Unit Economics
We pay $280 to acquire a customer who pays $40 a month — are we making money or lighting it on fire? Unit economics answers whether ONE customer is profitable, which decides whether growth helps or kills you.
What you'll learn
- What unit economics means and why it controls whether scaling helps or hurts
- ARPU, gross margin, churn — and how they combine into LTV
- CAC, payback period, and the 3× LTV:CAC rule of thumb
- Why computing LTV on revenue (not gross profit) is the most common — and dangerous — mistake
Before you start
Revenue growing 40% a year looks exciting until you ask: does each individual customer actually make us money? A business can scale rapidly while losing more on every customer it adds. Unit economics is the lens that catches this before it’s fatal.
Unit economics — the revenue and cost associated with a single unit of the business (here, one customer) — tells you whether your fundamental model works. If the unit is profitable, growth is an accelerant. If it isn’t, growth is gasoline on a fire.
The four numbers you need
ARPU — what one customer pays you each month
ARPU stands for Average Revenue Per User per month. It’s the simplest input: if 1,000 customers collectively pay $40,000 a month in subscription fees, ARPU = $40,000 ÷ 1,000 = $40/month.
ARPU is revenue, not profit. To get to profit we need the next piece.
Gross margin — how much you actually keep per dollar of revenue
Gross margin (expressed as a percentage) is the share of revenue left after the direct cost to serve each customer — the servers, support staff time, and payment-processing fees that scale with each user. If ARPU is $40 and those direct costs are $10, gross margin = ($40 − $10) ÷ $40 = 75%.
Monthly gross profit per customer = ARPU × gross margin = $40 × 75% = $30.
That $30 is the money that actually flows toward covering your business — and eventually toward profit.
Churn — how long a customer stays
Churn is the share of customers who cancel (or “churn out”) each month. If 4% of your customers cancel in January, monthly churn = 4%.
Churn controls how long a customer stays, and therefore how many months of gross profit you collect. Because customers leave at a roughly steady rate, the average customer lifetime follows a clean formula:
Average lifetime = 1 ÷ monthly churn rate = 1 ÷ 0.04 = 25 months.
At 4% monthly churn, the average customer sticks around for about two years.
CAC — what it costs to win one customer
CAC stands for Customer Acquisition Cost — the fully-loaded sales and marketing spend needed to win one new customer. “Fully-loaded” means everything: ad spend, sales salaries, commissions, trade-show booths, and the 20% of the CEO’s time spent on sales calls. Total sales + marketing spend last quarter ÷ new customers won = CAC.
For our example: CAC = $280.
LTV — the payoff from one customer’s whole life
LTV (Lifetime Value) is the total gross profit one customer brings from sign-up to cancellation.
LTV = monthly gross profit × average lifetime = $30 × 25 months = $750.
There’s a compact version of the same formula that’s easier to track on a dashboard:
LTV = (ARPU × gross margin) ÷ monthly churn = ($40 × 0.75) ÷ 0.04 = $750.
LTV vs. CAC — the moment of truth
Now we have both sides of the equation:
- LTV = $750 (what one customer is worth)
- CAC = $280 (what one customer costs to acquire)
- LTV:CAC ratio = $750 ÷ $280 ≈ 2.7×
The industry rule of thumb is LTV:CAC ≥ 3×. Why 3×? The 1× just covers the acquisition cost. The second × covers ongoing operating expenses (product, infrastructure, G&A) that aren’t in COGS. The third × is the buffer for risk, seasonality, and capital to fund growth. Below 3× you’re alive but thin; below 1× you lose money on every customer you keep.
At 2.7× this business is thin — not drowning, but not healthy either.
The second metric investors watch is the payback period — the months of gross profit needed to earn back the CAC:
Payback = CAC ÷ monthly gross profit = $280 ÷ $30 ≈ 9.3 months.
The rule of thumb for payback is under 12 months for most SaaS. At 9.3 months this passes, but the LTV:CAC ratio is still marginal.
Try it — fix the business with the explorer
The widget below shows LTV and CAC as bars, with a 3× CAC marker. The health verdict and payback period update live. The defaults match the numbers above — LTV $750 vs. CAC $280, ratio 2.7×, verdict “thin.”
Try each lever in isolation and notice which ones move the ratio most:
- Lower monthly churn from 4% to 2% — what happens to lifetime and LTV?
- Raise ARPU from $40 to $50 — same churn, what ratio do you reach?
- Cut CAC from $280 to $200 — does that alone get you to 3×?
- Improve gross margin from 75% to 85% — how much does LTV shift?
The churn experiment is usually the most surprising: halving churn from 4% to 2% doubles the average lifetime from 25 to 50 months and doubles LTV from $750 to $1,500 — pushing the ratio from 2.7× to 5.4× without touching price or CAC at all. Small churn improvements compound dramatically because churn sits in the denominator of the LTV formula.
The two rules of thumb — and their limits
| Metric | Rule of thumb | Our example | Verdict |
|---|---|---|---|
| LTV:CAC | ≥ 3× | 2.7× | Thin |
| CAC payback | under 12 months | 9.3 months | Passes |
These benchmarks come from decades of SaaS investing and are useful starting points, not laws of physics. A hardware business with long customer lifetimes might accept 18-month payback. A marketplace with negative churn (expansion revenue from existing customers) can operate at lower ratios. Always ask: what assumptions does our industry’s benchmark embed, and do they apply to us?
Unit economics in the wild
When a business is growing fast but burning cash, unit economics is the first question a board asks. If the unit is healthy (LTV:CAC safely above 3×), burning cash to acquire more customers is rational — you’re buying valuable long-term assets. If the unit is underwater, burning cash only digs the hole deeper. Growth doesn’t fix bad unit economics; it accelerates the problem.
This is why seed-stage investors often care more about unit economics than revenue. A $500K/year business with a 5× LTV:CAC ratio is fundable. A $5M/year business with a 0.8× ratio is not — at least not without a plan to fix the unit before scaling it.
Quick check
Next
Averages that lie — why the “average customer” usually doesn’t exist, and how cohort analysis reveals the customers who actually drive your unit economics.
Practice this in an interview
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