The LTV:CAC ratio is the most quoted number in SaaS and the most frequently miscalculated. Two errors are almost universal, and together they can double the reported figure.
#Error 1: computing LTV on revenue instead of gross profit
The version most people use:
LTV = ARPA / churn rateThe correct version:
LTV = (ARPA × gross margin) / churn rateLifetime value is the profit a customer generates, not the revenue they produce. Hosting, third-party APIs, payment processing, support and customer success all consume part of every dollar collected.
At a 78% gross margin the revenue version overstates LTV by 28%. For an infrastructure-heavy or support-heavy product running 60% margins, it overstates by 67%. Any deck reporting an LTV:CAC above 5:1 should be checked for this before anything else.
This matters more now than it did five years ago, because AI inference costs are pushing gross margins down in products that previously ran at 85%. If a meaningful share of your cost of goods sold is model inference, size it with the LLM API cost calculator and feed the resulting margin into your LTV.
#Error 2: understating CAC
CAC should include everything spent to acquire a customer:
- All paid acquisition spend
- Fully loaded salaries and commission for sales and marketing
- Marketing tooling and agency fees
- Content, events and sponsorships
- The portion of customer success required to get a customer live and productive
That last one is contested and it should not be. If a customer cannot reach value without a dedicated onboarding specialist, that specialist's cost is part of acquisition, not retention.
Excluding a sales development team, or treating a founder's selling time as free, produces a CAC that flatters the ratio and misleads the plan built on it.
#Getting churn right for the LTV denominator
1 / churn rate gives average customer lifetime, and it is highly sensitive to the churn figure you use.
- 2% monthly churn implies a 50-month lifetime
- 3% monthly churn implies 33 months
- 5% monthly churn implies 20 months
A one-point error in churn moves LTV by 30% or more. Two refinements are worth making:
Use revenue churn, not logo churn, when revenue is concentrated. If your top decile of customers represents 60% of revenue, logo churn dramatically understates the risk.
Cap the projection. Extrapolating 1% monthly churn to a 100-month lifetime assumes a customer relationship lasting more than eight years, which is unverifiable for a company that is three years old. Capping at 36 or 60 months produces a defensible figure that will survive diligence.
#Reading the corrected ratio
Below 1:1 — you lose money on every customer. Stop scaling acquisition and fix pricing or retention.
1:1 to 3:1 — inefficient. The business survives but cannot fund its own growth. Look at channel mix, sales cycle and win rate before adding headcount.
3:1 to 5:1 — healthy. The band most venture-backed SaaS targets.
Above 5:1 — frequently a sign of underinvestment. If each customer returns seven times acquisition cost in gross profit, you could profitably spend considerably more. Founders often read a high ratio as validation when it is really an unexploited opportunity.
#The ratio does not tell you about cash
Two businesses can both show 4:1 and have completely different funding requirements, because LTV:CAC says nothing about when the money arrives.
CAC payback period does:
CAC payback (months) = CAC / (ARPA × gross margin)Under 12 months is strong for SMB, under 18 acceptable for enterprise. Beyond 24 months the business structurally cannot self-fund growth — each new customer consumes cash for two years before contributing any, so growing faster burns cash faster.
Report both. LTV:CAC tells you whether the business model works. Payback tells you how much capital it needs to prove it.
#Segment before you conclude
An aggregate ratio across self-serve and enterprise is usually meaningless, because the two have entirely different acquisition costs, contract values and retention. A blended 3.5:1 might be enterprise at 6:1 and self-serve at 1.4:1 — which is a completely different business to run.
Run the SaaS metrics calculator once per segment and act on the segment-level numbers.
Frequently asked questions
What is a good LTV to CAC ratio?
Three to one is the widely accepted floor, meaning each customer returns three times acquisition cost in gross profit. Below three suggests unsustainable spend; above five often signals underinvestment in growth rather than excellence.
Should customer success costs be in CAC or COGS?
Split it. Onboarding and implementation work required to get a customer to first value belongs in CAC. Ongoing account management and support belongs in cost of goods sold, which reduces gross margin.
How do I calculate LTV without enough history?
Use current churn to estimate lifetime, but cap the projection at 36 or 60 months rather than extrapolating to a decade. A capped, defensible figure is far more useful in diligence than an optimistic one.