The Two Numbers Every Founder Must Know

The two numbers that determine whether your startup lives or dies are LTV and CAC. Lifetime Value divided by Customer Acquisition Cost. If that ratio is below 3:1, your business model does not work, no matter how fast you are growing.

I have seen this pattern kill more startups than bad products, bad timing, or bad teams. A founder raises a seed round, spends it on growth, and six months later realizes every customer costs more to acquire than they will ever be worth. By then the runway is gone.

What LTV and CAC actually measure

CAC measures how much it costs to acquire one customer. Not a lead. Not a signup. A paying customer.

Take your total sales and marketing spend in a given period. Divide it by the number of new paying customers you acquired in that period. That is your CAC.

Most founders get this wrong in two ways. They exclude salaries. And they count free trial signups as “customers.” Both mistakes make CAC look artificially low, which delays the moment you realize the model is broken.

LTV measures how much revenue one customer generates over their entire relationship with you, adjusted for gross margin. The simple formula is average revenue per customer times gross margin, divided by monthly churn rate.

The problem with the formula is that it assumes constant churn. Real churn is not constant. Early cohorts churn differently than late cohorts. Pricing changes shift the numbers. Cohort analysis is the only honest way to calculate LTV if you have more than six months of data.

Why the ratio matters more than either number alone

A CAC of 500 EUR is not inherently bad. A CAC of 50 EUR is not inherently good. It depends entirely on what that customer is worth.

At PizzaPortal, our CAC for restaurant partners was high. We had to send salespeople to individual restaurants, demo the tablet, and often visit twice. But the LTV was enormous because restaurants that switched to online ordering stayed for years. The ratio worked.

At another company I invested in, the CAC was 12 EUR per user. Looked cheap. But the LTV was 18 EUR. A ratio of 1.5:1. They were losing money on every customer and trying to make it up with volume. That company is gone.

The ratio tells you something neither number can tell you alone: whether your growth engine creates value or destroys it.

The 3:1 threshold

3:1 is not arbitrary. It comes from the economics of how venture-backed businesses work.

If your LTV:CAC is 3:1, roughly a third of the customer’s lifetime value goes to acquisition, a third covers the cost of serving them, and a third is your margin. Dip below that and the margin disappears.

Above 5:1 means one of two things. Either you have an extraordinarily efficient business model. Possible but rare. Or you are underinvesting in growth and a competitor will eventually outspend you.

The sweet spot for most B2B SaaS companies at Series A is between 3:1 and 5:1. Use the Two Numbers Calculator to see where your company falls.

How to improve the ratio

There are only two levers: increase LTV or decrease CAC.

To increase LTV, reduce churn. That is not a marketing problem. That is a product problem. The best way to reduce churn is to make your product part of the customer’s daily workflow so switching costs are high. Second option: expand revenue per customer through upsells, usage-based pricing, or additional product lines.

To decrease CAC, find channels where your customers already congregate. Paid acquisition is the most expensive channel for almost every business. Content, referrals, and partnerships have lower CAC, but they take longer to scale. CAC payback period is how you measure whether the lower CAC channels are worth the wait.

The mistake that kills most startups

The mistake is not getting the ratio wrong. Every early-stage company has a bad ratio at first.

The mistake is not measuring it, or measuring it with fake numbers. Founders who exclude headcount from CAC. Founders who use projected LTV instead of observed LTV. Founders who blend paid and organic acquisition to make the blended CAC look acceptable when the marginal CAC on paid is 10x organic.

If you are raising money, investors will ask for these numbers. If you do not know them, that is a red flag. If you know them and they look bad, that is fine, as long as you have a credible plan to fix them. What kills deals is founders who present fake unit economics and get caught.

Measure honestly. Fix systematically. The ratio is not a vanity metric. It is a survival metric.

/Lech