purchase value needs observed cohorts and a stated horizon, not an assumed lifetime multiplier. A strong blended month may hide a loss-making product group. Review that group before expanding discounts or assuming every new customer can be acquired at the current average.

Email retention flows can influence repeat sales without establishing new-customer advertising success. When ROAS rises but new-customer volume falls, inspect audience mix and branded demand. When CAC rises, validate customer identity and acquisition allocations before changing creative or offers. These are investigation paths, not certain causes inferred from one ratio.

Record a budget hypothesis, test scope, owner and stopping condition. Review confirmed customer quality, contribution, stock and cash alongside the three ratios. Set spending boundaries from the business’s own capacity. The decision file should explain what changed, which uncertainty remains and what evidence would justify the next increase or reduction.