Unit economics is the profit math on one unit of your business, usually one customer, one subscription or one order. You look at what it costs to acquire that unit, what contribution margin it leaves after variable costs, how much lifetime value it might produce, and how long it takes to earn the acquisition cost back. Mercury’s explainer calls this measuring profitability on a per-unit basis. Paddle’s educational pages walk through the same four pieces: customer acquisition cost (CAC), lifetime value (LTV), the LTV-to-CAC relationship, and CAC payback.
You’ll also hear contribution margin, LTV:CAC, or “the unit is profitable.” Those phrases only help if the unit is defined, the costs are fully loaded, and the average is not hiding a bad channel. This is education, not a fundraising forecast or a software recommendation. I’m not picking a billing tool or a benchmark that every company must hit.
Start with the unit, then contribution margin
Pick a unit that matches how you actually get paid. For a subscription product, that is usually a customer or an account. For commerce, it may be an order or a repeat buyer. If you mix those together, the ratios will lie to you.
Contribution margin is what is left from that unit’s net revenue after the costs that rise when you sell or serve it. Hosting that scales with usage, payment processing, delivery, usage-based contractors, and similar costs belong here. Your rent, your own salary as a fixed draw, and a brand campaign that would run anyway usually do not. Contribution margin per unit equals net revenue minus those variable costs. The percentage is that remainder divided by net revenue.
Gross profit and contribution margin are cousins, not twins. Gross profit subtracts cost of goods sold. Contribution often includes other variable costs of serving the customer. If you use one word for the other, payback and LTV will be too optimistic.
CAC: what it really cost to win the customer
CAC is the sales and marketing cost of acquiring new customers in a period, divided by the number of new customers you actually acquired. Paddle’s formula is the standard one: total sales and marketing cost divided by customers acquired. The argument is over the numerator.
Fully loaded CAC should include ads, agency fees, sales salaries and commissions, tools used to sell, events, and the discounts you gave to win the account, if you are going to treat those as acquisition costs. It should not quietly leave out the sales team and then look efficient. Use the same time window for spend and for new customers, and be honest about lag: a campaign you paid for in March may produce April customers.
Organic or word-of-mouth customers still have a cost, even if it is smaller. Blending a cheap organic cohort with an expensive paid cohort into one company-wide CAC is how founders convince themselves that “average CAC is fine” while the paid channel is underwater.
LTV and payback, with the assumptions in the open
Lifetime value, for unit economics, should be about contribution, not just revenue. A customer who pays a lot and costs a lot to serve is not as valuable as a revenue number suggests. One common subscription shortcut, when monthly churn is fairly stable, is:
LTV = average monthly revenue per customer × contribution margin percentage ÷ monthly logo churn.
If monthly revenue is $80, contribution margin is 80%, and monthly churn is 4%, then monthly contribution is $64, and LTV is $64 ÷ 0.04, or $1,600. That shortcut assumes churn and contribution stay roughly steady, and that expansion, contractions and one-year contracts are not distorting the average. If those assumptions fail, the formula is a slide, not a measurement. Observed retention by cohort is better than a guessed lifetime.
CAC payback is how long it takes the contribution from that customer to repay what you spent to get them. For a subscription, a simple version is CAC divided by monthly contribution margin per customer. Using the numbers above, a $400 CAC and $64 of monthly contribution implies a payback of 6.25 months. Paddle describes payback this way, and Mercury is right that the clock matters because cash leaves today and comes back later. A pretty LTV does not pay next month’s payroll if payback is 18 months and you do not have the cash.
Cash collection can move the clock. If customers pay a year up front, cash payback can be faster than a monthly accrual formula. If invoices sit for 60 days, it is slower. And if contribution per unit is negative, no CAC target will save the model. You lose more money on every additional customer until price, cost or mix changes.
A worked example that stays in one framework
Here is a labeled teaching example, not a real company’s books. A subscription product charges $80 a month. Variable costs to serve one customer are $16 a month, so contribution margin is $64, or 80%. In a quarter, the company spends $40,000 on fully loaded sales and marketing and adds 100 new paying customers, so CAC is $400. Monthly logo churn in the mature cohorts is 4%.
- Monthly contribution per customer: $64
- Simple LTV: $64 ÷ 0.04 = $1,600
- LTV:CAC: $1,600 ÷ $400 = 4
- CAC payback: $400 ÷ $64 = 6.25 months
Now split the same quarter, and keep the $64 monthly contribution. Sixty customers came from a paid campaign at a $600 CAC and churn at 6% a month. Forty came from referrals at a $100 CAC and churn at 2%. The company-wide CAC is still $400, so a blended ratio can still look healthy. The paid LTV is $64 ÷ 0.06, or about $1,067, so that channel’s LTV:CAC is about 1.8, and payback is 9.4 months. The referral LTV is $64 ÷ 0.02, or $3,200, so LTV:CAC is 32, and payback is under two months. The referrals are doing the heavy lifting. If you scale only the paid channel, you do not get the blend. You get the paid math.
There is no official “good” ratio
You will hear that a healthy LTV:CAC is 3:1, and that payback should come in under a year. Paddle’s educational material cites 3:1 as a SaaS benchmark, and similar rules of thumb show up all over investor decks. They are heuristics, not laws, and they break if LTV was built on a hopeful lifetime, if CAC was not fully loaded, or if cash collection lags billing.
A ratio above 1 only means the model can work if the assumptions hold. A ratio of 8 can still be a poor business if it takes four years to get the money back and you are funding that wait with expensive capital. A lower ratio can be acceptable early, or in a market where you are still learning, as long as you know you are buying information and you have the runway to support it. I would rather see one cohort table with real retention and a payback clock than a single company-wide ratio on a slide.
Investors read this math as a quality-of-growth test, which is why it belongs in the same conversation as what investors want to see in an early-stage product: evidence that the product is retained, not only that it can be sold once. And if you are bootstrapping a startup without investors, payback is even less of a vanity metric. A long recovery period can starve a company that does not have a fund to bridge the gap, even when the lifetime-value slide looks generous.
Price, retention and what you put in the CAC numerator are the levers. If contribution is thin, cheaper ads may not fix it. If churn is high, a bigger top-of-funnel spend will only buy a shorter relationship. Get the unit right, show the assumptions, and reread the numbers by channel and by cohort before you scale.
Frequently Asked Questions
What is unit economics?
Unit economics is the profit math on one unit of the business, usually a customer or an order. It ties together acquisition cost, contribution margin, lifetime value and the time it takes to recover CAC. The unit has to match how you actually get paid.
How do you calculate CAC?
Divide fully loaded sales and marketing costs for a period by the new customers acquired in that period. Include ads, sales pay, agencies, tools and, if you treat them as acquisition costs, discounts. Use the same window for spend and for new customers.
What is the difference between contribution margin and gross margin?
Gross margin is revenue minus cost of goods sold. Contribution margin is revenue minus the variable costs of selling and serving that unit, which can include more than COGS, such as payment fees or usage-based support. They are related, but not always the same number.
How is LTV calculated for a subscription business?
A common shortcut is monthly revenue times contribution margin percentage, divided by monthly churn. It assumes fairly stable churn and contribution. Cohort retention is more reliable than a guessed lifetime, especially when contracts, expansion or early churn distort the average.
What is CAC payback?
CAC payback is how long it takes a customer’s contribution to repay the cost of acquiring them. For subscriptions, a simple version is CAC divided by monthly contribution per customer. It is a cash question, not just a profitability question.
Is a 3:1 LTV to CAC ratio required?
No. 3:1 is a widely cited SaaS rule of thumb, not an official target. The ratio is only as honest as the LTV and CAC behind it, and a long payback can still strain cash even when the ratio looks strong.
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