What the LTV:CAC ratio measures
The LTV:CAC ratio compares how much gross profit a customer is expected to generate over their lifetime (LTV) with how much it cost to acquire that customer (CAC). A ratio of 4:1 means each dollar of acquisition spend is expected to return four dollars of gross profit over time. It is one of the most quoted SaaS unit-economics metrics, and one of the easiest to inflate by accident.
LTV:CAC = customer lifetime value ÷ customer acquisition cost
The ratio is only as reliable as its two inputs. LTV depends on a churn estimate that is often based on too little history, and CAC depends on which costs and which customers you include. This guide shows how to calculate each input, works through an example, explains the common rule of thumb and its limits, and argues that CAC payback period is often the more practical number for small software businesses.
To run your own numbers, use the LTV:CAC calculator.
How to calculate CAC
CAC = total sales and marketing cost in a period ÷ new paying customers acquired in that period
Include:
- Paid advertising and sponsorships.
- Sales and marketing salaries, contractors, and commissions (including a fair share of founder time if you want CAC at scale).
- Marketing and sales software, content production, and affiliate payouts.
Exclude costs that serve existing customers, such as customer success for renewals, unless you are deliberately calculating a fully loaded figure.
Blended vs paid CAC. Blended CAC divides all acquisition spending by all new customers, including organic signups. Paid CAC divides paid-channel spending by customers from those channels. Andreessen Horowitz recommends looking at paid CAC because blended figures, inflated by organic users, can hide whether paid campaigns actually work, and notes that acquisition costs usually rise as you try to reach larger audiences (a16z, "16 Startup Metrics").
Timing. If your sales cycle is weeks or months long, spending in one month produces customers in a later one. Use a quarterly window or lag spending by your typical cycle.
How to calculate LTV
The simplest common SaaS formula is:
LTV = (ARPA × gross margin %) ÷ monthly revenue churn rate
Where:
- ARPA is average monthly revenue per account: normalized MRR divided by paying accounts. If you take MRR from Stripe Billing, note that Stripe counts active and past-due subscriptions, excludes trialing subscriptions and one-time invoices, and normalizes annual plans to a monthly amount (Stripe Support, "Understanding MRR and ARR").
- Gross margin is
(revenue − cost of revenue) ÷ revenue, including hosting, third-party APIs, payment fees, and delivery support. - Monthly revenue churn is the share of starting MRR lost to cancellations and downgrades in a month.
Dividing by churn reflects the average customer lifetime: average lifetime (months) = 1 ÷ monthly churn. David Skok illustrates this with 3% monthly churn giving an average lifetime of roughly 33 months (For Entrepreneurs, "SaaS Metrics 2.0").
Why gross margin matters. Revenue-based LTV ignores the cost of serving the customer. A product with large AI inference or data costs can have a revenue LTV that looks excellent while the gross-profit LTV is modest. The a16z article goes further and argues that LTV should reflect the net profit of the customer, warning that revenue or gross-margin LTV suggests a higher limit on acquisition spend than the business can afford (a16z). At minimum, use gross margin.
Expansion revenue. If customers reliably upgrade, the simple formula understates LTV. Some teams use net revenue churn (churn minus expansion) in the denominator, but that formula breaks when net churn is zero or negative, producing an infinite LTV. Cap the customer lifetime (for example at 36 or 60 months) or model cohorts explicitly. See net revenue retention.
Worked example
A self-serve B2B tool has these quarterly figures:
- New paying customers: 120
- Paid ads: $9,000
- Content contractor: $4,500
- Marketing tools: $1,500
- Founder time on marketing: excluded for now
- ARPA: $60 per month
- Gross margin: 82%
- Monthly revenue churn: 3%
CAC:
CAC = ($9,000 + $4,500 + $1,500) ÷ 120 = $125
LTV:
LTV = ($60 × 0.82) ÷ 0.03 = $49.20 ÷ 0.03 = $1,640
LTV:CAC:
$1,640 ÷ $125 = 13.1:1
That looks exceptional, which is a signal to check the inputs. Suppose you add an honest founder-time cost of $10,000 per quarter: CAC becomes $25,000 ÷ 120 ≈ $208, and the ratio falls to about 7.9:1. Now suppose churn for new customers is really 5% because older, loyal customers are masking early churn: LTV becomes $49.20 ÷ 0.05 = $984, and the ratio is about 4.7:1. The same business can report 13:1 or under 5:1 depending on two assumptions. Document both.
CAC payback period
CAC payback (months) = CAC ÷ (ARPA × gross margin %)
In the adjusted example: $208 ÷ $49.20 ≈ 4.2 months.
Payback answers a simpler question: how long until the gross profit from a new customer repays what it cost to acquire them? It does not depend on a long-range churn forecast, so it is harder to inflate, and it directly drives cash needs. A bootstrapped company with short payback can fund growth from revenue; one with a long payback needs capital or patience. Model the cash impact with the startup runway calculator.
What is a good LTV:CAC ratio?
The most widely cited guideline comes from David Skok: LTV should be more than 3 times CAC, and months to recover CAC should be under 12 (For Entrepreneurs). Skok notes that the 3x guideline assumes a simple LTV calculation and gross margins of 80% or higher, and that enterprise "land and expand" businesses can reasonably run longer paybacks.
How to read your ratio:
- Below 1:1 — you lose money on each customer even before overhead. Fix retention, pricing, or channels before spending more.
- Around 1–3:1 — thin. Often acceptable early while you are learning, but growth will consume cash.
- Around 3–5:1 — commonly viewed as healthy for venture-style SaaS, if the inputs are honest.
- Well above 5:1 — either very efficient or, frequently, under-investing in growth or underestimating CAC or churn.
These are guidelines, not laws. A very high ratio for a profitable micro-SaaS can be perfectly rational if the founder prefers profit over growth.
Common mistakes
- Using too little history for churn. Churn measured over three months of a young cohort is noisy. Show the observation period.
- Using customer churn when revenue differs by plan. If large accounts churn less than small ones, logo churn and revenue churn diverge. Use revenue churn for revenue-based LTV. See how to calculate churn rate.
- Ignoring gross margin. Revenue LTV overstates value, especially for AI and data-heavy products.
- Blending organic and paid customers. Organic customers make paid channels look cheaper than they are.
- Ignoring founder time. Unpaid founder sales effort is a real cost a buyer would have to replace.
- Using net churn below zero without a lifetime cap. This produces infinite LTV.
- Using gross receipts instead of net revenue. Refunds and disputes reduce realized revenue. See gross vs net revenue.
How buyers and investors use LTV:CAC
Investors use the ratio to decide whether more capital will produce efficient growth. Acquirers of small SaaS businesses typically care more about whether existing marketing channels will keep working after a sale and what they cost. In diligence, expect requests for channel-level spend, cohort retention tables, and the raw data behind ARPA. A verified revenue profile can support the revenue side of the calculation, but RevenueBug does not see marketing spend or verify CAC, churn quality, or margins (methodology). For broader context see the SaaS metrics reference and startup buyer due diligence.
Frequently asked questions
What is the LTV:CAC formula?
LTV:CAC = LTV ÷ CAC, where LTV = (ARPA × gross margin) ÷ monthly revenue churn and CAC = sales and marketing spend ÷ new customers. State the period and whether CAC is blended or paid.
Is a 3:1 LTV:CAC ratio good?
It is the most common rule of thumb for healthy SaaS unit economics, popularized by David Skok. Treat it as a starting point; payback period, cash position, and growth goals matter as much.
Should LTV use revenue or gross profit?
Gross profit at minimum. Revenue-based LTV ignores delivery costs and can justify acquisition spending the business cannot afford.
How do I calculate LTV with very low churn?
Cap the customer lifetime at a defensible period, such as 36 to 60 months, or build LTV from actual cohort revenue curves. Very low churn measured over a short period is unlikely to hold indefinitely.
Is CAC payback better than LTV:CAC?
They answer different questions. Payback tells you how quickly acquisition spending returns cash and depends less on forecasts. LTV:CAC estimates long-run return. Small, self-funded companies often find payback more actionable.
Does LTV:CAC apply to one-time-purchase products?
Partially. With no recurring revenue there is no churn-based lifetime. Use expected gross profit per customer, including repeat purchases you can observe, divided by CAC.