CAC payback period calculator: how many months until the customer pays you back

This page answers when the money comes back, which is a question about cash and about a clock. It sits next to three others and overlaps none of them: the CAC calculator answers how much a customer costs to buy, and it is where the number for the first field here should come from; the LTV calculator answers whether they are worth what they cost, summing a whole lifetime into a ratio with no clock; the churn calculator answers how much is leaking from the base you already have. If you want the cost, start at CAC. If you want to know whether the business closes, go to LTV. If you want to know why the math closes and there is still no money in the bank, stay here.

An excellent customer can break your cash flow

The LTV:CAC ratio is a total value calculation: it sums everything the customer will leave behind and compares it to what they cost. It is a good calculation, and it has no clock. Payback puts the clock back in.

The gap shows up when the two disagree. A 5 to 1 ratio looks comfortable in any deck, but if that value is spread across three years of subscriptions, the company spends more than a year financing every customer it buys. The profit is real, it just lives in the future, while suppliers, payroll and media are paid in the present. That is why companies with healthy unit economics stall on cash: the model is not wrong, the timeline is.

The second mismatch is with margin. LTV:CAC is usually already calculated on margin, but payback almost never is: the version that circulates in spreadsheets is CAC divided by the subscription price, which ignores the cost of serving and returns a timeline that is far too pretty. With the default numbers on this page, revenue payback is 8.75 months and the real one is 12.91 months. Four months of difference from a single choice of numerator.

How to use it

1. Fill in CAC with the full cost of acquiring a customer, not just media. If you do not have that number yet, run the CAC calculator first and come back with the blended CAC for the period.
2. Enter revenue per customer per month, the ARPA. In subscription it is the average monthly price; on an annual contract, divide the year by 12; in ecommerce, use average order value times purchases per month.
3. Enter the gross margin of what you sell. It is mandatory for the math to mean anything: margin, not revenue, is what returns CAC. If in doubt, strip infrastructure, support, payment fees and third party licenses out of the price.
4. Adjust monthly churn and expansion. The two combine into a single cohort factor: the base shrinks with churn while whoever stays spends a little more through expansion. When expansion matches churn, monthly margin stops decaying and payback becomes the linear calculation.
5. Fill in new customers per month and your payback target. The first field does not change payback: it sizes the cash locked in the trough, which is the number that decides whether you can accelerate. The second returns the maximum CAC and the minimum ARPA that meet the target.

How it works: the formula

The naive calculation assumes the customer leaves the same margin every month forever. A real cohort shrinks and expands at the same time, so the margin in month n is the first month margin multiplied by a factor raised to n minus 1, and the cumulative total is the sum of a geometric progression.

That last line deserves a paragraph. If you acquire k customers every month starting in month 1, then in month T the oldest cohort is in its T month of life, the one before it in month T minus 1, and so on down to today cohort. The sum of margin across all of them is exactly k times the cumulative margin of a single customer over T months. Since the outflow is always k times CAC, the monthly cash flow of the acquisition engine stays negative while a single customer cumulative margin has not reached CAC, and it turns positive in the same month payback lands. The hole accumulated over that stretch is the working capital of your growth machine.

Worked example (it reproduces the default output)

The prefilled values describe a small B2B SaaS: $2,800 CAC, $320 ARPA per month, 75% gross margin, 2.5% monthly churn, 0.8% monthly expansion, 40 new customers a month and a 12 month payback target.

• First month margin: 320 × 0.75 = $240.00. Cohort factor: 0.975 × 1.008 = 0.9828, which is net revenue churn of 1.72% a month.
• Payback on gross revenue: 2,800 ÷ 320 = 8.75 months. Naive payback on margin: 2,800 ÷ 240 = 11.67 months. Both ignore the cohort shrinking.
• Cumulative margin over 12 months: 240 × (1 minus 0.9828 raised to 12) ÷ 0.0172 = $2,622.58, or 94% of CAC. Still short. Month 13 brings in $194.89 of margin and the math flips: real payback of 12.91 months, with churn adding 1.24 months to the naive number.
• Profit per customer at month 12: 2,622.58 minus 2,800 = minus $177.42. By month 24 cumulative margin reaches $4,752.25 and profit turns to $1,952.25 per customer.
• LTV in margin: 240 ÷ 0.0172 = $13,953.49, an LTV:CAC ratio of 4.98 to 1. By the value test, the business is excellent. By the timing test, it is still in the red a year after the sale.
• At 40 new customers a month, acquisition spend is $112,000 a month and the engine runs cash negative until month 13. The deepest point of the trough is month 12 and it is worth $640,455, or $16,011 locked per new customer of the month.
• To hit the 12 month target without changing anything else, CAC would have to drop to $2,622.58 or ARPA would have to rise to $341.65. Either target is small, and that is the point of those lines: turning a payback goal into an operational one.

Locked cash is what caps growth

The number in the middle of the calculator is usually the one that changes the conversation. A 12.91 month payback looks manageable until you see that keeping 40 new customers a month requires over six hundred thousand dollars in the bank just to cross the trough. That money is not a loss: it comes back, and after month 13 the engine pays for itself and then some. Until then, it has to exist.

From that comes the rule of thumb almost nobody states out loud: locked cash grows with acquisition pace and with payback at the same time. Doubling new customers a month doubles the trough. Doubling payback makes the trough grow more than twice, because you end up financing more months of cohorts, each with a bigger deficit. That is why, between cutting CAC and accelerating acquisition, the first option almost always frees more cash than the second one consumes.

The same reading applies to discounted annual plans. Swapping a monthly subscription for twelve months paid upfront shortens payback to nearly zero on the day of the sale, even with a discount on price. In many cases a 20% discount on the annual plan is worth more than raising working capital to finance the monthly one, and the comparison becomes obvious when you run the calculator with the lower ARPA and compare the two troughs.

How to read it and where it misleads

The first limit is a blended CAC applied to unequal customers. If half your base comes from organic and the other half from expensive media, a single payback hides two realities. The honest move is to run the calculator once per channel with that channel CAC and compare. It is usually the difference between a channel that pays back in six months and one that never does.

The second is gross margin guessed too high. Software margin looks great until human support, onboarding and the infrastructure bill of your largest customer come in. Payback is almost linearly sensitive to this field: every 10 points of margin lost push the timeline out by more than a month in the default scenario.

The third is constant churn. The model applies the same rate every month, while real churn is higher in the first months and settles as the customer beds in. With high early churn, real payback is worse than this calculation; with a mature base, it is better. If you have cohort data, use churn from the first twelve months, which is the window that decides payback, rather than the average of the whole base.

The fourth is the time value of money. This page applies no discount rate: a dollar received in month 13 counts the same as a dollar received today. Over a one year payback that changes little, but over long timelines future money is worth less than the math suggests. The LTV calculator has a discount rate field and is the right place for that version of the question.

The fifth is upfront payment. If the customer pays for the whole year at signup, cash arrives before the margin is generated, and cash payback is much shorter than the margin payback calculated here. In that case treat this model as a pessimistic floor and count the prepayment as the advantage it is.

From the number to the decision

• If payback went past 18 months, attack price and packaging before volume. SaaS pricing page optimization moves ARPA, which is the denominator of the whole calculation.
• If churn is what pushes the timeline out, the path is reducing churn with experimentation, and it is worth sizing the leak in the churn calculator before choosing where to act.
• If the bottleneck is cash rather than the model, look at the whole book in the MRR and ARR calculator to see whether net new MRR already sustains the acquisition pace you want to keep.
• If the problem shows up early, between the sale and first value delivered, the subject is activation: activation metrics and A/B testing covers what to measure before churn becomes a number.
• If the trial funnel drops customers you already paid media for, start with trial to paid conversion testing, because improving that conversion lowers CAC without touching the budget.

Frequently asked questions

What is CAC payback period?

It is the time it takes for a customer gross margin to return what you spent acquiring them. It is not the time until they pay you the CAC amount in revenue: it is the time until cumulative gross margin, meaning what is left after the cost of serving them, reaches CAC. A customer paying $320 a month at 75% margin returns $240 a month, not $320. That single distinction moves payback by several months in most businesses.

What is a good CAC payback period?

The benchmark that has circulated in SaaS since the writing of David Skok and Bessemer is 12 months for SMB sales and up to 18 months for enterprise deals with long contracts and upfront payment. Under 12 months the acquisition engine funds itself with little capital tied up. Above 18 months the business depends on outside cash to grow, because every month of acceleration digs the trough deeper before it turns.

Is CAC payback the same as the LTV:CAC ratio?

No, and the two can disagree. LTV:CAC asks whether the customer is worth more than they cost, summing their whole lifetime. Payback asks when that money arrives. You can have a 5 to 1 LTV:CAC with a 20 month payback: the customer is excellent and the company still runs out of cash, because the value is spread across three years of subscriptions. The default scenario on this page is exactly that case, with a ratio near 5 and the customer still in the red at month 12.

Should payback be calculated on revenue or on margin?

On gross margin. The revenue coming in is not all yours: part of it pays hosting, support, payment processing and third party licenses. Only what is left over returns CAC. Calculating on revenue is the mistake that shortens payback the most on paper, and the calculator shows both side by side precisely so you can size that gap in your own numbers.

How does churn change CAC payback?

The cohort you bought shrinks every month, so the margin in month 10 is smaller than in month 1, and the linear CAC divided by monthly margin math understates the timeline. The calculator applies a factor of (1 minus churn) times (1 plus expansion) every month, and separately reports how many months churn added to the naive payback. When cumulative margin stalls below CAC, payback simply does not exist: no amount of waiting recovers the investment.

Why does payback cap growth even when the customer is profitable?

Because until payback lands, every new customer is money going out. Acquiring the same number of customers every month, the acquisition engine runs cash negative until the first cohort pays for itself, and the hole accumulated over that stretch is the working capital the operation requires. Doubling acquisition doubles that hole. That is why companies that are profitable on paper stall for lack of cash, and it is the number this page puts front and center.

Keep going

Payback is the timeline; experimentation is how you shorten it without buying growth. Start with growth experimentation for SaaS to build the method, then use the experimentation ROI calculator to put a price on every point of conversion a test gives back.