What is CAC payback period?

CAC payback period is the number of months it takes for the cumulative gross profit earned from a new customer to equal the cost of acquiring that customer. A shorter payback period means you recover your acquisition spend faster and need less working capital to fund growth.

What is a good CAC payback period?

In SaaS, under 12 months is generally considered healthy, and under 6 months is excellent. Consumer businesses with high churn should target even shorter periods. Payback over 24 months is a warning sign that growth will strain cash flow — you are spending money for too long before seeing a return.

How is CAC calculated?

CAC equals total sales and marketing spend divided by the number of new customers acquired in the same period. For example, spending £50,000 to acquire 200 customers gives a CAC of £250.

What is LTV:CAC ratio and what does 3x mean?

LTV (lifetime value) is the total gross profit you expect to earn from an average customer before they churn. The LTV:CAC ratio compares that lifetime value to what you paid to acquire the customer. A ratio of 3x is a widely cited SaaS benchmark — it means you earn three pounds of gross profit for every pound spent on acquisition, leaving room for overheads and profit after recouping the CAC.

Why does gross margin matter for payback period?

Only gross profit — revenue minus direct costs — actually pays back your acquisition spend. A product with 40% gross margin earns half as much GP per month as the same ARPU product at 80% margin, so the payback period is roughly double. This is why infrastructure-heavy or service-heavy businesses often struggle with long payback even at high ARPU.

How does churn affect the payback calculation?

Higher churn means fewer customers are still active in later months, so cumulative gross profit grows more slowly. At 10% monthly churn roughly half your cohort has cancelled by month seven, making it much harder to recover a high CAC before most customers have left. The calculator models this with a retention curve so you see the real month-by-month picture rather than an optimistic zero-churn estimate.

CAC Payback Period Calculator

The CAC payback period is one of the most important metrics in subscription and SaaS businesses. It tells you how many months it takes to recover the money you spent acquiring a customer — and whether your unit economics are strong enough to support scaling.

This calculator combines four inputs — your customer acquisition cost (CAC), average revenue per user (ARPU), gross margin and monthly churn — into a full cohort model. You get a precise payback month, a lifetime value (LTV) estimate and an LTV:CAC ratio rated against the widely-used 3x benchmark.

How it works

Step 1 — CAC. Divide your total sales and marketing spend for a period by the number of new customers acquired in that same period:

CAC = Total S&M Spend / New Customers Acquired

If you spent £50,000 and won 200 customers, your CAC is £250.

Step 2 — Monthly gross profit per customer. Multiply ARPU by your gross margin percentage. This is the revenue contribution that actually flows toward recouping the CAC — COGS and direct service costs have already been stripped out.

Monthly GP per customer = ARPU x Gross Margin %

Step 3 — Theoretical payback (no churn). The floor estimate ignores churn entirely:

Payback (months) = CAC / Monthly GP per customer

At £250 CAC, £99 ARPU and 70% gross margin that is 250 / (99 x 0.70) = 3.6 months — but only if zero customers cancel.

Step 4 — Cohort payback with retention curve. In reality customers churn. The calculator applies a monthly retention factor of (1 − churn%)^t to each month, accumulates the actual gross profit earned as the cohort shrinks, and finds the month where cumulative GP first equals CAC. This is the real payback period you should plan around.

Step 5 — LTV and LTV:CAC. With a steady churn rate, LTV converges to:

LTV = Monthly GP per customer / Monthly Churn Rate

The LTV:CAC ratio divides that by your CAC. The 3x benchmark (LTV at least three times CAC) comes from David Skok’s SaaS metrics research and is used by most growth-stage investors as a minimum viability signal.

Worked example

A B2B SaaS company spends £80,000 on sales and marketing in Q1 and acquires 160 new customers.

Input	Value
S&M Spend	£80,000
New Customers	160
ARPU/month	£149
Gross Margin	72%
Monthly Churn	2%

CAC = 80,000 / 160 = £500

Monthly GP = £149 x 0.72 = £107.28/month

Theoretical payback = 500 / 107.28 = 4.66 months

With 2% churn the cohort retains 98% at month 1, 96% at month 2, and so on. Cumulative GP reaches £500 around month 5 — slightly longer than the zero-churn figure, confirming a healthy payback period.

LTV = 107.28 / 0.02 = £5,364

LTV:CAC = 5,364 / 500 = 10.7x — excellent.

Formula note

All five formulas follow standard SaaS unit-economics conventions:

CAC = S&M Spend / New Customers
Monthly GP per customer = ARPU x Gross Margin %
Payback (floor) = CAC / Monthly GP per customer
Retention at month t = (1 - Monthly Churn)^t
LTV = Monthly GP per customer / Monthly Churn Rate
LTV:CAC = LTV / CAC