What this calculator does
This calculator answers a cash question rather than a value one: how many months of gross profit it takes to recover what you spent winning a customer. Enter average revenue per customer, your acquisition cost, and gross margin, and it returns the payback period in months together with the monthly gross profit each customer produces.
Payback tells you how quickly the growth engine recycles cash. LTV:CAC indicates whether acquisition creates value eventually; payback indicates when the money returns. For a company running to a finite cash position, the second question is generally the more pressing one.
When to use it
Use it before scaling a channel. A channel that passes an LTV:CAC test can still be the wrong place for money if it ties up cash for 20 months and you hold 14 months of runway. Payback turns a marketing question into a treasury question.
It is also the calculation to run when considering an annual discount. Taking 20 percent off for annual prepayment lowers effective revenue per customer but collects a year of gross profit immediately. Comparing monthly payback against cash-basis annual payback settles that trade in a way general discussion of discounting rarely does.
Understanding the inputs
Average revenue per customer should be the recurring monthly subscription figure net of VAT and discounts. If you invoice annually, divide contract value by twelve for the monthly view and treat the cash timing as a separate question.
CAC must be comprehensive: media spend, agency fees, sales and marketing salaries including employer National Insurance and pension contributions, tooling, and events, divided by new customers won in the same period. Omitting employment costs is the usual distortion. Gross margin should reflect genuine cost to serve — hosting, support, third-party services — commonly 70 to 85 percent in SaaS.
How is this calculated?
Monthly Gross Profit = Revenue × Gross Margin. CAC Payback Months = CAC / Monthly Gross Profit.
A worked example
A UK SaaS business spent £84,000 on sales and marketing last quarter, salaries and employer NI included, and won 30 customers — a CAC of £2,800. Customers pay £250 a month at an 80 percent gross margin, contributing £200. Payback comes out at 14 months, which is workable but above the level most investors like to see.
Bring CAC down to £2,000 by shifting spend toward organic and referral, and payback falls to 10 months. Alternatively, moving those customers onto annual prepaid contracts collects £2,400 of gross profit on day one — or about £2,040 after a 15 percent annual discount — recovering roughly three-quarters of CAC in month one rather than month fourteen. That is a cash lever rather than an efficiency one, and usually the faster of the two.
Limitations and assumptions
The calculation assumes no churn during the payback window, which is optimistic. If customers leave in month eight of a fourteen-month payback, actual recovery is far worse than the figure shown. Always read payback alongside average customer lifetime rather than in isolation.
It uses a blended CAC across channels and segments, masking wide variation between self-serve and enterprise motions. It ignores expansion revenue, which shortens real payback, and any discount rate, which lengthens it. Use it to compare scenarios and test spending decisions, then break the figure down by segment before you act on it.
Common Questions
- What is a good CAC payback period?
- As a common rule of thumb, under 12 months is healthy for B2B SaaS and the strongest businesses sit closer to six. Enterprise models with large contracts often accept 18 to 24 months because retention is better. Consumer subscription products usually need three to six, given much higher churn.
- At what point does payback become a real problem?
- Beyond 24 months, and earlier where churn is high. The decisive test is whether payback exceeds average customer lifetime: if customers leave after 20 months and payback takes 22, the acquisition cost is never recovered. That comparison matters far more than any published benchmark.
- Why does the calculation use gross profit rather than turnover?
- Because turnover consumed in serving the customer cannot repay acquisition cost. A customer paying £250 a month at 80 percent gross margin returns £200 toward CAC. Using the full £250 shortens apparent payback by a fifth, which is precisely how an unsustainable channel escapes scrutiny.
- How does payback relate to runway?
- Directly, which is why it often matters more than LTV:CAC for an early-stage company. Payback is how long each pound of acquisition spend stays locked up. A business with 18-month payback and 12 months of runway cannot grow out of the problem — the cash runs out before the customers repay it.
- What costs belong in CAC?
- Everything in sales and marketing for the period: media spend, agency fees, salaries plus employer National Insurance and pension contributions for sales and marketing staff, tooling, events, and content. Divide by new customers won. Leaving out employment costs is the most common way CAC is understated, frequently by half.
- Does annual billing change the picture?
- Substantially, and favourably. An annual contract paid up front delivers twelve months of gross profit on day one, so payback on a cash basis can be immediate even where the monthly calculation suggests a year. If you sell annually, model cash payback separately — it is usually the best argument for an annual discount.
- How do I shorten payback in practice?
- Three levers, roughly in order of speed: raise price, which flows straight into gross profit; improve gross margin by cutting cost to serve; and reduce CAC by shifting mix toward cheaper channels. Moving customers to annual billing improves the cash position immediately without changing any of the three.
- Should payback be measured per segment?
- Yes, because blending conceals the issue. Enterprise deals carry heavy CAC and long payback but strong retention, while self-serve customers repay in weeks and churn quickly. A blended 14-month figure can hide one segment repaying in three months and another never repaying at all.
- Should VAT be included anywhere in this?
- No. Use net-of-VAT subscription revenue and net-of-VAT marketing costs, since VAT charged is collected for HMRC and VAT on inputs is reclaimable. Mixing gross and net figures across the numerator and denominator is a surprisingly common error that skews payback by several months.