Unit economics measures whether each customer makes you money or costs you money. The four numbers to know: CAC (cost to acquire), LTV (lifetime value), LTV:CAC ratio (target 3:1+), and CAC payback period (target under 12 months). Capital is more expensive in 2026 — investors scrutinise these earlier than ever.
The four numbers
Unit economics asks one question: does each customer make you money? Not in aggregate, not someday — per customer, today.
According to SaaS Capital’s 2025 data, the median SaaS company now spends $2.00 to acquire $1.00 of new ARR — a 14% increase from 2023. Median CAC payback has stretched from 14 months to 18. Companies that survive aren’t the ones with the most fuel — they’re the ones with the most efficient engines.
1. Customer Acquisition Cost (CAC)
What you spend to win one customer.
CAC = Total Sales & Marketing Spend ÷ New Customers Acquired
The trap: most founders calculate “blended CAC” — total spend across all channels. It hides which channels work. Calculate CAC per channel. You’ll often find one channel delivers 4:1 while another delivers 1.5:1 — that’s a budget reallocation waiting to happen.
2. Customer Lifetime Value (LTV)
LTV = (ARPU × Gross Margin %) ÷ Monthly Churn Rate
Revenue isn’t profit. A customer paying £200/month at 60% margin generates less than one paying £150/month at 85% margin (£120 vs £127.50/month). Use the margin-adjusted version for any acquisition spend decision.
3. The LTV:CAC ratio
LTV:CAC = LTV ÷ CAC
The single most important number. Target 3:1. Below that, you’re spending too much. Above 5:1, you may be underinvesting in growth.
4. CAC payback period
Payback (months) = CAC ÷ (Monthly ARPU × Gross Margin %)
For cash-constrained startups, this matters more than the ratio. A £50K LTV customer who takes 24 months to pay back is worse than a £30K customer who pays back in 8 — cash today beats theoretical lifetime value.
ApexSaaS, a fictional UK B2B startup. CAC = £450. ARPU = £75/mo. Gross margin = 78%. Monthly churn = 4%. Their LTV = (75 × 0.78) ÷ 0.04 = £1,463. LTV:CAC = 1,463 ÷ 450 = 3.25:1 — passing. CAC payback = 450 ÷ (75 × 0.78) = 7.7 months — passing. Ready to scale acquisition, not raise more.
What good looks like in 2026
| Metric | Concerning | Healthy | Best-in-class |
|---|---|---|---|
| LTV:CAC | <2:1 | 3:1 — 4:1 | 5:1+ |
| CAC payback | >24 months | 12 — 18 months | <12 months |
| Monthly churn | >5% | 2% — 5% | <2% |
| Gross margin | <60% | 70% — 85% | 85%+ |
Quick context: median B2B SaaS LTV:CAC is 3.2:1 across 939 companies (Optifai, 2026). Top quartile is at 4:1–6:1 (Bessemer 2026). Going from 2:1 to 3:1 can nearly triple your valuation. Stage matters — 5% monthly churn at pre-seed is normal; the same at Series A is a problem.
For a deeper benchmark breakdown by stage, segment, and channel, see our 2026 unit economics benchmarks.
The three levers
If your numbers don’t work, you have three things to change:
- Reduce CAC. Product-led growth (40–60% reduction common), better targeting, channel pruning. Easiest to act on. See our guide to reducing CAC for UK SaaS startups.
- Increase LTV. Reduce churn (1pp drop = ~33% LTV lift). Raise prices (most B2B SaaS underprice). Improve gross margin.
- Speed up payback. Activation flow audit. Most startups leak revenue in week 1 — reducing time-to-value from 14 days to 3 is one of the highest-leverage moves in any growth stack.
Five mistakes that kill the analysis
- Using revenue, not gross profit, in LTV. Inflates LTV by 15–30%.
- Calculating LTV from 6 months of data. Project a 5-year LTV from short history and you’re guessing. Use cohorts.
- Measuring blended CAC. Example: a fintech we audited had blended CAC of £620 — “fine.” Per-channel: Google £210, LinkedIn £1,400. Reallocation cut blended by 35% in 60 days.
- Forgetting fully-loaded CAC. Includes salaries + tooling, not just ad spend.
- Treating LTV as the input. LTV is downstream of CAC and retention. For early-stage, focus on what you control.
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