The SaaS business model — Business acumen: how your buyer's company actually works

Most UK health-tech and life-science software companies sell subscriptions. Investors judge them on a specific set of metrics, those metrics shape the deals you are allowed to sign, and knowing them is the fastest way to sound like someone who has done the job.

MRR — monthly recurring revenue; ARR — annual recurring revenue = MRR × 12. £25,000 MRR = £300,000 ARR. ARR excludes one-offs — implementation, training, bespoke work — which matters personally, since quotas and commission are usually written in ARR or ACV terms.

Bookings, billings, revenue and cash are four different numbers. Sign a three-year, £60k-a-year deal on 1 January, billed annually in advance: bookings £180k, billings £60k, revenue recognised in January £5k, cash £60k. Billing in advance creates deferred revenue — a liability for service not yet delivered, and why finance teams love annual-upfront terms.

Margins. In Benchmarkit's 2025 data the median total-revenue gross margin for B2B SaaS was 77%, subscription-only 81%, services around 30%. So every pound of discount costs roughly 80p of gross profit while every pound of services adds about 30p — which is why your company would rather give away implementation days than cut price.

CAC, payback and the cash-flow trough

CAC — customer acquisition cost is the spend required to win one customer. It is paid up front; the revenue arrives over years, and that mismatch defines the model.

Voice from the field
"The single most misunderstood software-as-a-service (SaaS) metric I've encountered is the CAC Payback Period" — "a compound metric that is generally defined as the months of contribution margin to pay back the cost of acquiring a customer". And on what it does not tell you: "Payback is all about how long your money is committed (so it can't be used for other projects) and at risk (meaning you might not get it back). Payback doesn't tell you anything about return."
— Dave Kellogg, Kellblog (2016)

Worked example 4 — how a discount moves payback. You win a customer at £60,000 ACV. Subscription gross margin 81% → £48,600 of gross profit a year, £4,050 a month. Your company spent £70,000 to acquire them.

Payback = £70,000 ÷ £4,050 = 17.3 months.
Apply a 15% discount: ACV £51,000 → £41,310 a year → £3,442.50 a month.
Payback = £70,000 ÷ £3,442.50 = 20.3 months.

A 15% discount added three months to payback. Said in an interview that sentence does more for you than any amount of enthusiasm; said internally, it wins the argument for holding price.

For context: in the 2026 SaaS & AI Performance Benchmarks from Aleph and Benchmarkit (2025 actuals, 342 companies), median CAC payback was 16 months, top quartile ≤6, bottom quartile ≥24, lengthening with deal size. The classic guidelines — recover CAC within about 12 months, lifetime value of at least 3× CAC — come from David Skok's SaaS Metrics 2.0, which explains why the model burns cash: SaaS businesses "have to invest heavily upfront to acquire the customer, but recover the profits from that investment over a long period of time", so "the faster the business decides to grow, the worse the losses become". That explains why your CFO wants annual-upfront billing and why a company can be growing beautifully and still be nervous.

Churn, GRR and NRR

GRR — gross revenue retention: retained revenue from the existing base counting churn and downgrades but excluding expansion; never above 100%. NRR — net revenue retention: the same including expansion; above 100% the company grows without selling anything new. (Module 06 goes deeper.)

Worked example 5. Opening ARR from existing customers £4,000,000; £400,000 churned, £100,000 downgraded, £700,000 expanded. GRR = (4,000 − 400 − 100) ÷ 4,000 = 87.5%. NRR = (4,000 − 400 − 100 + 700) ÷ 4,000 = 105%. Both sit near the 2024 Benchmarkit medians (GRR 88%, NRR 101%).

Why churn terrifies investors is compounding. Monthly churn of 2% is not 24% a year — it is (1 − 0.02)¹² = 78.5% retained, about 21.5% lost. At 1% monthly you retain 88.6%, losing 11.4%. Churn drags on lifetime value, on payback and on the growth needed just to stand still: a high-churn company is pouring acquisition spend into a leaking container.

The rule of 40 — defined honestly

Voice from the field
"The 40% rule is that your growth rate + your profit should add up to 40%. So, if you are growing at 20%, you should be generating a profit of 20%. If you are growing at 40%, you should be generating a 0% profit. If you are growing at 50%, you can lose 10%."
— Brad Feld, Feld Thoughts, 3 February 2015

Feld was passing on a heuristic from a late-stage investor. Applied: 25% growth at a −10% margin scores 15; 30% growth at a 12% margin scores 42. Stating it, applying it and naming its faults separates a candidate who read a blog post from one who thought about it. "Profit" is undefined — EBITDA, operating margin and free cash flow give different answers. It treats growth and margin as interchangeable — 60% growth at −15% and 20% growth at +25% both score 45 and are nothing alike. It does not apply pre-scale. And it ignores retention. Use it to open a conversation, not reach a verdict.

Your deal structure moves all of this. Contract length drives revenue predictability, churn exposure and valuation multiple, so multi-year deals are worth real concessions. Billing frequency drives cash timing and deferred revenue, often worth more to your CFO than price. Discount costs ~80p of gross profit in the pound and lengthens payback; ramped pricing delays it too, but is sometimes the only way a constrained buyer can start. Services fees are non-recurring, ~30% margin and usually outside ARR. An AE who says "I'd rather give them quarterly billing and hold price than discount 15% and add three months to payback" is describing the job accurately.