How to Calculate LTV and CAC: A Practical Guide for SaaS Founders
I'm not a finance person. I build tools. But somewhere between building and shipping, you realize that if you don't understand unit economics, you're basically flying blind. So I learned. Here's what I wish someone had told me when I started.
The Four Numbers That Matter
LTV (Lifetime Value) — How much profit a customer generates over their entire relationship with you.
CAC (Customer Acquisition Cost) — How much you spend to get one new customer.
LTV:CAC Ratio — Whether your acquisition spending makes sense.
Payback Period — How long until you recoup your acquisition cost.
These aren't investor metrics to game. They're operational reality. If you're spending $500 to acquire a customer who generates $200 in profit over their lifetime, you have a problem whether investors notice or not.
Calculating LTV
The basic formula:
LTV = (Average Revenue Per User × Gross Margin %) / Churn Rate
Let's work through an example:
- MRR: $50,000
- Customers: 100
- ARPU: $500/month
- Gross Margin: 70%
- Monthly Churn: 5%
ARPU = $50,000 / 100 = $500/month
Monthly gross profit per customer = $500 × 0.70 = $350
Customer lifetime = 1 / 0.05 = 20 months
LTV = $350 × 20 = $7,000
Each customer generates $7,000 in gross profit over their lifetime.
Why Gross Margin Matters
LTV uses profit, not revenue. This trips people up.
A company with 90% gross margins has money to reinvest in growth. A company with 40% margins does not, even if they have the same revenue.
For SaaS, gross margin typically includes hosting, third-party services, and payment processing. It doesn't include marketing, sales, or engineering salaries.
Calculating CAC
CAC = Total Acquisition Spend / New Customers Acquired
What's in acquisition spend? The honest answer: whatever you can track that directly led to new customers. This includes paid ads, content marketing (if trackable), events, sales team costs, and tools used for acquisition.
What's not included? Customer support, retention programs, general brand marketing that you can't tie to specific customers.
Blended vs Organic
Most companies track two CAC numbers:
Blended CAC — All spend, all customers Organic CAC — Only inbound/word-of-mouth customers
If organic is a meaningful part of your growth, tracking both reveals your true efficiency. Organic customers who found you through content or referrals cost less than paid customers from Google Ads.
The LTV:CAC Ratio
LTV:CAC = LTV / CAC
Using our example:
- LTV = $7,000
- CAC = $1,000
LTV:CAC = 7:1
The benchmark is generally 3:1. Below that, you're spending too much relative to what customers generate. Above 5:1, you might be underinvesting in growth.
But benchmarks are guidelines, not rules. A 2:1 ratio might make sense if you're in a high-growth phase and proving product-market fit. A 10:1 ratio might mean you're being too conservative.
Payback Period
Payback Period = CAC / Monthly Gross Profit per Customer
Using our numbers: $1,000 / $350 = 2.86 months
You recoup your acquisition cost in under 3 months. That's healthy—12 months or less is generally considered good for SaaS.
Payback period tells you about cash flow. LTV:CAC tells you about long-term efficiency. Both matter.
The Rule of 40
Rule of 40 = Growth Rate (%) + Profit Margin (%)
For profitable companies, use actual margin. For unprofitable ones, analysts often substitute churn rate.
If your growth rate is 15% and your profit margin is 10%, your Rule of 40 score is 25%. Below 40% suggests you need either stronger growth or better profitability.
Where People Go Wrong
Using revenue instead of profit for LTV. A customer paying $500/month isn't worth $500/month. They're worth $500 minus the cost to deliver your service.
Ignoring churn in lifetime. The formula 1/churn gives you average lifetime, but early churn is often higher than late churn. Cohort analysis is more accurate but harder.
Blending inbound and outbound CAC. If you have both, track them separately. Blending masks the fact that one channel might be printing money while the other is bleeding you dry.
Using fully-loaded CAC. Your CAC should include salaries, benefits, tools, and overhead. Underestimating true acquisition cost leads to overestimating efficiency.
Using the Calculator
The SaaS Economics Calculator handles all of this. Input your numbers, get LTV, LTV:CAC, payback period, and Rule of 40 instantly. The hard part isn't the math—it's getting accurate numbers into the tool in the first place.
Start With What You Have
You don't need perfect data to start. Estimate if you have to. The exercise of thinking through these numbers reveals more than the precision of the output.
If you don't know your churn rate, pick a number that's probably wrong and see how sensitive your model is to it. If your LTV:CAC changes from 3:1 to 1:1 when you adjust churn by 2%, you know you need better data before making big decisions.
The metrics aren't the goal. Understanding your business well enough to improve it—that's the goal.