Unit economics for startups: CAC, LTV, margin and payback

Why unit economics matter before scale

A business can grow quickly and still lose money on every customer. Growth then accelerates losses rather than fixing them. Unit economics check the underlying arithmetic of one customer: what it costs to get them, what they pay, what it costs to serve them and how long they stay.

Early on, your numbers will be rough and based on small samples. That is fine. The point is to know which numbers matter, track them from the first customer, and spot a structural problem, such as acquisition costs that can never be recovered, before it becomes expensive.

The core metrics and formulas

The table below defines each metric and works through an example for a hypothetical subscription product. Every number is illustrative, chosen to make the arithmetic easy to follow.

Customer acquisition cost

CAC is the total spent on winning new customers divided by the number won in the same period. Include advertising, sales salaries and commissions, tools, content production and agency fees. Excluding staff time is the most common way founders make CAC look better than it is.

Track CAC by channel where you can. A blended figure hides the fact that one channel may be profitable while another is burning money. Also be aware of timing: if your sales cycle is long, spend this month may produce customers next quarter, so compare spend and new customers across a sensible window.

Gross margin and contribution margin

Gross margin is revenue minus the direct cost of delivering the product, as a percentage of revenue. For software, direct costs include hosting, third-party services and sometimes customer support. For physical products, they include materials, manufacturing and shipping.

Contribution margin goes further by subtracting all variable costs that rise with each additional customer, such as payment processing fees, per-customer support and usage-based third-party charges. It tells you how much each customer contributes towards fixed costs like salaries and rent. If contribution margin is thin or negative, adding customers will not make you profitable, no matter how many you win.

Churn, lifetime and LTV

Churn is the share of customers who leave in a period. It drives lifetime, and lifetime drives LTV, so small changes in churn have large effects. In the example, monthly churn of 5% gives an average lifetime of 20 months. If churn rose to 10%, lifetime would halve to 10 months and LTV would halve to 400, turning a comfortable 4 to 1 ratio into a marginal 2 to 1.

Be cautious with LTV early on. With only a few months of history, the formula assumes churn stays constant, which is rarely true. Early customers are often more committed than later ones, so churn can rise as you grow. Treat early LTV as an optimistic estimate and consider capping assumed lifetime, for example at two or three years, rather than trusting a figure that implies customers stay for a decade.

Payback and the LTV to CAC ratio

CAC payback is how many months of margin it takes to recover the cost of acquiring a customer. It matters as much as LTV because it determines how much cash you need to grow. A business with a strong LTV but a long payback has to fund acquisition for many months before each customer becomes profitable, which can exhaust cash quickly.

The LTV to CAC ratio summarises whether acquisition is worth it. A ratio around 3 to 1 is a commonly cited rule of thumb for subscription businesses, but treat it as a rough guide rather than a law. A very high ratio can mean you are under-investing in growth. A ratio near or below 1 to 1 means each customer costs about as much as they ever return, which is unsustainable.

Common unit economics mistakes

• Using revenue instead of margin in LTV — A customer paying 50 a month with high serving costs is worth far less than the revenue suggests.
• Leaving salaries out of CAC — Founder and sales time are real acquisition costs, even if unpaid now.
• Trusting LTV from three months of data — Early churn rarely predicts later churn.
• Ignoring payback — A good ratio with slow payback can still run you out of cash.
• Blending very different customers — Small and large customers often have completely different economics; calculate them separately.
• Forgetting runway — Unit economics show whether each customer works; burn and runway show whether you survive long enough to find out.

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