By the time you’re raising a Series B, the fundraising conversation has shifted significantly. Investors are no longer betting on potential – they’re analysing your business with the rigour they’d apply to a late-stage investment. The metrics they care about have changed, the benchmarks are higher, and the questions are sharper.

Here’s what UK Series B investors are actually scrutinising, and how to put yourself in the strongest position before you start conversations.

What UK Series B Investors Are Looking For

UK Series B rounds typically range from £10 million to £30 million, and the investors writing those cheques – Balderton, Octopus Ventures, Index Ventures, Notion Capital – evaluate businesses against a consistent set of benchmarks. They want to see that you’ve found product-market fit, built a repeatable growth model, and have unit economics that justify deploying significant capital.

The metrics that matter most at this stage aren’t a mystery. They’re worth knowing before you build your investor materials.

ARR Growth and Net Revenue Retention

For SaaS and subscription businesses – which make up the majority of UK Series B raises – Annual Recurring Revenue (ARR) growth rate is the primary revenue signal. UK Series B investors typically expect to see 2-3x year-on-year ARR growth, though the precise benchmark depends on your market and the fundraising environment at the time.

Equally important – and consistently underweighted by founders – is Net Revenue Retention (NRR). NRR measures how much revenue you retain and expand from your existing customer base after accounting for churn and contraction. An NRR above 100% means your existing customers are spending more over time, which signals that your product is genuinely delivering value. Above 120% is considered excellent at Series B and will materially strengthen your valuation narrative. Below 100% means you’re losing ground with your existing base before acquiring a single new customer – investors will push hard on this, and rightly so.

Burn Multiple and the Rule of 40

Burn rate on its own is a less useful metric than it once was. The question UK Series B investors increasingly ask is not “how much are you burning?” but “how much new ARR are you generating per pound of net burn?” This is the burn multiple: net burn divided by net new ARR added in the same period. A burn multiple below 1x is excellent. Between 1x and 2x is acceptable for a growth-stage business. Above 2x suggests capital inefficiency that will need a credible explanation.

For SaaS businesses, the Rule of 40 provides a complementary benchmark: your ARR growth rate plus your EBITDA margin should total 40% or higher. A business growing at 60% year-on-year with a -20% EBITDA margin passes; one growing at 25% with a -25% margin doesn’t. It’s a useful shorthand for whether you’re achieving an acceptable balance between growth and efficiency – and it’s one UK growth-stage investors use regularly.

Unit Economics: CAC Payback and LTV:CAC

LTV:CAC ratio remains a standard Series B metric – a 3:1 ratio or higher is the baseline expectation. But UK investors are increasingly focused on CAC payback period alongside the ratio, because payback period is a more direct measure of capital efficiency. A CAC payback of under twelve months means you’re recovering acquisition spend quickly enough to fund growth without excessive external capital. Above eighteen months starts to raise questions about how efficiently you’re deploying marketing and sales investment.

If your payback period is long, the conversation shifts to whether your LTV is high enough to justify it, and whether your gross margin can absorb the wait.

Gross Margin and Path to Profitability

Gross margin is the foundation that everything else sits on. For SaaS businesses, Series B investors expect to see gross margins of 70% or above – anything below that suggests structural cost issues that will compound at scale. For other business models the benchmark varies, but the direction of travel matters: margins should be improving as you grow, not compressing.

EBITDA-level profitability is rarely expected at Series B, but a credible path to it is non-negotiable. Investors want to see a financial model that shows how the capital from this round moves you towards profitability within a defined timeframe, with clear assumptions and milestones attached. Vague projections don’t survive scrutiny at this stage – the model needs to be detailed enough to be challenged and still hold up.

Getting Your Financial Model Series B-Ready

The financial model you bring to Series B conversations is as important as the metrics themselves. It needs to demonstrate not just where you are now, but how the business grows from here – with the assumptions behind each driver made explicit. Investors at this stage will stress-test your numbers, and a model that can’t survive that process will undermine an otherwise strong story.

If your current model was built for seed or Series A conversations, it’s worth rebuilding it with Series B-level depth before you start pitching. The right model doesn’t just support your narrative – it shapes it.