CAC, LTV, and Payback Period: Calculated Correctly for Once
CAC, LTV, and payback period are the three most cited unit economics metrics in venture capital conversations --- and the three most frequently calculated incorrectly. CAC must include all acquisition costs, not just ad spend. LTV must reflect actual customer behaviour, not a theoretical maximum. Payback period must use gross profit contribution, not revenue. Getting these right is not academic precision: it changes the story the numbers tell about whether the business is capital efficient.
Author: Yanni Papoutsi · Fractional VP of Finance and Strategy for early-stage startups · Author, *Raise Ready*
Published: 2025-03-08 · Last updated: 2025-03-08
Reading time: \~8 min
What Each Metric Is Actually Measuring
CAC (Customer Acquisition Cost): The total cost of acquiring one new paying customer. This includes all sales and marketing spend that contributed to that acquisition, divided by the number of new customers acquired in the same period.
LTV (Lifetime Value): The total gross profit contribution expected from a customer over the entire duration of their relationship with the business. Not revenue. Gross profit.
Payback period: The number of months required for a customer to generate enough gross profit to recover the CAC invested to acquire them.
The relationship between them:
CAC | Total S&M spend ÷ New customers Using ad spend only, not acquired | total S&M
LTV | Avg monthly gross profit per | Using revenue instead of customer ÷ Monthly churn rate | gross profit
LTV:CAC | LTV ÷ CAC | Both inputs wrong = ratio
How to Calculate CAC Correctly
The most common CAC error is including only paid advertising spend. The correct denominator includes every cost that contributed to acquiring that customer:
All paid media (search, social, display)
Sales team salaries, commissions, and on-target earnings
Sales tools and CRM costs (allocated to acquisition)
Marketing team salaries (for marketing-led growth models)
Events, conferences, and trade show costs
Content and SEO investment (if it drives direct acquisition) Agency fees
The period of calculation matters too. A company that hires a sales team in month one and closes the first customers in month three should not calculate CAC only in month three (which would look artificially low)
cost.
For early-stage businesses with small numbers, use a rolling 3-6 month CAC to smooth out timing mismatches between spend and customers acquired.
CAC formula:
CAC = Total Sales & Marketing Spend (period) ÷ New Customers Acquired (period)
How to Calculate LTV Correctly
The two most common LTV errors: using revenue instead of gross profit, and using an overly optimistic churn rate.
The gross profit distinction:
LTV is the value the business retains from each customer, not the revenue the customer pays. A customer paying £1,000 per month in a business with 70% gross margins contributes £700 per month in gross profit. Using revenue overstates LTV by the inverse of the gross margin. At 70% gross margin, revenue-based LTV is 43% higher than the correct gross-profit-based LTV.
The churn rate input:
For a subscription business, the simplest LTV model is:
LTV = Average Monthly Gross Profit per Customer ÷ Monthly Churn Rate This formula assumes constant gross profit per customer and constant churn, which is an approximation. It is a starting point, not a precise model.
For a business with significant churn seasonality, cohort-specific churn rates, or a known expansion revenue component, build a more detailed LTV model using cohort analysis rather than the simple formula. The churn error:
Using an annual churn rate in a monthly formula (or vice versa) without converting. Monthly churn of 2% is not the same as annual churn of 24%
annual churn.
How to Calculate Payback Period Correctly
Payback period is the number of months to recover CAC through gross profit contribution.
Payback Period (months) = CAC ÷ Average Monthly Gross Profit per Customer
Using revenue instead of gross profit understates the payback period --- which is the same as overstating capital efficiency. This error is common in pitches because the shorter payback number looks better. Payback period benchmarks by business model:
SaaS | < 12 months | 12-18 months | > 18 months Marketplace | < 6 months | 6-12 months | > 12 months E-commerce | < 3 months | 3-6 months | > 6 months
The LTV:CAC Ratio: What It Actually Measures
The LTV:CAC ratio measures the return on customer acquisition investment. A ratio of 3:1 means every pound invested in acquiring a customer returns three pounds in lifetime gross profit. Industry convention treats 3:1 as a minimum threshold for a sustainable SaaS business.
But the ratio is only as meaningful as its inputs. LTV:CAC calculated with revenue-based LTV and ad-spend-only CAC can look like 5:1 when the correct calculation is 2:1. The ratio is a useful benchmark when both components are calculated correctly.
What the ratio does not tell you:
How long it takes to realise the LTV (a 3:1 ratio over 3 years is
different from 3:1 over 8 years)
Whether the churn assumption is realistic
Whether CAC is trending in the right direction at scale
The payback period addresses the first problem. Cohort analysis addresses the second. CAC by channel and acquisition cohort addresses the third.
Key insight: The LTV:CAC ratio and payback period are companion metrics, not substitutes. A business with a great LTV:CAC ratio but a 36-month payback is capital-intensive --- it requires a lot of cash before the acquisition investment is recovered. A business with a 3:1 ratio and 8-month payback is capital-efficient. Both metrics together tell the full story.
Frequently Asked Questions
Should LTV include expansion revenue?
Yes, if it is genuinely predictable and part of the typical customer journey. Expansion revenue from upsells, seat additions, or usage growth that has been demonstrated in historical cohorts can be included in LTV. Expansion revenue based on aspirational assumptions should not be. Include it only if the cohort data supports it.
How do you calculate CAC for a product-led growth business?
PLG businesses acquire customers through the product itself --- free trials, freemium tiers, viral loops. CAC for PLG companies typically includes the cost of free users (infrastructure, support), the cost of the product features that drive conversion, and any sales cost associated with converting free-to-paid. The denominator is paying customers, not free users.
What is a \"blended\" vs. \"unblended\" CAC?
Blended CAC combines spend across all acquisition channels and divides by total new customers. It is useful for an overall efficiency view. Unblended CAC calculates separately by channel (paid search, organic, outbound, referral). Unblended is more useful for optimisation because it reveals which channels are efficient and which are subsidised by others.
Summary
CAC must include all sales and marketing spend, not just ad spend. LTV must be calculated on gross profit, not revenue, with a realistic churn rate. Payback period must use gross profit per customer. The LTV:CAC ratio is only meaningful when both inputs are correct. The payback period and LTV:CAC ratio together tell the full capital efficiency story: the ratio shows the return, the payback period shows how long it takes to realise it. Get these three metrics right before any investor conversation that references unit economics.
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