Unit economics: CAC, LTV, payback and contribution margin

Revenue growth is easy to see and easy to celebrate. Whether each sale actually makes money is harder to see, and it is the question that decides whether growth helps or hurts. Unit economics answers it by looking at one unit, usually one customer or one order, and asking what it brings in and what it costs.

Contribution margin

Contribution margin is the price of a unit minus the variable costs of producing and delivering it. Variable costs are the ones that rise with each sale: materials, packaging, delivery, payment fees, marketplace commission, and the support time each customer needs. Rent, salaries for the core team and software you would pay for anyway are fixed costs and stay out.

Contribution margin is what each unit contributes towards those fixed costs and then profit. If it is negative, selling more makes things worse.

Customer acquisition cost (CAC)

CAC is the cost of winning one new customer. The simple version is marketing spend in a period divided by new customers won in that period. A fuller version adds sales salaries, agency fees and tools used for acquisition. Use the fuller version for decisions, and say which one you are quoting.

Calculate CAC by channel where you can. A blended CAC that includes word-of-mouth customers can hide a paid channel that is losing money.

Lifetime value (LTV)

LTV is the total contribution a customer is expected to bring over the whole relationship. For a subscription, a simple version is monthly contribution per customer multiplied by the expected number of months they stay. For repeat purchases, use average contribution per order multiplied by expected orders.

The most common error is calculating LTV from revenue rather than contribution. That can make LTV look several times larger than it really is.

Payback period

Payback is the number of months it takes for a customer’s contribution to earn back what you spent to win them: CAC divided by monthly contribution. It matters because LTV is a forecast, while payback is about cash. A small business can run out of money waiting for customers to become profitable, however large their eventual LTV.

A worked example (illustration only)

These figures are invented to show the arithmetic.

Had LTV been calculated on revenue (£30 × 20), it would show £600, a quarter higher than the real figure.

How to read the results

• Contribution margin negative: fix price or variable cost before spending on growth.
• Payback longer than your cash runway: growth will drain cash. Slow paid acquisition, raise prices, or ask for annual upfront payment.
• LTV close to CAC: little room for error. Small changes in churn or ad costs can turn it negative.
• One channel much worse than others: move budget, or test whether that channel brings customers who stay longer.

Many investors look for LTV comfortably above CAC, often quoting around three to one as a rough guide. Treat that as a conversation starter, not a rule, and look at payback alongside it.

Mistakes to avoid

• Using revenue instead of contribution in LTV.
• Leaving payment fees, returns, discounts or support out of variable costs.
• Counting only ad spend in CAC and ignoring sales time and agency fees.
• Assuming customer lifetime from a few months of data. Early customers often behave differently from later ones.
• Calculating once and never again. Prices, ad costs and churn all move.

Where to start

1. Pick one unit and define it clearly.
2. List variable costs and calculate contribution margin.
3. Calculate CAC for your biggest channel.
4. Work out payback in months.
5. Only then estimate LTV, and show it as a range.

Which lever to pull

Once the numbers are on one page, each weak spot points to a specific fix. To improve contribution margin, raise prices, cut payment or delivery costs, or reduce the support each customer needs. To lower CAC, move budget to the channels that convert best, improve the landing page, or ask happy customers for referrals. To shorten payback, offer annual plans paid upfront, or charge a setup fee that covers part of acquisition. To raise LTV, work on why customers leave: talk to the ones who cancelled and fix the most common reason first. Change one lever at a time, so you can see which one moved the numbers.