Marketing ROI seems simple on paper. In practice, it's the hardest measurement question in marketing because most purchases involve multiple touchpoints across multiple channels over weeks or months. Getting it right requires combining attribution, lifetime value, and honest accounting.
The basic formula
Marketing ROI = (Revenue from Marketing − Marketing Cost) ÷ Marketing Cost
Example: $100,000 in attributable revenue from a $20,000 marketing investment = ($100K - $20K) ÷ $20K = 400% ROI, or 4:1 in ROAS terms.
The attribution problem
When a customer:
- Sees an Instagram ad (no click)
- Searches your brand on Google (organic click)
- Visits your blog 3 weeks later from a LinkedIn post
- Receives 4 newsletter emails over 2 months
- Returns via Google Ads brand search
- Books a demo
- Closes a sale 6 weeks later
...which channel deserves credit for the revenue?
Common attribution models
- Last-click — all credit to the final touchpoint before conversion. Simple but undervalues awareness channels.
- First-click — all credit to the first touchpoint. Overvalues awareness, undervalues closing channels.
- Linear — equal credit across all touchpoints. Fair but doesn't reflect actual influence variation.
- Time-decay — touchpoints closer to conversion get more credit. Often closer to reality.
- Position-based (U-shaped) — first and last touchpoints get most credit (40% each), middle gets remaining 20%.
- Data-driven attribution — algorithmic, based on actual conversion path patterns. GA4's default. More accurate but requires sufficient data.
The lifetime value override
For businesses with repeat purchases or recurring revenue, ROI based on first purchase dramatically underestimates true marketing value. A customer acquired for $200 who generates $2,400 in lifetime value produces 12:1 LTV:CAC even if their first purchase was barely profitable.
What to actually measure
- CAC by channel — total spend on a channel divided by customers acquired from it
- LTV:CAC ratio — lifetime value compared to acquisition cost (3:1+ healthy for subscription; varies by category)
- Payback period — how long until a customer's revenue covers their CAC (under 12 months ideal for SaaS)
- Marketing-influenced revenue — total revenue from customers who had any marketing touch, even if attributed elsewhere
- Marketing-sourced revenue — customers who entered the funnel through marketing channels
- Pipeline contribution — for B2B, how much qualified pipeline marketing generated
The practical answer for most SMBs
- Set up GA4 with proper conversion tracking
- Use UTM parameters on every paid campaign and email link
- Tie at least some conversions to revenue (ecommerce direct; lead-gen via CRM integration)
- Run two attribution models (data-driven for the standard view, first-click to understand awareness channels)
- Calculate ROI quarterly, not weekly — longer windows smooth out noise
- Accept that some channels (PR, brand, awareness content) have hard-to-measure influence that's real but won't show in last-click reporting