SaaS Unit Economics Explained: The Numbers That Determine If Your Business Is Viable
Unit economics tell you whether each customer makes or loses you money. Here's how to calculate contribution margin, magic number, and the efficiency metrics that separate fundable startups from money pits.
What Are Unit Economics?
Unit economics answer one question: do you make money on each customer, or lose it? It sounds basic, but a shocking number of startups — even funded ones — can't clearly answer this.
At the simplest level: if you spend $100 to acquire a customer who generates $300 over their lifetime and costs $50 to serve, your unit economics are positive ($300 - $100 - $50 = $150 profit per unit). If those numbers were reversed, you'd be burning cash on every customer you add.
The Key Unit Metrics
Gross Margin
Gross Margin = (Revenue - Cost of Goods Sold) ÷ Revenue
For SaaS, COGS includes hosting, infrastructure, payment processing, and direct support costs — everything required to deliver the product. Healthy SaaS gross margins are 70-85%. Below 65% is a red flag that usually means your infrastructure costs don't scale well or you're including too much manual service.
Contribution Margin
Goes deeper than gross margin by including sales and marketing costs per customer:
Contribution Margin = (LTV × Gross Margin) - CAC
If LTV is $1,000, gross margin is 80%, and CAC is $200, contribution margin is ($1,000 × 0.80) - $200 = $600 per customer. That's the actual profit contribution after acquiring and serving each customer.
The Magic Number
Measures sales and marketing efficiency for growth:
Magic Number = Net New ARR (this quarter) ÷ Sales & Marketing Spend (last quarter)
| Magic Number | Meaning | Action |
|---|---|---|
| Below 0.5 | Inefficient — spending too much for too little growth | Fix funnel, reduce CAC, improve product |
| 0.5 - 0.75 | Moderate — room for improvement | Optimize channels, test new approaches |
| 0.75 - 1.0 | Good — efficient growth | Scale carefully, maintain efficiency |
| Above 1.0 | Excellent — invest more aggressively in growth | Pour fuel on the fire |
CAC Payback Period
Payback = CAC ÷ (ARPU × Gross Margin)
If CAC is $150, ARPU is $40/month, and gross margin is 80%, payback is $150 ÷ ($40 × 0.80) = 4.7 months. Anything under 12 months is healthy. Under 6 is excellent. Over 18 means you need to fix either pricing, acquisition costs, or both.
How Unit Economics Change Over Time
Unit economics should improve as you scale. If they don't, something is wrong:
- CAC should decrease as brand awareness grows and organic channels mature
- LTV should increase as you reduce churn and add expansion revenue
- Gross margin should improve as infrastructure costs spread across more users
If unit economics worsen with scale, you have a structural problem — usually pricing too low for the service level required, or CAC rising faster than LTV.
Modeling Unit Economics
The InnovexFlow Revenue Modeler calculates LTV, LTV:CAC ratio, and payback period automatically across all three scenarios. The data table shows these metrics at each year mark so you can verify your unit economics improve over the 5-year projection.
For the complete picture of how unit economics fit into your financial model, start with your revenue model inputs and work through the projections to see contribution margins at scale.