The short answer
ROAS (return on ad spend) compares the conversion value credited to an ad, campaign, or channel with the cost of those ads. MER (marketing efficiency ratio) compares total business revenue with total marketing spend over the same period. ROAS helps you diagnose campaign performance; MER shows the blended relationship between marketing investment and revenue. Neither metric, on its own, measures profit or proves that marketing caused the sales.
What is ROAS, and how do you calculate it?
ROAS = revenue or conversion value attributed to ads ÷ ad spend.
If a paid-search campaign spends $80,000 and the ad platform credits it with $400,000 of purchase value, its reported ROAS is 5.0x (or 500%). That means the platform reports $5 in conversion value for every $1 spent on those ads. In Google Ads, the underlying measure is conversion value divided by cost; conversion value can represent purchases or values assigned to other actions. A lead assigned an estimated value is not the same thing as collected revenue.
ROAS is useful for comparing campaigns within a consistent tracking setup. Its numerator depends on conversion tracking, attribution rules, time windows, and the values passed to the platform. Two platforms may each claim credit for the same purchase, so adding their reported revenue figures can double-count sales.
What is MER, and how do you calculate it?
MER = total business revenue ÷ total marketing spend.
If the business records $1,200,000 in revenue and spends $300,000 on marketing during the same month, MER is 4.0x. The business recorded $4 in revenue for every $1 of marketing spend. MER is sometimes called blended ROAS, but state exactly what is included in its denominator: some teams count only paid media, while others also count creative, agencies, software, and marketing staff. Those versions should not be compared as if they were identical.
Use a consistent revenue definition too. For an ecommerce business, net sales after discounts and returns often give a more useful operating view than gross order value; decide separately whether shipping and tax belong in the numerator. For a B2B business, distinguish recognized revenue, signed contract value, invoiced revenue, and pipeline value. Pick the measure that matches the decision, label it, and use the same basis each period.
MER vs ROAS: the key differences
Revenue measured
- ROAS: Conversion value attributed to specific ads.
- MER: Total business revenue, including sales not attributed to an ad platform.
Spend measured
- ROAS: The spend on the measured ads, campaign, or channel.
- MER: The defined total marketing spend across the business.
Best use and main limitation
- ROAS helps diagnose campaigns, but attribution can miss or duplicate sales.
- MER tracks blended business efficiency, but cannot identify which activity drove a change.
- Neither ratio measures profit.
The two metrics answer different questions. A strong ROAS can coexist with a weakening MER if other marketing costs rise faster than total revenue, if channels claim the same orders, or if the business relies heavily on existing customers. Conversely, MER may improve while one campaign's reported ROAS falls because organic demand, retention, pricing, or another channel changed. A ratio alone does not establish the cause.
A worked revenue example
Suppose one month produces the following results (all figures are illustrative):
- Net business revenue: $1,200,000.
- Total marketing spend: $300,000, including $200,000 in paid media and $100,000 in creative, agency, and tool costs.
- Paid-search spend: $80,000, included in the paid-media total.
- Purchase value attributed to paid search: $400,000.
Paid-search ROAS = $400,000 ÷ $80,000 = 5.0x. MER = $1,200,000 ÷ $300,000 = 4.0x. The $400,000 is a platform-attributed slice of activity, not an extra amount to add to the $1,200,000 in business revenue. The $80,000 is already part of the $300,000 marketing total; do not count it twice.
These calculations describe revenue relationships, not earnings. If the business has a 35% contribution margin before marketing, the $1,200,000 in revenue leaves $420,000 after the variable costs covered by that margin. Subtracting $300,000 in marketing leaves $120,000 before fixed overhead and other costs. At a 25% pre-marketing contribution margin, the same 4.0x MER would leave no contribution after marketing. This is why a universal “good MER” or “good ROAS” target is unreliable.
How the ratios affect revenue planning
You can rearrange the formulas to set a revenue hurdle:
- Revenue required at a target MER = target MER × planned total marketing spend. A 4.0x target on a $300,000 marketing budget implies a $1,200,000 total-revenue goal.
- Attributed conversion value required at a target ROAS = target ROAS × planned ad spend. A 5.0x target on $80,000 of ads implies $400,000 in attributed conversion value.
These are planning identities, not forecasts. A target ratio does not make revenue happen, and attributed value is not necessarily incremental revenue. Forecast using historical performance, sales capacity, seasonality, margin, and the delay between spend and sale. When testing a budget increase, watch incremental revenue and contribution: did the extra spend produce enough additional business value after relevant costs?
A practical measurement framework
- Choose one revenue source of truth. Use finance, commerce, or CRM records for total revenue; document how refunds, discounts, tax, shipping, and currency conversion are handled.
- Define marketing spend once. Decide whether MER includes media only or the full marketing function. Keep campaign spend inside that total, never alongside it as an additional cost.
- Align time periods. Compare revenue and spend over the same window, then account for returns and delayed sales. In long-cycle B2B, compare cohorts or rolling periods rather than judging one week's spend against one week's closed revenue.
- Use ROAS to investigate channels. Check conversion-value quality, attribution settings, new versus returning customers, and whether campaigns are reaching the intended audience.
- Use MER with margin to make business decisions. Review total revenue, contribution margin, customer acquisition cost, repeat purchase or expansion revenue, and cash payback before scaling the budget.
For B2B teams, a platform may optimize to lead submissions while the business earns revenue months later. Report lead or pipeline value separately from closed-won or recognized revenue. Calling a lead-value ROAS “revenue ROAS” makes the comparison with MER misleading.
Which metric should guide your next decision?
Use ROAS when deciding which campaign, creative, audience, or paid channel needs attention. Use MER when asking whether the whole marketing system is producing enough revenue relative to its cost. For a scale decision, put both alongside contribution margin and a measure of incremental growth. The goal is not to make one ratio look impressive; it is to turn marketing spend into sustainable business value.
Sources and further reading
- Google Ads Help: Glossary and About conversion values.
- Shopify: Marketing Efficiency Ratio and Finance Reports.