The formula
Blended ROAS = Total revenue (all sources)
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Total ad spend (all networks)The numerator is the important half. It has to come from one place that sees every sale — your store, your payment processor, or your subscription platform. The moment you build it by summing what each ad platform reports, you have counted some conversions two or three times.
Why the two numbers never match
Meta's default attribution is 7-day click and 1-day view. Google UAC models conversions across a 30-day window. TikTok has its own. A user who saw a Meta ad on Monday, tapped a Google ad on Wednesday, and bought on Friday shows up as a conversion in both accounts — and neither platform is wrong by its own rules.
Sum them and you get a number larger than your bank statement. The gap is usually 20–60% for accounts running four or more networks. It widens as you add networks, which means the error grows exactly as the allocation decision gets harder.
Blended ROAS and MER
They're the same ratio. MER — Marketing Efficiency Ratio — is the term used when the whole marketing budget is treated as one input against total business revenue. Blended ROAS is the term used when comparing against paid spend specifically. Pick one and use it consistently; the arguments come from two people using different denominators.
A note for subscription apps
If you sell subscriptions, blended ROAS on a calendar month will understate you badly. Revenue from a March cohort keeps arriving in April, May, and June, while March spend is fully booked in March. Use a cohort view instead — spend and revenue both anchored to install date, measured at a fixed day count. That method is covered in D30 realized LTV, and the store-commission correction in how to calculate true ROAS from RevenueCat.
Common questions
- What is blended ROAS?
- Blended ROAS is total revenue divided by total ad spend across every channel, measured against revenue your business actually recorded rather than what each ad platform claims. It is the same calculation as MER (Marketing Efficiency Ratio).
- How is blended ROAS different from platform ROAS?
- Platform ROAS is calculated inside a single ad account using that platform's own attribution window. Because those windows overlap, the same conversion can be claimed by Meta, Google, and TikTok at once. Blended ROAS uses one revenue figure for the whole business, so nothing is counted twice.
- What is a good blended ROAS?
- It depends on margin and business model. An e-commerce brand with a 60% gross margin breaks even near 1.7x. A subscription app buying a renewal stream may run below 1.0x on day 30 and still be profitable by day 180, because the first month captures only part of the customer's revenue.
- Should I subtract the app store commission?
- Yes, if you sell through the App Store or Google Play. Apple and Google keep 30%, or 15% under the small-business programs and on subscriptions after the first year. ROAS calculated on gross revenue overstates returns by roughly that amount.
- Why is the sum of my platforms' revenue higher than my real revenue?
- Overlapping attribution windows, view-through conversions, and modeled conversions mean multiple platforms claim credit for the same purchase. The gap between the summed platform figure and your real revenue is the over-attribution gap — it is normal, and it is why budget should not be allocated on platform-reported ROAS alone.