Unit economics for SaaS: CAC, LTV and why payback beats margin

We break down the metrics on a real B2B SaaS with 40k EUR MRR. What to track every month.

"We have good margins" is one of the most dangerous phrases you can hear from a SaaS founder. Because high margin doesn't mean the business scales profitably. It doesn't mean customer acquisition pays off. And it definitely doesn't mean the company isn't burning cash on growth that will never break even.

Unit economics is a system of metrics that answers one key question: do we earn more from each customer than we spend acquiring them? And if so — over what period?

In this article we break down all the key metrics using a real B2B SaaS with 40k EUR MRR as the example. Not theory — concrete numbers, formulas, and what to track every month.

Meet our example

To stay concrete, let's take a specific company. We'll call it FlowDesk — a B2B SaaS for document workflow automation, 2.5 years on the market.

Current numbers:

• MRR: 40,000 EUR
• Active customers: 115
• Average contract: annual subscription
• Team: 12 people
• Acquisition channels: content + cold outreach + partners

We'll use these figures throughout the article. By the end you'll understand whether this company is healthy from a unit economics perspective — and what it should change.

Metric 1. MRR and ARR — the base everything counts from

MRR (Monthly Recurring Revenue) — monthly recurring revenue. The money the company receives each month from active subscriptions.

ARR (Annual Recurring Revenue) — annual recurring revenue. For most SaaS this is simply MRR × 12.

For FlowDesk: MRR = 40,000 EUR, ARR = 480,000 EUR.

But MRR isn't just one number. It needs to be broken down into components every month:

MRR start of month:           40,000 EUR
+ New MRR (new customers):    +4,200 EUR
+ Expansion MRR (upsells):    +800 EUR
– Churned MRR (lost):         –1,600 EUR
– Contraction MRR (downgrade):–200 EUR
= MRR end of month:           43,200 EUR

This is MRR Movement. It shows not just "how much we earned" but where growth came from and how much we lost.

In FlowDesk's case the 3,200 EUR MRR growth looks good. But pay attention: churned MRR is 1,600 EUR — that's 4% of current MRR every month. An important figure we'll come back to.

ARPU (Average Revenue Per User) — average revenue per customer per month.

ARPU = MRR / Number of customers
ARPU FlowDesk = 40,000 / 115 = 348 EUR/mo

Metric 2. Churn Rate — the most important SaaS health indicator

Churn Rate — the percentage of customers or MRR the company loses each month. The most critical SaaS metric, because it determines whether you can build a scalable business at all.

There are two types of churn:

Customer Churn — how many customers left:

Customer Churn Rate = Customers lost / Customers at start of month
FlowDesk: 5 customers lost / 115 = 4.3% per month

Revenue Churn (MRR Churn) — how much MRR was lost:

MRR Churn Rate = Churned MRR / MRR at start of month
FlowDesk: 1,600 / 40,000 = 4% per month

Benchmarks for B2B SaaS:

• Excellent: < 1% per month (< 12% per year)
• Good: 1–2% per month
• Acceptable: 2–3% per month
• Problem: > 3% per month

FlowDesk's 4% monthly churn is a warning. At that rate the company loses about 40% of its customer base per year. To maintain current MRR it has to constantly acquire new customers just to stand still — the leaky bucket effect.

Metric 3. LTV — how much one customer brings over their entire lifetime

LTV (Lifetime Value) — total revenue the company earns from one customer over their entire stay.

Basic formula:

LTV = ARPU / MRR Churn Rate
LTV FlowDesk = 348 EUR / 0.04 = 8,700 EUR

Or via average customer lifespan:

Avg lifespan = 1 / Churn Rate = 1 / 0.04 = 25 months
LTV = ARPU × Avg lifespan = 348 × 25 = 8,700 EUR

Both formulas give the same result: an average FlowDesk customer stays 25 months and brings 8,700 EUR over that time.

But LTV is a gross figure. For real understanding you need Gross Margin LTV — LTV adjusted for the cost of serving the customer.

Gross Margin % FlowDesk = 72%
(hosting, support, infrastructure per customer — 28% of revenue)

Gross Margin LTV = 8,700 × 0.72 = 6,264 EUR

It's Gross Margin LTV that's compared against CAC — not raw LTV. This matters.

Metric 4. CAC — how much it costs to acquire one customer

CAC (Customer Acquisition Cost) — the full cost of acquiring one new customer.

Typical mistake: counting only the ad budget. In reality CAC includes everything related to marketing and sales.

CAC = (Marketing spend + Sales spend) / Number of new customers

FlowDesk last quarter:
– Marketing salaries (2 people): 6,000 EUR/mo × 3 = 18,000 EUR
– Sales salaries (2 people): 5,500 EUR/mo × 3 = 16,500 EUR
– Ad budget: 4,500 EUR
– Conferences and events: 2,000 EUR
– Tools (CRM, automation): 900 EUR
Total for the quarter: 41,900 EUR

New customers in the quarter: 19

CAC = 41,900 / 19 = 2,205 EUR

Important nuance: if salespeople also work with existing customers (upsells, renewals) — split their time between new and existing customers and only count the relevant share.

Metric 5. LTV/CAC — the headline unit economics ratio

LTV/CAC — the key ratio that shows whether customer acquisition pays off.

LTV/CAC FlowDesk = 6,264 / 2,205 = 2.8

Benchmarks:

• < 1: disaster — you spend more on acquisition than you earn
• 1–2: danger zone — almost no margin for operational mistakes
• 3: gold standard for SaaS
• 5: either highly efficient acquisition, or growth that's too conservative

FlowDesk's LTV/CAC of 2.8 is close to normal but slightly below the benchmark. The main reason is high churn, which shortens LTV. If churn dropped from 4% to 2%, LTV would double and LTV/CAC would become 5.7.

That's why reducing churn is the best investment for most SaaS companies at this stage.

Metric 6. Payback Period — and why it beats margin

Payback Period — how many months it takes to recoup CAC from one customer's contribution margin.

Payback Period = CAC / (ARPU × Gross Margin %)
Payback Period FlowDesk = 2,205 / (348 × 0.72) = 2,205 / 251 = 8.8 months

So FlowDesk recovers its acquisition cost in roughly 9 months.

Why is payback more important than margin?

Margin shows product profitability. Payback shows speed of capital turnover. They're different things.

Imagine two businesses:

• Business A: 80% margin, 36-month payback
• Business B: 60% margin, 8-month payback

Business A looks more attractive at first glance. But Business B turns capital around 4.5× faster. As it grows it needs far less external capital — because it self-funds its own growth.

Business A "freezes" cash for three years per acquired customer. At fast growth that means a constant need for outside funding.

Benchmarks for B2B SaaS payback:

• Excellent: < 12 months
• Good: 12–18 months
• Acceptable for enterprise: up to 24 months
• Problem: > 24 months

FlowDesk's payback of ≈ 9 months is a good result. Today it's the company's strongest metric.

Putting it all together: FlowDesk diagnosis

Now the full picture:

Metric	FlowDesk	Benchmark	Verdict
MRR	40,000 EUR	—	—
ARPU	348 EUR/mo	—	—
Gross Margin	72%	> 70%	✅
MRR Churn	4%/mo	< 2%	⚠️
LTV (gross margin)	6,264 EUR	—	—
CAC	2,205 EUR	—	—
LTV/CAC	2.8	> 3	⚠️
Payback Period	8.8 mo	< 12 mo	✅

Verdict: FlowDesk is a healthy company with one serious problem. Monthly churn of 4% is a hole in the bucket — it shortens LTV and pulls LTV/CAC below the gold standard.

If the team focuses on retention and reduces churn to 2%, the picture changes dramatically:

Metric	Today	With 2% churn
Avg customer lifespan	25 mo	50 mo
LTV (gross margin)	6,264 EUR	12,528 EUR
LTV/CAC	2.8	5.7

No change in CAC, no change in margin, no new channels — just better retention doubles LTV and lifts the company to excellent unit economics.

What to track monthly: minimum dashboard

You don't need to track all of these weekly. Here's what to monitor every month without fail:

Monthly — operational pulse:

1. MRR Movement (new / expansion / churned / contraction)
2. Customer Churn Rate
3. MRR Churn Rate
4. ARPU (and its trend — rising or falling)

Quarterly — strategic metrics:

5. CAC (per channel where possible)
6. LTV / Gross Margin LTV
7. LTV/CAC
8. Payback Period

Quarterly — cohort analysis: split customers into cohorts by acquisition month and see how MRR holds up in each cohort after 3, 6, 12 months. The most accurate way to see real churn and its trend.

Example cohort table:

Cohort	Mo 0	Mo 3	Mo 6	Mo 12
Jan 2024	100%	88%	79%	68%
Apr 2024	100%	91%	84%	—
Jul 2024	100%	93%	—	—

Here you can see a positive trend: newer cohorts retain better. That means product or onboarding has improved — and it's reflected in real numbers.

How to calculate CAC by channel

A blended CAC is useful but not enough to make decisions. If you know CAC = 2,200 EUR but don't know where the cheapest customers come from, you can't optimize the marketing budget.

FlowDesk has three channels: content, outreach and partners. Here's CAC by channel:

Channel	Spend/quarter	New customers	CAC	LTV/CAC
Content / inbound	8,500 EUR	7	1,214 EUR	5.2
Cold outreach	24,000 EUR	9	2,667 EUR	2.3
Partners	9,400 EUR	3	3,133 EUR	2.0
Total	41,900 EUR	19	2,205 EUR	2.8

The picture gets much more interesting. Content generates the cheapest customers with the best LTV/CAC — but only 7 of 19 new customers. The partner channel is the most expensive and least efficient.

The takeaway for FlowDesk: invest more in content and rethink the partner program. If half the customers came through inbound, blended CAC would drop from 2,205 to ~1,700 EUR and LTV/CAC would rise to 3.7.

Three mistakes in unit economics math

Mistake 1. Not including salaries in CAC. The most common one. "Our CAC is 300 EUR" — because only the ad budget is counted. But if there's a sales manager, an SDR, a marketer — their time costs money. Real CAC is often 3–5× higher than "ad CAC".

Mistake 2. Calculating LTV without gross margin. LTV = ARPU / Churn looks pretty. But if you ignore the cost of serving the customer, you overstate real profit and get a false sense of unit economics health.

Mistake 3. Ignoring expansion revenue when calculating LTV. If customers upgrade plans or buy additional modules over time — that's expansion MRR, which raises real LTV. For companies with strong upsell, LTV can be 1.5–2× higher than the basic calculation based on month-one ARPU.

A more accurate LTV formula with expansion:

LTV = (ARPU + Avg expansion per month) / Churn Rate × Gross Margin

A ready template to calculate unit economics

Calculating all these metrics by hand every month is tedious and error-prone. We've prepared a ready Excel template that automatically computes all the key SaaS metrics: MRR Movement, Churn Rate, CAC by channel, LTV, LTV/CAC, Payback Period and cohort analysis.

Plug in your numbers — get the full unit economics dashboard in 30 minutes.

👉 Get the SaaS unit economics template →

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Previous in the series — "Cash flow without gaps: 3 rules of the payment calendar". Next — "Multi-currency accounting: how not to lose margin on FX differences".