The seed stage was about finding product-market fit. Series A is about proving you can turn product-market fit into a scalable, repeatable growth engine. The bar changes completely. Seed investors bet on potential. Series A investors bet on evidence — and the evidence they demand is unit economics.
At this stage, your intangible assets are no longer theoretical. Your technology is generating revenue. Your customer relationships are deepening. Your brand is reducing acquisition costs. The question is no longer "does this work?" but "does this work economically at scale?" That question is answered by five numbers: LTV, CAC, the ratio between them, payback period, and burn rate.
Series A is the round where your business must prove it can acquire customers profitably. Every metric in this lesson — LTV, CAC, Rule of 40, burn rate — is a different lens on the same fundamental question: do the economics of your growth engine work?
Unit Economics: The Language of Series A
Unit economics strip away the complexity of your P&L and ask a simple question: on a per-customer basis, does your business make money? If the lifetime value of a customer exceeds the cost to acquire them by a sufficient margin, your business model works. If it does not, scaling will accelerate losses rather than generate profits.
LTV: Lifetime Value
Customer Lifetime Value is the total revenue you expect to earn from a customer over the entire duration of their relationship with your business. For SaaS, the standard formula uses monthly churn as the basis.
LTV = Average Revenue Per Account (ARPA) × Gross Margin % / Monthly Churn Rate. For a customer paying £700/month at 80% gross margin with 2% monthly churn: LTV = £700 × 0.80 / 0.02 = £28,000. With expansion revenue factored in (using NRR), the formula adjusts to reflect growing customer value.
CAC: Customer Acquisition Cost
CAC measures the fully loaded cost to acquire a single customer. It includes all sales and marketing spend — salaries, commissions, advertising, tools, content production — divided by the number of new customers acquired in the same period.
The Metrics That Matter
| Metric | Formula | NovaTech's Numbers | Series A Benchmark |
|---|---|---|---|
| ARPA | Total MRR / Customer count | £1,750/month | Varies by segment |
| LTV | ARPA × Gross Margin / Monthly Churn | £84,000 | Stage-dependent |
| CAC | Total S&M spend / New customers | £21,000 | Stage-dependent |
| LTV:CAC Ratio | LTV / CAC | 4.0× | >3.0× |
| CAC Payback | CAC / (ARPA × Gross Margin) | 14 months | <18 months |
| Gross Margin | (Revenue − COGS) / Revenue | 80% | >70% |
Why VCs Obsess Over LTV:CAC
The LTV:CAC ratio is the single most referenced metric in Series A due diligence. It answers a question that no other metric addresses so directly: for every pound you spend acquiring a customer, how many pounds do you get back?
A ratio below 1:1 means you are paying more to acquire customers than they are worth — your business destroys value with every sale. Between 1:1 and 3:1, the economics are marginal. Above 3:1, you have a profitable growth engine. Above 5:1, you should ask yourself whether you are underinvesting in growth.
A very high LTV:CAC ratio (above 7:1) is not always a positive signal. It can indicate that you are spending too little on acquisition and leaving growth on the table. VCs want to see efficient growth, not capital hoarding. The sweet spot for Series A is typically 3:1 to 5:1 with an accelerating growth rate.
The payback period adds a time dimension. A 4:1 LTV:CAC ratio with a 24-month payback means you need two years of cash runway per customer before seeing returns. A 4:1 ratio with a 12-month payback means you recoup your investment twice as fast, freeing capital for the next customer. VCs strongly prefer payback periods under 18 months.
The Rule of 40
The Rule of 40 is a heuristic used across the SaaS industry to assess the balance between growth and profitability. The formula is straightforward: add your year-over-year revenue growth rate to your profit margin (typically EBITDA margin). If the sum equals or exceeds 40, you are performing well.
Rule of 40 Scenarios
| Company | Revenue Growth | Profit Margin | Rule of 40 Score | Verdict |
|---|