Every SaaS metrics conversation eventually collapses into four numbers: what it costs to acquire a customer (CAC), what that customer is worth over their lifetime (LTV), the ratio between the two (LTV:CAC), and how long it takes to earn back the acquisition cost (CAC payback period). Founders usually learn these as four separate facts to report on a board slide. They are not separate — they are one system, and moving any one of them moves the others. Get the relationships wrong and you can hit every individual "healthy" benchmark while the business is quietly starving.
CAC: what actually goes into it
Customer acquisition cost is fully-loaded sales and marketing spend for a period, divided by the number of new customers acquired in that period. The mistake that inflates or deflates the number beyond usefulness is scoping it wrong: it should include ad spend, sales and marketing salaries and commissions, tools (CRM, marketing automation), and a reasonable share of overhead for those teams — not just the media buy. It should also match the period to the sales cycle; dividing this month's spend by this month's new customers is fine for a self-serve product with a same-day close, but it's misleading for anything with a multi-month sales cycle, where the spend that closes a customer this month was largely incurred last quarter.
CAC = (Total sales & marketing spend, fully loaded) / (New customers acquired in the same period). Include salaries, commissions, ad spend, and tooling for sales and marketing. Exclude customer success and support cost — that belongs in COGS, not CAC, and folding it in understates how expensive acquisition really is.
LTV: the part everyone gets wrong
Lifetime value is the total gross margin a customer generates over the time they stay a customer. The simplest version is average revenue per account divided by churn rate, multiplied by gross margin percentage — but each of those three inputs is where the number quietly goes wrong. Using gross revenue instead of gross-margin-adjusted revenue overstates LTV, because it ignores what it costs to actually serve the customer. Using a blended churn rate across every cohort hides that new cohorts often churn faster than mature ones, which means LTV computed today from historical averages can overstate what a customer acquired today is actually worth. And because LTV is a projection built on a churn rate that itself has a confidence interval, it should be treated as a working estimate that gets revised as more cohort data comes in, not a fixed constant.
WITH cohort_spend AS (
SELECT
DATE_TRUNC('month', c.started_at) AS cohort_month,
COUNT(DISTINCT c.customer_id) AS new_customers,
SUM(s.sales_marketing_spend) AS sm_spend
FROM customers c
JOIN spend_ledger s
ON DATE_TRUNC('month', s.spend_date) = DATE_TRUNC('month', c.started_at)
GROUP BY 1
),
cohort_economics AS (
SELECT
cohort_month,
new_customers,
sm_spend,
ROUND(sm_spend / NULLIF(new_customers, 0), 2) AS cac,
AVG(mrr) * 0.8 AS avg_margin_per_customer,
ROUND(AVG(mrr) * 0.8 / NULLIF(AVG(monthly_churn_rate), 0), 2) AS ltv
FROM cohort_spend
JOIN customers USING (cohort_month)
GROUP BY 1, 2, 3
)
SELECT
cohort_month,
cac,
ltv,
ROUND(ltv / NULLIF(cac, 0), 2) AS ltv_to_cac,
ROUND(cac / NULLIF(avg_margin_per_customer, 0), 1) AS payback_months
FROM cohort_economics
ORDER BY cohort_month;The LTV:CAC ratio, and why 3:1 is the commonly cited line
LTV:CAC above 3:1 is the commonly cited health threshold in SaaS — spend a dollar on acquisition, get back three dollars of gross-margin value over the customer's lifetime. Below roughly 3:1, the business is spending too much to acquire customers relative to what they're worth, and growth is expensive growth. Above roughly 5:1 is often actually a warning sign in the other direction: it can mean the company is under-investing in growth relative to the opportunity, leaving cheap, profitable customers on the table that a competitor will pick up instead. The ratio is a range to manage inside, not a single number to maximize — pushed too high, you're being too conservative with a lever (sales and marketing spend) that could be compounding revenue faster.
A 5:1 LTV:CAC ratio looks excellent on a slide, but if CAC payback takes 24 months, the company still needs to fund two years of runway per customer before that value shows up as cash. Ratio measures whether acquisition is profitable in the long run; payback period measures whether the business can survive long enough to collect on it. Both have to hold at the same time.
CAC payback period: the cash question
CAC payback period is how many months of gross margin from a customer it takes to recover what was spent acquiring them — CAC divided by average monthly gross margin per customer. This is the metric that determines how much cash a company burns to grow, independent of whether the eventual LTV is good. A healthy payback period for most SaaS businesses is commonly cited as under 12-18 months; enterprise sales motions with long contracts can tolerate longer paybacks because contract length and expansion revenue make up for it, while self-serve, low-ACV products need a much shorter payback because there's no sales team margin to subsidize a long wait. A business can have excellent unit economics on paper and still run out of money if payback period is long and growth is fast, because every new cohort of customers requires fresh cash before it turns profitable.
How the four numbers move together
These aren't four independent dials. Raising CAC by spending more on sales and marketing to grow faster directly worsens both the LTV:CAC ratio and the payback period, unless it's offset by acquiring higher-LTV customers in the process — which is exactly what happens when growth spend shifts from broad-based ads toward channels that land better-fit customers. Improving retention (lowering churn) raises LTV, which improves the ratio without touching CAC at all, and is usually the highest-leverage lever precisely because it's the one that doesn't cost more to acquire the next customer. Raising prices raises both LTV and monthly margin per customer, which improves the ratio and shortens payback simultaneously — it's rare to find a lever that helps both numbers at once, which is part of why pricing work is underrated as a unit-economics fix compared to sales efficiency work.
Wrapping up
CAC, LTV, LTV:CAC, and payback period describe the same acquisition decision from four angles: what it costs, what it's worth, whether it's worth doing at all, and whether the company can afford to wait for the payoff. A ratio above roughly 3:1 with payback under 12-18 months is the commonly cited healthy zone, but the ratio alone can hide a cash problem and the payback period alone can hide a churn problem — track both, and remember that fixing retention usually moves both numbers at once, which is the cheapest lever most SaaS companies aren't pulling hard enough.
Independent software engineer in Nairobi specialising in Acumatica customisations, Laravel backends, and tax fiscalisation integrations across East and Southern Africa.