SaaS businesses do not die from bad products; they die from inverted unit economics — paying $600 to acquire a customer worth $400. CAC, LTV, and churn are one equation, not three KPIs. Here is the math that decides whether growth is an engine or a bonfire.
1. CAC: The Fully-Loaded Price of a Customer
CAC is total sales and marketing spend divided by new customers acquired — fully loaded: ads, salaries, commissions, tools, agency fees. A startup spending $120,000/month on S&M to win 200 customers has a CAC of $600, no matter what the ad dashboard claims the "cost per click" was.
The common failure is partial accounting: counting ad spend but not the team running it. If your "CAC" excludes salaries, it is not CAC — it is a media metric wearing a costume.
2. LTV: Revenue Is Vanity, Margin Times Lifetime Is Sanity
Lifetime value is not "revenue until they leave" — it is gross profit until they leave. A $50/month customer at 80% gross margin contributes $40/month; at 2% monthly churn, the average lifetime is 1/0.02 = 50 months, and LTV is $40 × 50 = $2,000.
// Core unit economics (per customer)
LTV = ARPU * gross margin / monthly churn
// e.g. $50 ARPU, 80% margin, 2% churn:
// LTV = 50 * 0.8 / 0.02 = $2,000
//
// CAC = total sales + marketing spend / new customers
// Payback (months) = CAC / (ARPU * gross margin)
//
// Rules of thumb: LTV/CAC >= 3 | payback <= 12 monthsNotice what the formula exposes: churn sits in the denominator. Halving churn doubles LTV without touching price, sales, or product marketing. This is why retention work compounds and acquisition work merely adds.
CAC & LTV CalculatorLTV/CAC ratio and payback from your ARPU, margin, churn, and S&M spend — the investor-grade sanity check.→SaaS LTV & Churn CalculatorLTV under different churn scenarios — watch what a single point of monthly churn does to lifetime value.→3. The Ratio That Decides: LTV/CAC ≥ 3, Payback ≤ 12 Months
The venture-grade heuristics exist for a reason. LTV/CAC below 1 means you burn money on every customer — growth accelerates the bleeding. Between 1 and 3 the model works but is fragile: any churn spike or CAC inflation erases the margin of safety. Above 3 with a payback under 12 months, growth spend is a machine: put a dollar in, get three out within the customer’s lifetime, with the first one back inside a year.
Above 5 is not "great" — it usually means you are under-investing in growth and leaving the market to competitors. The ratio is a steering wheel, not a trophy.
4. Churn Compounds: The Monthly-to-Annual Trap
Monthly churn is quoted small because it sounds small. 3% monthly churn is 31% annual (1 − 0.97¹²); 5% monthly is 46% annual — you replace nearly half your customer base every year just to stand still. At 8% monthly, the base fully turns over annually and the business is a treadmill, not a flywheel.
The operational threshold: consumer subscriptions live with 5–7% monthly; SMB SaaS must stay under 3%; enterprise with annual contracts needs logo churn in single digits per year. If your monthly churn is above 4%, stop buying growth and fix the leak — every acquired dollar is pouring into a bucket with a hole.
5. From Unit Economics to Company Economics
Unit economics scale up into the two numbers operators actually run: contribution margin (does each customer pay back before they leave?) and the break-even point (at how many customers does the machine cover fixed costs?). A $40/month contribution per customer against $100,000/month of fixed costs means the company needs 2,500 paying customers before it earns its first dollar of operating profit.
Break-Even CalculatorFixed costs vs per-unit contribution — the customer count where your SaaS stops burning and starts earning.→Profit Margin CalculatorGross, operating, and net margin from any cost/price pair — the m in the LTV formula, verified.→The Monthly Ritual
- Recompute CAC fully loaded — include salaries, or the number lies.
- Recompute LTV with trailing 3-month churn; a single month of churn data is noise.
- Watch payback period drift — it degrades silently as ad auctions inflate.
- Convert monthly churn to annual in every deck; honesty compounds too.
- When in doubt: retention first, acquisition second. The denominator decides.